Fully differential, switched capacitor, operational amplifier circuit with common-mode controlled output

[0002] The present invention refers to a fully differential, switched capacitor, operational amplifier circuit with common-mode controlled output.

[0003] As is known, in the manufacture of numerous switched-capacitor circuits advantageously switched operational amplifiers with fully differential topology are used, i.e., having two output terminals supplying differential voltages that are symmetrical with respect to a common-mode voltage. Switched operational amplifiers, in fact, overcome certain drawbacks typical of traditional switched-capacitor circuits, especially if they operate at a low supply voltage (for example, just over 1 V) and with a low consumption. These drawbacks, in particular, are due to the difficulty, in the presence of low supply voltages, of efficiently driving switches connected to terminals having wide dynamics that are comparable to the rail-to-rail supply voltage, such as in case of the output of an operational amplifier. The fully differential topology, moreover, allows the manufacture of integrated circuits having small overall dimensions, for implementing, for instance, high order active filters.

[0004] In order to prevent distortion of the signals, however, it is necessary to associate fully differential switched operational amplifiers to circuits that perform an accurate control of the common-mode voltage. A control circuit of such a type is described in U.S. Pat. No. 5,973,537 issued on Oct. 26, 1999, according to which the outputs of a fully differential switched operational amplifier are connected, via respective switched capacitors, to the inverting input of a standard operational amplifier, in an integration configuration. Thereby, the output of the standard operational amplifier supplies a voltage that is proportional to the common-mode voltage and that can be used for modifying the biasing of the fully differential switched operational amplifier and for obtaining a preset common-mode voltage. In addition, the inverting input of the standard operational amplifier is alternately connected to ground and to the supply voltage via a further switched capacitor in order to prevent voltage jumps when the switched operational amplifier is turned on.

[0005] According to a different known solution, the outputs of the fully differential switched operational amplifier are connected to the supply line via a capacitive divider. An intermediate node of the capacitive divider, supplying a voltage correlated to the common-mode voltage and with a reduced dynamics with respect to the differential voltages of the switched operational amplifier, is connected to the inverting input of a standard operational amplifier, which also receives, on its non-inverting input, a reference voltage. The output voltage of the standard operational amplifier, correlated to the difference between the common-mode voltage and the reference voltage, is used for modifying the biasing of the switched operational amplifier and for obtaining a preset common-mode voltage, as in the previous case.

[0006] The above solutions, however, have a number of drawbacks. In both cases, in fact, an additional standard operational amplifier must be used, which increases circuit consumption and, furthermore, limits the performance of the circuit in terms of frequency response. The increase in power absorbed due to the standard operational amplifier is particularly disadvantageous when the circuit is used inside a very low consumption device. The second solution, moreover, requires the use of supply voltages higher than that of the switched operational amplifier.

[0007] The disclosed embodiments of the present invention provide a control circuit that overcomes the above described drawbacks and, in particular, results in a very low consumption, uses the same supply voltage as for the switched operational amplifier, and reduce the number of components.

[0008] In accordance with one embodiment of the present invention, a fully differential, switched capacitor, operational amplifier circuit with common-mode controlled output is provided, the operational amplifier circuit including a control circuit having a capacitive detecting network and a biasing control circuit. The capacitive detecting network includes a first capacitive element connected between a first differential output of the operational amplifier and a common-mode node, a second capacitive element coupled between the second differential output of the operational amplifier and the common-mode node, and a third capacitive element connected between the common-mode node and a biasing node that is connected to a first reference potential line in a first operative condition and a second reference potential line in a second operative condition. The biasing control circuit includes a control input connected to the common-mode node and an output connected to a control terminal of the operational amplifier. A switchable biasing circuit is further included that is activated for biasing the common-mode node at a desired voltage in a first operative condition and is deactivated in a second operative condition to leave the common-mode node in a floating condition.

[0009] For a better understanding of the invention, an embodiment thereof is now described, purely to furnish a non-limiting example, with reference to the attached drawings, wherein:

[0010] FIG. 1 shows a block diagram of a circuit according to the present invention;

[0011] FIG. 2 illustrates a detailed circuit diagram of the circuit of FIG. 1; and

[0012] FIG. 3 shows plots of quantities of the circuit of FIG. 1.

[0013] FIG. 1 shows a switched capacitor circuit 1, of a fully differential type, with common-mode controlled output, comprising an operational amplifier 100, that is switched and has fully differential topology, and a common-mode control circuit 2.

[0014] The operational amplifier 100 has a first and a second inputs 3a,3bforming inputs of the switched capacitor circuit 1 and receiving respective differential input voltages V_I1, V_I2, and first and second differential outputs 4a,4bforming outputs of the switched capacitor circuit 1 and supplying, respectively, first and second differential output voltages V_O1, V_O2, which are symmetrical with respect to a common-mode voltage V_CM. Each of the differential outputs 4a,4bis connected to a respective input 3a,3bthrough a feedback network 5 of a capacitive and/or resistive type. Furthermore, the operational amplifier 100 has a control terminal 6 for regulating the biasing and an activation terminal 7 which receives a first phase signal F1 generated by a timing circuit (known and not illustrated). The first phase signal F1 causes the operational amplifier 100 to be alternately switched on and off.

