Integrated circuit having a comparator circuit including at least one differential amplifier

[0002] This is a continuation of application Ser. No. 08/308,426, filed Sep. 19, 1994 now U.S. Pat. No. 6,320,429, which is a continuation-in-part of Ser No. 07/904,648, filed Jun. 26, 1992 now abandoned, which are incorporated herein by reference.

[0003] 1. Field of the Invention

[0004] The present invention relates to an integrated circuit having a comparator circuit including at least one differential amplifier, and more particularly to an integrated circuit having a comparator, which is adaptable for a 1-bit A/D convertor or a digitizing circuit for translating the output signals of sensors, entering keys, or the like, and delivering a control signal to a switch for controlling a load.

[0005] 2. Discussion of the Related Art

[0006] Output signals received from sensors, entering keys, or the like, which are treated as digital signals, may be considered to be, in the strict sense, analog signals having gentle rise profiles. Before such output signals are supplied to post-processing by a microcomputer, for example, the output signals must be converted into normal digital signals (binary logic signals) with high precision by a 1-bit A/D converter or a quantizing circuit. It is a common practice to use a buffer circuit for the digitizing circuit or the quantizing circuit. The buffer circuit has a unique threshold value. Because of this, the timing of changes in the logic level of the digital signal is unconditionally determined and fixed by the threshold value of the buffer circuit. As a result, it is impossible to adjust the rise point and the fall point of the digital signal. To cope with this, it is desirable to use a comparator constructed using a differential amplifier circuit of high gain. In this respect, there has been demanded the development of a semiconductor integrated circuit containing a plurality of comparators of that type in order to digitize a plurality of signals.

[0007] A conventional comparator using a high-gain differential amplifier circuit is shown in FIG. 12. As shown, the comparator is made up of a current mirror circuit M, a differential amplifier circuit S₁with two current paths L₁and L₂, and an inverter INV. The output section of the current mirror circuit operates as a constant current source for the differential amplifier circuit.

[0008] A drain current i₀of an enhancement type (E-type) MOS transistor 2 flows, as a reference current, from a voltage source V_DDto the input section of the current mirror circuit M, through a load MOS transistor 1 of the depletion type (D-type). A drain current i₃, which depends on the reference current i₀, flows through an E-type MOS transistor 10 in the output section.

[0009] In the first current path L₁of the differential amplifier circuit S₁, a drain current i₁flows from the voltage source V_DDand passes through a D-type MOS transistor 11, which serves as a load transistor, and an E-type MOS transistor 15, which serves as an amplifying transistor. In the second current path L₂, a drain current i₂flows from the voltage source V_DDand passes through a D-type MOS transistor 12, which serves as a load transistor, and an E-type MOS transistor 16, which serves as an amplifying transistor. The gates of the amplifying transistors 15 and 16 are respectively connected to input terminals in1 and in2 as the input terminals of the differential amplifier circuit S₁. The current paths L₁and L₂are connected to the output section (as a constant current source) of the current mirror circuit M. Accordingly, the following equation holds:

[0010] i₃=i₁+i₂  (1)

[0011] When the voltage applied to the input terminal in1 is equal to the voltage applied to the input terminal in2, the following equation holds:

[0012] i₁=i₂i₃/2  (2)

[0013] Under this condition, the differential amplifier circuit S₁is in a balanced state.

[0014] When the voltage (input voltage V_in) applied to the input terminal in2 is higher than the voltage (reference voltage V_ref) applied to the input terminal in1 the current i₂flowing through the MOS transistor 16 is increased by an increment Δi, which depends on the difference between the input voltages. Correspondingly, the current i₁flowing into the MOS transistor 15 is decreased by Δi, as seen from the equation (1). A voltage drop across the load transistor 12 increases and the source potential (potential at node 102) consequently drops.

[0015] When the input voltage V_inis lower than the reference voltage V_ref, the voltage drop across the load transistor 12 decreases and the source potential (potential at node 102) rises. Accordingly, the potential (at node 102) of the load transistor 12 is the amplified voltage, which depends on the difference between the voltages applied to the input terminals in1 and in2, with the balancing voltage being the voltage drop caused by the balancing current (i₃/2).

[0016] The inverter INV is made up of a MOS transistor 4, which serves as a switching element, and a MOS transistor 3, which serves as a load transistor, wherein both the MOS transistor 3 and the MOS transistor 4 are connected in series. The gate of the MOS transistor 4 receives the output voltage (differential amplifier voltage) V₁₀₂present at the node 102. The inverter INV produces a digitized signal V_OUTas a binary signal having a high or low logic level which depends on the amplitude of the input voltage V_inrelative to that of the reference voltage V_ref. As recalled, the input voltage V_inis applied to the input terminal in2 and the reference voltage V_refto the input terminal in1.

[0017] FIG. 13(a) is a graph showing variations of the input potentials (V_inand the reference voltage V_ref) of the differential amplifier circuit S₁with respect time t wherein the reference voltage V_refat the input terminal in1 is higher than the respective threshold voltages V_th15and V_th16of the amplifying transistors 15 and 16. At point A, the reference voltage V_refis equal to the input voltage V_in(V_ref=V_in). At point P, the input voltage V_inis equal to the threshold voltages V_th15and V_th16. FIG. 13(b) is a graph showing variations of the output voltages (voltage V₁₀₁, at node 101 and voltage V₁₀₂at node 102) of the differential amplifier circuit S₁with respect to units of time t corresponding to the units of time of FIG. 13(a) wherein the input voltages of differential amplifier circuit S₁are those shown in FIG. 13(a). A line consisting of alternating long and two short dashes indicates the threshold voltage V_th4of the MOS transistor 4. Point B indicates a voltage balanced state (V₁₀₁=V₁₀₂), which corresponds to the voltage balanced state at point A. At point C, the voltage V₁₀₂at the node 102 is equal to the threshold voltage V_th4.

