Semiconductor Differential Amplifier

The present invention relates to a semiconductor differential amplifier.

FIG. 2 shows a prior art semiconductor differential amplifier. In FIG. 2, the reference characters Q1 and Q2 designate a third and a fourth load P channel MOS transistor having an equal gate width and gate length, respectively. The reference characters Q3 and Q4 designate a first and a second driving N channel MOS transistor having an equal gate width and gate length, respectively. The reference characters D and D designate a pair of input terminals. The reference character N designates a connection node of the drains of the transistors Q1 and Q3. The reference character RD designates a connection node of the drains of the transistors Q2 and Q4, RD being an output terminal of this differential amplifier. The reference character Q5 designates a power cut N channel MOS transistor. The reference character SE designates a power cut internal signal. The reference character Vcc designates a power supply voltage, and the reference character GND designates a ground voltage.

The device operates as follows.

Supposing that the power cut internal signal SE is at "H" level, the transistor Q5 is turned on an the differential amplifier is activated.

In such a state, the transistor Q1 has a common gate drain, and therefore the variation of the voltage of the node N is small relative to the variation of the voltage of the input terminal D. The node N also functions as a gate input of the transistor Q2 and if the voltages of the input terminals D and D are equal to each other, the output at node RD will be the same voltage as that of the node N. The voltages of the input terminals D and D and the sizes of the transistors Q1 to Q4 are established such that the transistors Q2 and Q4 enter an equilibrium state at the pentode region. In this state, when a slight voltage difference arises between input terminals D and D the transistors Q2 and Q4 go out of equilibrium, thereby resulting in a large variation of the voltage at the output node RD.

Furthermore, when the power cut internal signal SE is at an "L" level thereby to turn off the transistor Q5, the quiescent currents which penetrate through the transistors Q1 to Q3, or the transistors Q2 to Q4, respectively, are eliminated, whereby the device enters into the power cut state.

In the prior art semiconductor differential amplifier of such construction, the voltage difference between the input terminal pair D and D is received only by the N channel MOS transistor, and thus the ability of pulling up the voltage at the node N or the output RD to the power supply voltage side is small, therefore it is not possible to obtain a high sensitivity against a small voltage difference between the terminal pair D and D.

An object of the present invention is to provide an improved semiconductor differential amplifier having a high sensitivity.

Other objects and advantages of the present invention will become apparent from the detailed description given hereinafter; it should be understood, however, that the detailed description and specific embodiment are given by way of illustration only, since various changes and modifications within the spirit and scope of the invention will become apparent to those skilled in the art from this detailed description.

According to the present invention, there is provided a semiconductor differential amplifier for amplifying the voltage difference between a first and a second input terminal and providing an output signal on an output terminal representing said difference, which comprises first conductivity type first and second MOS transistors and second conductivity type third to sixth MOS transistors; the gates of said first and fifth MOS transistors being connected to said first input terminal; the gates of said second and sixth MOS transistors being connected to said second input terminal; the drains of said first, third, and fifth MOS transistors and the gates of said third and fourth MOS transistors being connected with each other; the drains of said second, fourth, and sixth transistors being connected to an output terminal; the sources of said first and second MOS transistors being connected to a first constant voltage source; and the sources of said third to sixth MOS transistors being connected to a second constant voltage source.

FIG. 1 is a circuit diagram showing a semiconductor differential amplifier of a preferred embodiment of the present invention;

FIG. 2 is a circuit diagram showing a prior art semiconductor differential amplifier;

FIG. 3 is a diagram showing the input vs output transmission characteristics of the prior art device and the device of the present invention; and

FIG. 4 is a diagram showing a circuit in which the amplification factor is enhanced with the use of the prior art differential amplifier shown in FIG. 2.

In order to explain the present invention in detail, reference will be particularly made to FIG. 1.

FIG. 1 shows a semiconductor differential amplifier as an embodiment of the present invention. In FIG. 1, the same reference numerals as those shown in FIG. 2 are used to designate the same elements. In FIG. 1, the reference characters Q6 and Q7 designate P channel fifth and sixth MOS transistors having an equal gate width and gate length, respectively. The fifth transistor Q6 is connected between the power supply voltage Vcc and the node N, and the gate thereof is connected to the input terminal D. The sixth transistor Q7 is connected between the power supply voltage Vcc and the output terminal RD, and the gate thereof is connected to the other input terminal D.

The device is operated as follows.

The two P channel MOS transistors Q6 and Q7 are provided symmetrically at the left and right, and the node N and the output RD are kept at equal voltages when the input terminals D and D are at equal voltages similarly as in the prior art device.

Now suppose that a slight voltage difference arises between the input terminal pair D and D. The transistor Q6 becomes more conductive when the voltage of the terminal D decreases, and the transistor Q6 becomes more non-conductive when the voltage of the terminal D rises. Accordingly, transistor Q6 has an effect of increasing the variation of the voltage at the node N, that is, the gate voltage of the transistor Q2. Furthermore, because the direction of the change of the input terminal D, that is, the gate voltage of the transistor Q7 is the same as the direction of the change of the gate voltage of the transistor Q2, the voltage variation at the output node RD increases as a result of any of the transistors Q6 and Q7.