[0015] The common-mode control circuit 2 comprises a first and a second read switched capacitors 9, 10, a compensation switched capacitor 11, a voltage source 12, and a control transistor 13.

[0016] The first and second read switched capacitors 9, 10, which have the same capacitances, are connected between a common-mode node 15 and the first differential output 4a,respectively the second differential output 4bof the operational amplifier 100.

[0017] The compensation switched capacitor 11 has a first terminal connected to the common-mode node 15 and a second terminal connected to a node A, which is connected alternately to ground through a first compensation switch 18 and to a supply line 16 through a second compensation switch 19. The supply line 16 supplies a supply voltage V_DD(for example, 1.2 V). In particular, the first and second compensation switches 18, 19 are opened and closed in counterphase by a second and a third phase signals F2, F3 supplied to respective control terminals 18a,19a.

[0018] The voltage source 12, which supplies a desired voltage V_D, preferably equal to V_DD/2, is connected to the common-mode node 15 and may be alternately switched on and off by an enabling switch 20 which receives the second phase signal F2 on an own control terminal 20a.

[0019] The control transistor 13 has its gate terminal connected to the common-mode node 15, its drain terminal connected to the control terminal 6 of the operational amplifier 100, and its source terminal connected to ground via a control switch 22 receiving the first phase signal F1 on an own control terminal 22a.

[0020] Operation of the switched capacitor circuit 1 is the following.

[0021] The first, second and third phase signals F1, F2, F3 are periodic (see also FIG. 3). In particular, in a first half-period the first phase signal F1 switches the operational amplifier 100 off and controls opening of the control switch 22, turning the control transistor 13 off; the second phase signal F2 closes the first compensation switch 18 and the enabling switch 20; and the third phase signal F3 controls opening of the second compensation switch 19.

[0022] Consequently, the voltage source 12 is connected to the common-mode node 15, and the compensation switched capacitor 11 is connected between the common-mode node 15, which is at the desired voltage V_D, and ground. The compensation switched capacitor 11 is then charged to the voltage V_D. In addition, the differential outputs 4a,4bof the operational amplifier 100, which is off, are brought to the supply voltage V_DDin a known way, and consequently a voltage of V_DD−V_Dis present across the first and second read switched capacitors 9, 10.

[0023] In a second half-period, the first phase signal F1 closes the control switch 22 and activates the operational amplifier 100. Thereby, the control transistor 13, turning on, starts to conduct a control current I_C, and the first and second differential outputs 4a,4bof the operational amplifier 100 respectively reach the first and the second differential voltages V_O1, V_O2, which, in absence of a signal, are equal to V_DD/2. At the same time, the second phase signal F2 controls opening of the first compensation switch 18 and of the enabling switch 20, and the third phase signal F3 closes the second compensation switch 19, so that the switched compensation capacitor 11 is connected between the common-mode node 15 and the supply line 16, and the voltage source 12 is not connected to the common-mode node 15.

[0024] In the second half-period, the capacitive circuit including the read switched capacitors 9, 10 and the compensation switched capacitor 11 has a dual biasing condition with respect to the first period, with node A set at the supply voltage V_DDand, in absence of a signal on the outputs 3a,3b,with the differential outputs 4a,4bset at V_DD/2. Therefore, charge is distributed between the capacitors 9, 10 and 11, which, having equal capacitances, maintain the common-mode node 15 at the desired voltage V_D. It should be noted that, as regards the voltage stability of the common-mode node 15, the condition with no signals on the inputs 3a,3bof the operational amplifier 100 is the most critical because both of the differential outputs 4a,4bare lowered by V_DD/2.

[0025] On the other hand, if, for example, because of disturbance or temperature variations, the common-mode voltage V_CMvaries, the gate-to-source voltage V_GSof the control transistor 13 is modified, and thus also the control current I_Csupplied to the control terminal of the operational amplifier 100 varies. The control current I_Ccan be used, in a per se known manner, to modify biasing of the operational amplifier 100 and to bring the common-mode node voltage V_CMback to the desired voltage V_D. For example, if the common-mode voltage V_CMincreases, the gate-to-source voltage V_GSof the control transistor 13 increases, and thus also the control current I_Cincreases.

[0026] A more detailed circuit diagram of the operational amplifier 100 and of the common-mode control circuit 2 is shown in FIG. 2, in which parts already illustrated in FIG. 1 are designated by the same reference numbers.

[0027] In particular, the operational amplifier 100 comprises a differential stage 25 including a first and a second input NMOS transistors 27, 28, that have respective source terminals connected to the drain terminal of the control transistor 13 and, in practice, defining the control terminal 6. The gate terminal and the drain terminal of the first input transistor 27 respectively define the first input 3aand the first differential output 4aof the operational amplifier 100; the gate terminal and the drain terminal of the second input transistor 28 respectively define the second input 3band the second differential output 4bof the operational amplifier 100.