[0018] When the input voltage V_inis lower than the threshold voltages V_th15and V_th16, the amplifying MOS transistor 16 is in an off state, and the amplifying MOS transistor 15 is in an on state. The node 102 is pulled up to the source potential V_DD, and the voltage V₁₀₁at the node 101 is in the lowest level V_LOW. When the input voltage V_inexceeds the threshold voltages V_th15and V_th16, the amplifying MOS transistor 16 is also turned on, a normal differentially amplified output signal is present at the nodes 101 and 102. When the input voltage V_inis lower than the threshold voltages V_th15and V_th16, the output signal of the differential amplifier circuit S₁, in the strict sense, is not the output signal when the differential amplifier circuit operates as a linear differential amplifier. However, this is not problematic in the digitizing operation because under this condition, the amplifier circuit has output characteristics which depend upon the input voltage difference.

[0019] In the case where the input voltage V_inis lower than the threshold voltages V_th15and V_th16, the circuit shown in FIG. 12 exhibits the following problems. FIG. 14(a) is a graph showing variations of the input potentials (V_inand the reference voltage V_ref) of the differential amplifier circuit S₁with respect to time t wherein the reference voltage V_refat the input terminal in1 is lower than the threshold voltages V_th15and V_th16of the amplifying transistors 15 and 16. At point A, the reference voltage V_refis equal to the input voltage V_in. At point P. the input voltage V_inis equal to the threshold voltages V_th15and V_th16. FIG. 14(b) is a graph showing variations of the output voltages (voltage V₁₀₁at node 101 and voltage V₁₀₂at node 102) of the differential amplifier circuit S₁with respect to units of time t corresponding to the units of time of FIG. 14(a) wherein the input voltages V_inand V_refof differential amplifier circuit S₁are those shown in FIG. 14(a). When the input voltage V_inis lower than the threshold voltages V_th15and V_th16, the amplifying MOS transistors 16 and 15 are both in an off state. Accordingly, the nodes 101 and 102 are both pulled up to the source potential V_DD. When the MOS transistors 16 and 15 are both inoperable in this low level region, and there is a voltage difference present between the input voltage V_inand the reference voltage V_ref, a differentially amplified output signal based on the voltage difference cannot be obtained. In other words, when the reference voltage V_refis set to be lower than the threshold voltages V_th15and V_th16, the differential amplifier circuit loses its differential amplifying function for the input voltage V_inwithin the low level region, and consequently it loses its digitizing function.

[0020] FIG. 13(c) is a graph showing variations of the output voltage V_OUTof the inverter INV with respect to units of time t corresponding to the units of time of FIGS. 13(a) and 13(b) wherein the input voltages V_inand V_refof differential amplifier circuit S₁are those shown in FIG. 13(a) and wherein the reference voltage V_refat the input terminal in1 is higher than the threshold voltages V_th15and V_th16of the amplifying transistors 15 and 16. When the input voltage V_inrises from ground potential, the voltage V₁₀₂at the node 102 falls, as seen from FIG. 13(b). When the voltage V₁₀₂falls and its locus crosses the level of the threshold voltage V_th4(point C) of the MOS transistor 4, the output voltage V_OUTof the inverter INV rises from a low potential level (referred to as an L level) to a high potential level (referred to as an H level). The balancing potential (point B) of the differential amplifier circuit S₁is determined by the element characteristics of the differential amplifier circuit S₁. The threshold voltage V_th4(point C) is a property of the MOS transistor 4 of the subsequent stage which depends, for example, on the semiconductor fabricating process used to produce MOS transistor 4. Therefore, the points B and C are generally not coincident with each other in position, and it is almost impossible to coincide point B with point C. For this reason, the logic level of the V_OUTremains unchanged until the input voltage V_inexceeds the reference voltage V_refplus an offset voltage V_off2. Presence of the offset voltage _off2owing to the threshold voltage of the inverter INV brings about retardation (phase delay) of the rise or advancement (phase advancement) of the fall of the output digital signal with respect to the input voltage V_in, and formation of an insensitive region for digitalization.

[0021] FIG. 14(c) is a graph showing variations of the output voltage V_OUTof the inverter INV with respect to units of time t corresponding to the units of time of FIGS. 14(a) and 14(b) wherein the input voltages V_inand V_refof differential amplifier circuit S₁are those shown in FIG. 14(a) and wherein the reference voltage V_refat the input terminal in1 is lower than the threshold voltages V_th15and V_th16of the amplifying transistors 15 and 16. Since the reference voltage V_refis lower than the threshold voltage V_th15,16of the amplifying MOS transistor 15, the transistor is always in an off state. When the input voltage V_inrises from the ground potential, and reaches the threshold voltage V_th15,16, the MOS transistor 16 is turned on and the output voltage V₁₀₂at the node 102 becomes a low voltage V_LOW. The transistor 4 connected to the node 102 is then turned on, so that the output voltage V_0UTof the inverter INV rises from an L level to an H level. Although the output voltage V_OUTtakes a digital form, the logic level changing point of the output voltage is determined by the threshold voltages V_th15,16which are properties of the MOS transistors. When the input voltage V_inexceeds the reference voltage V_ref, the logic state of the output voltage V_OUTremains unchanged. When the input voltage V_inexceeds the reference voltage V_refplus an offset voltage V_off1, the output voltage V_OUTchanges. Presence of the offset voltage V_off1owing to the threshold voltage of the differential amplifier circuit S₁also brings about retardation (phase delay) of the rise or advancement (phase advancement) of the fall of the output digital signal with respect to the input voltage V_in, and formation of an insensitive region for digitalization.