FIG. 3 is a diagram showing the input vs output transmission characteristics of the pior art device and the present embodiment. In FIG. 3, the broken lines show the characteristics of the prior art device, and the solid lines show the characteristics of the present embodiment. In FIG. 3, the reference characters V.sub.D, V.sub.D, and V.sub.RD designate voltages of the terminals D, D, and RD. Herein, the gate length L and the gate width W of the transistors are established such that L=2.0 .mu.m and W=25.0 .mu.m for transistors Q1 and Q2, L=1.5 .mu.m and W=6.0 .mu.m for transistors Q3 and Q4, and L=2.0 .mu.m and W=20.0 .mu.m for transistors Q6 and Q7. For example, when the voltage V.sub.D varies from 3.8 V to 3.7 V in a case where V.sub.D =3.8 V, the variation quantity of the output voltage V.sub.RD is 0.6 V in the present embodiment as opposed to that of the prior art device which is 0.35 V. Thus, the amplification factor is enhanced about 70%.

There is conventionally known a method of constructing a two stage differential amplifier in order to enhance the sensitivity of the differential amplifier. FIG. 4 shows such a construction. In FIG. 4, the reference characters SA.sub.1 to SA.sub.3 designate a first to a third differential amplifier portion. The reference characters D.sub.inl and D.sub.inl designate an input terminal pair of the first differential amplifier portion SA.sub.1. Similarly as above the reference characters D.sub.in2, D.sub.in2 and D.sub.in3, D.sub.in3 designate input terminal pairs of the second and the third differential amplifier portions SA.sub.2 and SA.sub.3, respectively. Furthermore, the reference characters D.sub.out1, D.sub.out2, and D.sub.out3 designate output terminals of the differential amplifier portions SA.sub.1, SA.sub.2, and SA.sub.3, respectively. The reference character SA designates a differential amplifier constituted by the first to third differential amplifier portions SA.sub.1 to SA.sub.3.

The input terminals D and D of the differential amplifier SA are connected to the terminals D.sub.in1, D.sub.in2 and D.sub.in1, D.sub.in2, respectively, and the terminals D.sub.out1 and D.sub.out2 are connected to the terminals D.sub.in3 and D.sub.in3, respectively. The terminal D.sub.out3 is an output RD of the differential amplifier SA.

In this differential amplifier, first of all, amplifications are conducted by first stage differential amplifier portions SA.sub.1 and SA.sub.2 in directions reverse to each other, thereby enlarging the voltage difference between the terminals D.sub.out1 and D.sub.out2 relative to the voltage difference between the input terminals D and D, the voltage difference being further amplified by the second stage differential amplifier portion SA.sub.3.

In this conventional method, however, three stages of differential amplifiers SA.sub.1, SA.sub.2, and SA.sub.3 as fundamental units are used, resulting in an increase in the area of the circuit. To the contrary, in the present invention, only two P channel MOS transistors are added to a differential amplifier as a fundamental unit, and the amplification factor is enhanced while suppressing the increase in area to a minimum.

Herein, the ratio W/L of the gate width W and the gate length L of the MOS transistor is called a conductance factor A. When the conductance factors A6 and A7 of the transistors Q6 and Q7 are larger than the conductance factors A1 and A2 of the transistors Q1 and Q2, the circuits comprising the transistors Q4 and Q7 and the transistors Q3 and Q6, respectively, exhibit dominant CMOS inverter characteristics. Therefore, although the amplification factor when the input voltage is matched with the characteristics of the differential amplifier is large, the allowable range of the input voltage for obtaining matching characteristics becomes narrow, therefore the function as a differential amplifier which is operated based on only the relative difference of the input voltages becomes weak. Accordingly, the conductance factors A6 and A7 are desirably made smaller than the conductance factors A1 and A2, respectively.

In the above-illustrated embodiment, an N channel transistor is used as a driving transistor and a P channel MOS transistor is used as a load transistor, but the present invention can be applied to a case where a P channel MOS transistor is used as a driving transistor and an N channel MOS transistor is used as a load transistor.

In the above-illustrated embodiment, the conductance factors A of the transistors Q1 and Q2, the transistors Q3 and Q4, and the transistors Q6 and Q7 are made equal to each other, respectively, but it is only required that the conductance factor ratios A of the transistors Q1, Q3, and Q6 are equal to the conductance factor ratios A of the transistors Q2, Q4, and Q7, that is, A1/A2=A3/A4=A6/A7, with no unfavorable influence on the DC current differential amplification characteristics.

Furthermore, in the illustrated embodiment, an N channel MOS transistor Q5 is used as a power cut MOS transistor and this is inserted between the remainder of the circuit and ground, but any type of power cut MOS transistor which cuts off the current penetrating from the P channel MOS transistor to the N channel MOS transistor in the differential amplifier can be used.

As is evident from the foregoing description, according to the present invention, signal inputs from the input terminal pair are designed to be received by the N channel and the P channel MOS transistor, whereby quite a small voltage difference between the input terminals is sensed with a high sensitivity.