[0028] The differential stage 25 further comprises a pair of load transistors 29 which have source terminals connected to the supply line 16, gate terminals connected to ground, and drain terminals respectively connected to the first and to the second differential outputs 4a,4b.

[0029] The voltage source 12 of the common-mode control circuit 2 is formed by a current mirror circuit 30 connected to a resistive transistor 32. The current mirror circuit 30 includes a first and a second mirror transistors 33, 34 having gate terminals connected to each other and source terminals connected to the supply line 16 via respective mirror switches 35 formed by PMOS transistors that receive, on respective gate terminals, a fourth phase signal F4, also generated in a known way. The first mirror transistor 33 is diode-connected. In addition, the source terminals of the first and second mirror transistors 33, 34 are respectively connected to a reference current source 37 and to the drain terminal of the resistive transistor 32.

[0030] The resistive transistor 32, which is of NMOS type and is diode-connected, has drain and gate terminals connected together and to the common-mode node 15 of the common-mode control circuit 2, and a source terminal connected to ground via the enabling switch 20. The enabling switch 20 is formed by an NMOS transistor that has source terminal connected to ground and gate terminal defining the control terminal 20a.

[0031] The first compensation switch 18 is formed by an NMOS transistor having source terminal connected to ground, drain terminal connected to the compensation capacitor 11, and gate terminal defining the control terminal 18a(which receives the second phase signal F2).

[0032] The second compensation switch 19 is formed by a PMOS transistor having source terminal connected to the supply line 16, drain terminal connected to the compensation capacitor 11, and gate terminal defining the control terminal 19a(which receives the third phase signal F3).

[0033] The control switch 22 is formed by an NMOS transistor having source terminal connected to ground, drain terminal connected to the source terminal of the control transistor 13, and gate terminal defining the control terminal 22a(which receives the first phase signal F1).

[0034] FIG. 3 shows the plots of the first, second, third and fourth phase signals F1, F2, F3, F4 in a period T. In particular, the first phase signal F1 and the second phase signal F2 close the respective switches, formed by NMOS transistors, when they have a value equal to the supply voltage V_DD(namely, during the second half-period and the first half-period, respectively); the third phase signal F3 and the fourth phase signal F4 control closing of the respective switches, formed by PMOS transistors, when their amplitude is 0 V (namely, during the second half-period and the first half-period, respectively).

[0035] Consequently, during the first half-period the enabling switch 20, the mirror switches 35 and the first compensation switch 18 are closed, whereas all the other switches are open. The current mirror circuit 30 can thus conduct and inject current into the resistive transistor 32. Thereby, the current source 12 is enabled and forces the common-mode node 15 to the desired voltage V_D. Since, instead, the control switch 22 is open, no current flows in the differential stage 25, and the differential outputs 4a,4bof the operational amplifier 100 reach the supply voltage V_DD. In addition, the compensation switched capacitor 11 is connected between the common-mode node 15 and ground, as mentioned previously.

[0036] During the second half-period, the control switch 22 and the second compensation switch 19 are closed, and all the other switches are open. The current mirror circuit 30 is thus off so as to disable the voltage source 12, while the control transistor 13 conducts the control current I_C, activating the differential stage 25. In addition, as mentioned previously, variations in the common-mode voltage V_CMwith respect to the desired voltage V_Dgive rise to variations in the gate-to-source voltage V_GSof the control transistor 13, and thus to variations in the control current I_C. Consequently, also the current flowing in the differential stage 25 varies, and the common-mode voltage V_CMvaries with opposite sign and returns to the desired voltage V_D. For example, an increase in the common-mode voltage V_CMcauses an increase in the gate-to-source voltage V_GSof the control transistor 13, and thus of the control current I_C. Since also the current flowing in the differential stage 25 increases, the voltage drops on the load transistors 29 increase by equal amounts and reduce the first and second differential output voltages V_O1, V_O2. Also the common-mode voltage V_CMthus decreases until it reaches the desired voltage V_D.

[0037] The circuit according to the invention has the following advantages.

[0038] First, control of the common-mode voltage V_CMis performed with the exclusive use of passive components (switched capacitors), switches, and a control transistor, without using additional operational amplifiers. It is thus possible to obtain both a considerable reduction in absorbed power and an improvement in performance, which are factors of particular importance in case of use in very low power-consumption devices.

[0039] In addition, all the switches that are present are formed either by NMOS transistors having source terminals connected to ground or by PMOS transistors having source terminals connected to the supply line. The switches can therefore be driven by a minimal supply voltage, i.e., equal to the one necessary for the operational amplifier 100.

[0040] A reduced number of components is required for implementing the circuit according to the invention. In fact, the voltage source 12 and the control transistor 13 can be formed by components already present in the operational amplifier 100.

[0041] Finally, it is clear that modifications and variations may be made to the circuit described herein, without thereby departing from the scope of the present invention. In particular, the switched operational amplifier with fully differential topology may be of any type and have an architecture different from the one illustrated herein (for example, it could be a two-stage amplifier, in which case the common-mode control circuit 2 controls the first stage). Thus, the invention is to be limited only by the scope of the appended claims and the equivalents thereof.