[0022] The dependency of the offset voltage V_off2on the reference voltage V_refnow will be described. First, consider the balancing potential of the amplifying voltages V₁₀₁and V₁₀₂at which the references voltage V_refis equal to the input voltage V_in, and the differential amplifier circuit S₁is in a balanced state. In the current path L₁of the differential amplifier circuit S₁shown in FIG. 12, when the reference voltage V_refincreases, the impedance of the MOS transistor 15 decreases. At this time, in order to compensate for a fixed balancing current (i₃/2) flowing through the MOS transistor 10, which serves as the constant current source, the impedance of the load transistor 11 increases. As a result, the voltage drop across the load transistor 11 increases, and the potential V₁₀₁at the node 101 falls. Accordingly, as the reference voltage V_refincreases, the amplified voltages V₁₀₁and V₁₀₂decrease. That is, the balancing potential is varied. Accordingly, as shown in FIG. 13(a), when the reference voltage V_refis at first higher than the threshold voltages V_th15and V_th16of the MOS transistors 15 and 16, and then is decreased, the voltage at point A (V_in=V_ref) drops. With this, the potential at point B (V₁₀₁=V₁₀₂) which represents the balancing potential of the differential amplifier circuit S₁, also changes. When the balancing potential at point B changes, the offset voltage V_off2, which represents a voltage to be added to the one of the input voltages V_inor V_refsuch that the output voltage V_OUTchanges logic levels when at a time when V_in=V_ref, also changes. Thus, the offset voltage V_off2depends on the reference voltage V_refof the differential amplifier circuit S₁. In other words, V_off2cannot be determined by only the threshold voltage of the inverter INV. This fact implies that when the reference voltage V_refis lowered to approach to the threshold voltage V_th15,16in order to operate the differential amplifier circuit in the low level region, the retardation (phase delay) of the rise of the output digitized signal with respect to the input voltage V_in, and the like increase, impairing the digitalization precision.

[0023] FIG. 15 shows an integrated circuit 400 having a comparator circuit 405 for controlling a switch element 410, which either permits or prevents a load current from flowing through a load 420, which may consist of a coil, via a voltage source 421. Typically, switch element 410 is a vertical voltage control switch element, such as a power MOSFET that may consist of an n channel DMOS (NDMOS).

[0024] When utilizing a conventional comparator circuit, such as that shown in FIG. 12, as the comparator circuit 405 in the integrated circuit 400 shown in FIG. 15, a problem arises due to the presence of PMOS and NMOS transistors in the conventional self-isolation CMOS comparator circuit.

[0025] FIG. 16 shows a cross-section of integrated circuit 450 constructed utilizing a comparator circuit 455 including both PMOS (460) and NMOS (470) transistors and a vertical voltage control switch element 480 all formed on an n⁻ epitaxial layer 490, which is formed on an n⁺ substrate 491. PMOS transistor 460 includes p-type source and drain diffusion layers 462 formed in the surface of n⁻ epitaxial layer 490, and a gate electrode 465 formed on a gate insulation film (not shown) bridging source and drain diffusion layers 462. NMOS transistor 470 includes a p-type base layer 471 formed in the surface of n⁻ epitaxial layer 490, n-type source and drain diffusion layers 472 formed in the surface of p-type base layer 471, and a gate electrode 475 formed on a gate insulation film (not shown) bridging source and drain diffusion layers 472. Vertical voltage control switch element 480 includes a p-type base layer 481 formed in the surface of n⁻ epitaxial layer 490, n-type source diffusion layers 482 formed in the surface of p-type base layer 481, gate electrodes 485 formed on a gate insulation film (not shown), and a drain electrode 486 connected to the opposite surface of n⁺ substrate 491.

[0026] If such an integrated circuit is prepared as shown in FIG. 16, the circuit cannot be operated due to the forward bias present at the p-n junction between epitaxial layer 490 and p-type source diffusion layer 462 of PMOS transistor 460. The forward bias results from the 0.1 volt on-state voltage of the vertical voltage control switch 490, which produces a potential of 0.1 V in n⁻ epitaxial layer 490. When this forward bias is present, comparator circuit 455 cannot supply a normal gate voltage to the gate of vertical voltage control switch 480.

[0027] Thus, to operate a comparator circuit of the type shown in FIG. 12 in the integrated circuit shown in FIG. 15, the integrated circuit must be designed to include isolation layers between the comparator circuit 455 and vertical voltage control switch 480. An example of such an integrated circuit is shown in FIG. 17 and is generally designated by reference numeral 550.

[0028] Integrated circuit 550 includes a comparator circuit 555 including both PMOS (560) and NMOS (570) transistors and a vertical voltage control switch element 580 all formed on a p-type substrate 590. PMOS transistor 560 includes p-type source and drain diffusion layers 562 formed in the surface of a first n-type epitaxial layer 591, and a gate electrode 565 formed on a gate insulation film 566 bridging source and drain diffusion layers 562. NMOS transistor 570 includes a p-type base layer 571 formed in the surface of first n-type epitaxial layer 591, n-type source and drain diffusion layers 572 formed in the surface of p-type base layer 571, and a gate electrode 575 formed on gate insulation film 566 bridging source and drain diffusion layers 572. Vertical voltage control switch element 580 includes a p-type base layers 581 formed in the surface of a second n-type epitaxial layer 592, n-type source diffusion layers 582 formed in the surface of one of the p-type base layers 581, gate electrodes 585 formed on gate insulation film 566 bridging p-type base layers 581, and an n-type drain diffusion layer 595. By providing two separate n-type epitaxial layers 591 and 592 isolated by a portion of p-type substrate 590 for comparator circuit 555 and vertical voltage control switch 580, the p-n junction between first n-type epitaxial layer 591 and p-type source diffusion layer 562 of PMOS transistor 560 will remain inverse biased and comparator circuit 555 can supply a normal gate voltage to the gate of vertical voltage control switch 580.

[0029] However, the integrated circuit design shown in FIG. 17 is disadvantageous in that the isolating portions of the substrate make manufacturing and miniaturization difficult. Because the use of PMOS transistors in the comparator circuit necessitate the use of isolation layers, it is preferable to not use PMOS transistors in the comparator circuit to avoid such problems.

[0030] The present invention has been made in view of the above circumstances and has as an object to provide an integrated circuit including a comparator circuit and a vertical voltage control switch element formed on a single substrate that is compact and easy to manufacture.

[0031] Another object of the present invention is to provide a differential amplifier circuit and a comparator that can ensure high digitalization precision for the differential input voltage V_inin the low level region. To ensure high digitalization precision, it is desirable to provide a comparator circuit in which an offset voltage, if present, does vary with respect to a reference voltage applied to the comparator.

[0032] Additional objects and advantages of the invention will be set forth in part in the description which follows and in part will be obvious from the description, or may be learned by practice of the invention. The objects and advantages of the invention may be realized and attained by means of the instrumentalities and combinations particularly pointed out in the appended claims.

[0033] To achieve the objects and in accordance with the purpose of the invention, as embodied and broadly described herein, the integrated circuit of this invention comprises a substrate, a comparator circuit, formed on the substrate, and a switch, formed on the substrate and connected to a load, for continuing or discontinuing the current supplied to the load in response to an output signal from the comparator circuit. The comparator circuit comprises a constant current source, input means for inputting a reference voltage signal and an input voltage signal, a differential amplifier including at least two parallel current paths each including a load element, and a MOS transistor having a gate connected to the input means to receive one of the input voltage signal and the reference voltage signal, a drain connected to said load element, and a source connected to the constant current source. The comparator circuit further comprises output means, connected to one of the current paths, for outputting the output signal, which is representative of the comparison of the input voltage signal and the reference voltage signal.

[0034] The accompanying drawings, which are incorporated in and constitute a part of this specification illustrate embodiments of the invention and, together with the description, serve to explain the objects, advantages and principles of the invention. In the drawings,

[0035] FIG. 1 is a circuit diagram showing a comparator using a differential amplifier circuit according to a first embodiment of the present invention;

[0036] FIGS. 2(a)-2(c) are graphs showing signal waveforms at key points in the first embodiment;

[0037] FIG. 3 is a circuit diagram showing a comparator using a differential amplifier circuit according to a second embodiment of the present invention;

[0038] FIG. 4 is a circuit diagram showing a comparator using a differential amplifier circuit according to a third embodiment of the present invention;

[0039] FIGS. 5(a)-5(f) are graphs showing signal waveforms at key points in the second embodiment;

[0040] FIG. 6 is a circuit diagram showing a comparator using a differential amplifier circuit according to a fourth embodiment of the present invention;

[0041] FIG. 7 is a circuit diagram showing a comparator using a differential amplifier circuit according to a fifth embodiment of the present invention;

[0042] FIGS. 8(a)-8(f) are graphs showing signal waveforms at key points in the fourth embodiment;

[0043] FIG. 9 is a circuit diagram showing a comparator using a differential amplifier circuit according to a sixth embodiment of the present invention;

[0044] FIG. 10 is a circuit diagram showing a comparator using a differential amplifier circuit according to a seventh embodiment of the present invention;

[0045] FIG. 11 is a circuit diagram showing a comparator using a differential amplifier circuit according to an eighth embodiment of the present invention;

[0046] FIG. 12 is a circuit diagram of a comparator using a conventional differential amplifier circuit;

[0047] FIGS. 13(a)-13(c) are graphs showing signal waveforms at key points when the reference voltage is set to be higher than the threshold value of the amplifying element in the conventional comparator;

[0048] FIGS. 14(a)-14(c) are graphs showing signal waveforms at key points when the reference voltage is set to be lower than the threshold value of the amplifying element in the conventional comparator;

[0049] FIG. 15 is a circuit diagram of an integrated circuit including a comparator circuit and a switch element connected to a load;

[0050] FIG. 16 is a partial cross sectional view of an integrated circuit including a comparator circuit having a PMOS transistor and a switch element connected to a load;

[0051] FIG. 17 is a partial cross sectional view of an integrated circuit including a comparator circuit and a switch element connected to a load where the comparator circuit and a switch element are isolated from one another;

[0052] FIG. 18 is a partial cross sectional view of an integrated circuit according to a ninth embodiment of the present invention; and

[0053] FIG. 19 is a partial cross sectional view of an integrated circuit according to a tenth embodiment of the present invention.

[0054] A first embodiment of the invention is a differential amplifier having a constant current source and a differential amplifier circuit including at least two current paths arranged in parallel, each current path includes a load section and an amplifying element with a control input terminal, wherein D-type MOS transistors having no threshold voltage V_th, are used as the amplifying elements instead of E-type MOS transistors which have a threshold voltage V_th. A comparator for digitizing an input signal further includes switching means operable in response to the output signal of any one of the amplifying elements. The switching means may be an inverter circuit. The comparator is preferably constructed on a semiconductor integrated circuit.

[0055] In the differential amplifier circuit of the first embodiment, which uses D-type MOS transistors having no threshold voltage V_th, the MOS transistor is in an on state regardless of the voltage level applied to the control input terminal of the amplifying element. Accordingly, if an input voltage having a potential level within a subthreshold region, which ranges from the reference voltage for the E-type MOS transistor to the threshold voltage V_th, is applied to a pair of control input terminals of the differential amplifier circuit using D-type MOS transistors as the amplifying elements, the amplifying elements operate in the differential amplifying mode so that a balancing potential can always be obtained. Thus, a digital signal can be produced without using a level shift circuit, for example, even in a case where the reference voltage is set to be extremely biased towards the source potential.

[0056] FIG. 1 is a circuit diagram showing a comparator using a differential amplifier circuit according to the first embodiment of the present invention. As shown, the comparator is made up of a current mirror circuit M, a differential amplifier circuit D₁with two current paths L₁and L₂, and an inverter INV. The output section of the current mirror circuit M operates as a constant current source for the differential amplifier circuit D₁.

[0057] A drain current i₀of an E-type MOS transistor 2 flows, as a reference current, from a voltage source V_DDto the input section of the current mirror circuit M, through a load MOS transistor 1 of the D-type. A drain current i₃, which depends on the reference current i₀, flows through an E-type MOS transistor 10 in the output section.

[0058] In the first current path L₁of the differential amplifier circuit D₁, a drain current i₁flows from the voltage source V_DDand passes through a D-type MOS transistor 11, which serves as a load transistor, and an E-type MOS transistor 13, which serves as an amplifying transistor. In the second current path L₂, a drain current i₂flows from the voltage source V_DDand passes through a D-type MOS transistor 12, which serves as a load transistor, and an E-type MOS transistor 14, which serves as an amplifying transistor. The gates of the amplifying transistors 13 and 14 are respectively connected to input terminals in1 and in2 as the input terminals of the differential amplifier circuit D₁. The current paths L₁and L₂are connected to the output section (as a constant current source) of the current mirror circuit M. Accordingly, equation (1) discussed above holds true for the circuit of FIG. 1 as well as the circuit of FIG. 12. When the voltages applied to the input terminals in1 and in2 are equal to each other, equation (2) discussed above, also holds for this circuit.

[0059] The inverter INV is made up of a MOS transistor 4, which serves as a switching element, and a MOS transistor 3, which serves as a load transistor, wherein both the MOS transistor 3 and the MOS transistor 4 are connected in series. The gate of the MOS transistor 4 receives the output voltage (differential amplifier voltage) V₁₀₂present at the node 102. The inverter INV produces a digitized signal V_OUTas a binary signal having a high or low logic level, which depends on the amplitude of the input voltage V_inrelative to that of the reference voltage V_ref. As recalled, the input voltage V_inis applied to the input terminal in2 and the reference voltage V_refto the input terminal in1.

[0060] N-channel D-type MOS transistors 13 and 14 are used for the amplifying elements of the differential amplifier circuit D₁. Even in the subthreshold region ranging from the ground potential V_SSto the threshold voltage V_th(normally 0 to about +1.5 V), the D-type MOS transistors 13 and 14 are in an on state. Accordingly, the differential amplifier circuit D₁can produce a differential amplifier output signal, which is based on the linear characteristics of the differential amplifier circuit even in the low level region, as will be described below.

[0061] FIG. 2(a) is a graph showing variations of the input potentials (V_inand the reference voltage V_ref) of the differential amplifier circuit D₁with respect to time t wherein the reference voltage V_refat the input terminal in1 is lower than a subthreshold region (referred to as a SUB region). At point A, the reference voltage V_refis equal to the input voltage V_in(V_ref=V_in). FIG. 2(b) is a graph showing variations of the output voltages (voltage V₁₀₁at node 101 and voltage V₁₀₂at node 102) of the differential amplifier circuit D₁with respect to units of time t corresponding to the units of time of FIG. 2(a) wherein the input voltages V_inand V_refof differential amplifier circuit D₁are those shown in FIG. 2(a). A line consisting of alternating long and two short dashes indicates the threshold voltage V_th4of the MOS transistor 4. Point B indicates a voltage balanced state (V₁₀₁=V₁₀₂), which corresponds to the voltage balanced state at point A. At point C, the voltage V₁₀₂at the node 102 is equal to the threshold voltage V_th4. FIG. 2(c) is a graph showing variations of the output voltage V_OUTof the inverter INV with respect to the input potential V_inat the input terminal in2.

[0062] As shown in FIG. 2(a), also when the reference voltage V_refis set in the SUB region, the D-type MOS transistors 13 and 14 of the differential amplifier circuit D₁are in an on state. Therefore, as shown in FIG. 2(b), the linear characteristic of the differential amplifier circuit D₁is maintained also in this low level region. As will be described later, the fact that the linearity is ensured in the low level region indicates that the logic change point can be set at the balancing point in the cascade-connected differential amplifier circuits. As shown in FIG. 2(c), the offset voltage V_off2caused by the threshold value of the inverter INV is present, but the offset voltage V_off1shown in FIG. 14(c), which is caused by the threshold voltage of the differential amplifier circuit, is not present.

[0063] A second embodiment of the invention includes “n” stages of the differential amplifier circuits connected in cascade fashion and having the same construction as that of the differential amplifier circuit in the first embodiment. The amplifying elements in the first amplifier stage have a control input terminal. The output signal of the amplifying element in the i-th stage of said differential amplifier circuit is applied as a control signal to the amplifying element in the (i+1)th differential amplifier circuit. The amplifying elements of the first amplifier stage may be D-type MOS transistors. The amplified output signal of the n-th amplifier stage is a digital signal. To match the output signal with the logic level of a TTL family, switching means operable in response to the output signal of any one of the “n” amplifying elements may be provided. The switching means may be an inverter circuit. Such a comparator is preferably constructed on a semiconductor integrated circuit.

[0064] According to the arrangement of the second embodiment, if the difference between the voltages applied to the pair of control input terminals of the first stage of the cascade-connected differential amplifier circuits is small, the differential amplified output signal of the final amplifier stage is extremely large in amplitude and has a steep rise and fall waveform profile. In each amplifier stage, no current flows into the amplifying element of one of the paired current paths connected in parallel. Accordingly, no voltage drop takes place across the load, so that the output signal is equal to the source potential in level and provides the upper limit of the output characteristic. In such a case, a maximum current flows into the amplifying element in the other current path. The output signal reaches such a potential level as to cause a maximum voltage drop across the load, and provides the lower limit of the output characteristic. Thus, the amplified voltage signal output from the differential amplifier circuit is confined by the upper and the lower limits. Accordingly, the output voltage signal, which otherwise is an analog signal, can be treated as a digital signal.

[0065] In the conventional comparator, discussed above with reference to FIG. 12, a digital signal is obtained through a switching circuit from the amplified voltage signal output from a single differential amplifier circuit. Therefore, the conventional comparator further suffers from the following problems. Between the single differential amplifier circuit and the switching circuit, the balancing potential of the differential amplifier circuit is not coincident, with the threshold voltage characteristic of the switching element. When the reference voltage is varied, the balancing potential of the differential amplifier circuit also varies. In the present invention, because multiple stages of differential amplifier circuits are connected in cascade fashion, a digital signal can be realized without the need of a switching circuit. Therefore, the problems as stated above will not be created in the present invention. Further, in the present invention, the logic level changing point of the digital signal coincides with a comparison point where the input voltage V_inis equal to the reference voltage V_refso that high precision digitalization is realized.

[0066] FIG. 3 is a circuit diagram showing a comparator using differential amplifier circuits according to the second embodiment of the present invention. As shown, the comparator includes a multiple of stages of differential amplifier circuits D₁to D_nconnected in cascade. The comparator, includes a current mirror circuit M, and at least one differential amplifier circuit D₁with two current paths L₁and L₂, which respectively have input terminals in1 and in2. The output section of the current mirror circuit operates as a constant current source for the differential amplifier circuit D₁. The MOS transistors 15 and 16, which serve as the amplifying elements of the differential amplifier circuit D₁, are of the E-type. Nodes 101 and 102, which represent the amplified output voltages of the differential amplifier circuit D₁of a first stage in the cascade arrangement, are respectively connected to the gates of the amplifying MOS transistors 26 and 25 of the differential amplifier circuit D₂of a second stage. Nodes 201 and 202 of the differential amplifier D₂are respectively connected to the gates of the amplifying MOS transistors 36 and 35 of the differential amplifier circuit D₃of a third stage. The node-to-gate connection is continued when the n-th amplifier stage is reached. In this way, a differential amplifier arrangement consisting of multiple stages is constructed. MOS transistors 20 to n0, which serve as the constant current sources of the differential amplifier circuits D₂an D_n, each make up a current mirror circuit in cooperation with the MOS transistor 2. In this embodiment, the constant current sources are arranged in parallel for the output section of the current mirror circuit M. Alternatively, the constant current source may be arranged in a hierarchical structure for the output section. The output signal OUT is derived from a node n01 of the final differential amplifier stage D_n.

[0067] In this embodiment, since the N channel E-type MOS transistors are used for the amplifying elements of the differential amplifier circuit, the amplified voltage loses its linearity in the subthreshold voltage region of the voltage applied to the input terminals in1 and in2, as in the conventional differential amplifier circuit. Therefore, the digital signal cannot be produced in the SUB region. However, when the reference voltage V_refis higher than the threshold voltages V_th15and V_th16of the amplifying MOS transistors 15 and 16, it can produce a high precision digital signal.

[0068] FIG. 5(a) is a graph showing variations of the input potentials (V_inand the reference voltage V_ref) of the differential amplifier circuit D₁with respect to time t wherein the reference voltage V_refat the input terminal in1 is higher than the threshold voltages V_th15and V_th16of the amplifying MOS transistors 15 and 16. At point A, the reference voltage V_refis equal to the input voltage V_in(V_ref=V_in). FIG. 5(b) is a graph showing variations of the output voltages (voltage V₁₀₁at node 101 and voltage V₁₀₂at node 102) of the differential amplifier circuit D₁with respect units of time t corresponding to the units of time of FIG. 5(a) wherein the input voltages V_inand V_refof differential amplifier circuit D₁are those shown in FIG. 5(a). Point B₁indicates a voltage balanced state (V₁₀₁=V₁₀₂), which corresponds to the voltage balanced state at point A. FIG. 5(c) is a graph showing variations of the output voltages (voltage V₂₀₁at node 201 and voltage V₂₀₂at node 202) of the differential amplifier circuit D₂with respect to units of time t corresponding to the units of time of FIGS. 5(a) and 5(b) wherein the input voltages V_inand V_refof differential amplifier circuit D₁are those shown in FIG. 5(a). Point B₂indicates a voltage balanced state (V₂₀₁=V₂₀₂), which corresponds to the voltage balanced state at point A. FIG. 5(d) is a graph showing variations of the output voltages (voltage V₃₀₁at node 301 and voltage V₃₀₂at node 302) of the differential amplifier circuit D₃with respect to units of time t corresponding to the units of time of FIGS. 5(a)-5(c) wherein the input voltages V_inand V_refof differential amplifier circuit D₁are those shown in FIG. 5(a). Point B₃indicates a voltage balanced state (V₃₀₁=V₃₀₂), which corresponds to the voltage balanced state at point A. FIG. 5(e) is a graph showing variations of the output voltages (voltage V_nat node n01 and voltage V_nat node n02) of the differential amplifier circuit D_nwith respect to units of time t corresponding to the units of time of FIGS. 5(a)-(d) wherein the input voltages V_inand V_refof differential amplifier circuit D₁are those shown in FIG. 5(a). Point B_nindicates a voltage balanced state (V_nn), which corresponds to the voltage balanced state at point A.

[0069] It is assumed that in FIG. 5(a), the difference of the voltages applied to the differential amplifier circuit D₁is |V_ref−V_in|. When the reference voltage V_refis equal to the input voltage V_in, |V_ref−V_in|0. The amplified voltage differences are also zero in a balanced state at the respective amplifier stages. Accordingly, the balanced states at points A to B_ntake place at the same thing.

[0070] Assuming that the differential amplifier gain is G, the output voltage difference |V₁₀₁−V₁₀₂| of the differential amplifier circuit D₁is given by

[0071] |V₁₀₁−V₁₀₂|=G×|V_ref−V_in|  (4)

[0072] The output voltage difference |V_n−V_n| of the n-the stage differential amplifier circuit D_nis given by

[0073] |V₁₀₁−V₁₀₂|Gⁿ×|V_ref−V_in|  (5)

[0074] Therefore, the output voltage characteristics of the differential amplifier circuits D₁, D₂, D₃, . . . , D_nare as shown in the figures. As the number of differential amplifier stages is increased, the output voltages more steeply change with increase of the amplified voltage, with respect to the balancing points B₁, B₂, B₃, . . . , B_n. The upper limit of the output voltage of each differential amplifier circuit is at the source voltage V_DD, while the lower limit is at the voltage V_LOW, which is the lowest voltage relative to the ground potential. Consequently, the output characteristic profile of the final amplifier stage D_nmay be considered to be the profile of a digitized signal. The logic state of the digitized signal changes invariably at the balancing point B_n, so that the signal is not attended by the offset voltage V_off1, which depends on the threshold voltage of the subsequent inverter. In other words, since the digitized signal can be obtained without the switching circuit, for example, the inverter of the subsequent stage, it is not attended by the offset voltage V_off1and hence high precision digitalization is realized. As in the conventional circuit, as the reference voltage V_refis set to be lower, the balancing potential rises to a certain degree. However, the digitalization precision is almost invariable against the variation of the reference voltage V_refsince the changes in logic level take place at the timing of the balancing point. Therefore, in this embodiment, there is eliminated the dependency of the digitalization precision on the reference voltage V_ref. When the input voltage V_inis lower than the threshold voltage V_th15and V_th16the MOS transistor 16 is in an off state, while the MOS transistor 15 is in an on state. Accordingly, the node 102 is pulled up to the source voltage V_DDand the voltage V₁₀₂at the node 101 is at the lowest level V_LOW. The differential characteristic is not linear as shown in FIGS. 5(b) and 5(c). However, this is not problematic in the digitizing operation since the output characteristic depends on the polarity of the input voltage difference.

[0075] FIG. 4 is a circuit diagram showing a comparator using the differential amplifier circuit according to the third embodiment of the invention. The comparator, which is based on the cascade-connected arrangement of the differential amplifier circuits shown in FIG. 3, further includes an inverter INV. FIG. 5(f) is a graph showing variations of the output voltage V_OUTof the inverter INV with respect to units of time t corresponding to the units of time of FIGS. 5(a)-(e) wherein the input voltages V_inand V_refof differential amplifier circuit D₁are those shown in FIG. 5(a) and wherein the reference voltage V_refat the input terminal in1 is higher than the threshold voltages V_th15and vth₁₆of the amplifying MOS transistors 15 and 16. The logic levels of the output voltage signal of the final amplifier stage are such that a high logic level is at the source potential V_DDand a low logic level is at the lowest level V_LOWof the amplifier output signal. To match it with the logic levels of the TTL logic family, for example, the inverter INV is provided so as to set the low logic level closer to the ground level V_SS.

[0076] FIG. 6 is a circuit diagram showing a comparator using the differential amplifier circuit according to the fourth embodiment of the invention.

[0077] This embodiment is constructed by using D-type MOS transistors 13 and 14 for the amplifying elements 15 and 16 in the first amplifier stage D₁in the comparator shown in FIG. 3. FIG. 8 shows graphs of signal waveforms at the key points of the comparator. As shown in FIG. 8(b), also in this embodiment, the differential amplifying function operates in the SUB region closer to the ground potential V_SS, as in the first embodiment. Accordingly, the digitalization is possible even in the low level region. Further, because of the cascade connection of the differential amplifier circuits, the offset voltage is not caused and the logic level changing point resides at the balancing point of the differential amplifier circuit. Accordingly, this embodiment has the advantageous features of both the first and second embodiments.

[0078] FIG. 7 is a circuit diagram showing a comparator using the differential amplifier circuit according to the fifth embodiment of the invention.

[0079] In this embodiment, an inverter INV is used in addition to the comparator based on the cascade connection of differential amplifier circuits shown in FIG. 6. The reason why the inverter INV is additionally used is to match it with the logic level of the TTL family, for example, the low logic level is made to more approach to the ground potential V_SS.

[0080] In the respective embodiments as mentioned above, the D-type MOS transistor is used for the load element. When the differential amplifiers and the comparators of the invention are fabricated into an integrated circuit, an E-type MOS transistor or a diffusion resistor, in place of the D-type MOS transistor, may be used for the load element.

[0081] FIG. 9 is a circuit diagram showing a comparator using the differential amplifier circuit according to the sixth embodiment of the invention. In this embodiment, an E-type MOS transistor 7 is used for the load element in the current mirror circuit M.

[0082] FIG. 10 is a circuit diagram showing a comparator using the differential amplifier circuit according to the seventh embodiment of the invention. In this embodiment, E-type MOS transistors 17 and 18 are used for the load elements in the differential amplifier circuit D₁.

[0083] FIG. 11 is a circuit diagram showing a comparator using the differential amplifier circuit according to the eight embodiment of the invention. In this embodiment, E-type MOS transistors 17, 18, 27, 28, 37, 38, n7, and n8 are used for the load elements in the differential amplifier circuit D₁to D_n.

[0084] It will be evident to those skilled in the art that the N channel MOS transistors in the embodiments as mentioned above may be replaced by P channel MOS transistors. In this case, the source voltage V_DDis a negative voltage relative to the ground potential V_SS, normally 0 V. Accordingly, the potential in the SUB region is a negative potential relative to the ground potential V_SS. It will be understood that based upon symmetric properties, if the reference voltage is set to be closer to the high potential side, the digital signal can be realized with high precision.

[0085] As seen from the foregoing description, in the present invention, the depletion type MOS transistors are used for the amplifying elements in the differential amplifier circuit. Accordingly, the following advantageous effects can be achieved.

[0086] Since the amplifying element has no threshold value, even if the reference voltage is extremely biased toward the source potential, the differential amplifying characteristics can be realized. Accordingly, the digitized signal in the low level region, which cannot be obtained by the conventional technique, can be obtained without using a level shift circuit, for example, so that the analog signal can be digitized with high precision, with removal of the offset voltage.

[0087] The multistage arrangement of the differential amplifier circuits connected in cascade allows the analog signal to be digitized without the switching means provided at the subsequent stage. Accordingly, the circuit is not attended by the offset voltage owing to the threshold value of the switching means, so that the high precision digitalization is realized free from the phase distortion. If the balancing points of the differential amplifier circuits are dislocated in accordance with the value of the reference voltage, the timings of the balancing points are invariable. The dislocation of the balancing points will not influence the digitalization precision.

[0088] Use of the different embodiments described above expands the range of the input voltages to be digitized and realizes high precision digitalization.

[0089] FIG. 18 shows the cross-section of an integrated circuit constructed according to a ninth embodiment of the present invention, which may employ the various embodiments of the comparator circuit described above in the manner described above with respect to FIG. 15.

[0090] The integrated circuit shown in FIG. 18 includes an n+substrate 710, an n⁻ epitaxial layer 715, a comparator circuit 755 including NMOS transistors 760 and 770, and a switching transistor 780.

[0091] NMOS transistor 760 includes a p-type base layer 762 formed in epitaxial layer 715, an n-type source diffusion layer 763 formed in p-type base layer 762, a source electrode 764 connected to n-type source diffusion layer 763, an n-type drain diffusion layer 765 formed in p-type base layer 762, a drain electrode 766 connected to n-type drain diffusion layer 765, a gate insulation film 767 covering portions of p-type base layer 762,-n-type source diffusion layer 763, and n-type drain diffusion layer 765, and a gate electrode 768 formed on a region of gate insulation film 767 bridging p-type base layer 762, n-type source diffusion layer 763, and n-type drain diffusion layer 765.

[0092] NMOS transistor 770 includes a p-type base layer 772 formed in epitaxial layer 715, an n-type source diffusion layer 773 formed in p-type base layer 772, a source electrode 774 connected to n-type source diffusion layer 773, an n-type drain diffusion layer 775 formed in p-type base layer 772, a drain electrode 776 connected to n-type drain diffusion layer 775, gate insulation film 767, which covers portions of p-type base layer 772, n-type source diffusion layer 773, and n-type drain diffusion layer 775, and a gate electrode 778 formed on a region of gate insulation film 767 bridging p-type base layer 772, n-type source diffusion layer 773, and n-type drain diffusion layer 775.

[0093] Switching transistor 780 is preferably a power MOSFET formed as an n channel DMOS (NDMOS) including a p-type base layer 782 formed in epitaxial layer 715, n-type source diffusion layers 783 formed in p-type base layer 782, source electrodes 784 connected to n-type source diffusion layers 783, a drain electrode 786 connected to the opposite surface of n⁺ substrate 710, gate insulation film 767, which covers portions of p-type base layer 782 and n-type source diffusion layers 783, and gate electrodes 788 formed on a region of gate insulation film 767 bridging p-type base layer 782 and n-type source diffusion layers 783.

[0094] FIG. 19 illustrates an integrated circuit constructed according to a tenth embodiment of the present invention, which may employ the various embodiments of the comparator circuit described above.

[0095] The tenth embodiment of the integrated circuit shown in FIG. 19 is similar to the ninth embodiment shown in FIG. 18 except that n⁺ substrate 710 is replaced with a p-type substrate 810. By using p-type substrate 810, switching transistor 780, functions as an n channel insulated gate bipolar transistor (IGBT).

[0096] The foregoing description of preferred embodiments of the invention has been presented for purposes of illustration and description. It is not intended to be exhaustive or to limit the invention to the precise form disclosed, and modifications and variations are possible in light of the above teachings or may be acquired from practice of the invention. The embodiments were chosen and described in order to explain the principles of the invention and its practical application to enable one skilled in the art to utilize the invention in various embodiments and with various modifications as are suited to the particular use contemplated. It is intended that the scope of the invention be defined by the claims appended hereto, and their equivalents.