JP2000201038A - Gain variable arithmetic amplifier - Google Patents

Abstract

PROBLEM TO BE SOLVED: To compensate a phase, and to obtain satisfactory responsive characteristics even in the case of high gain by connecting a capacitor variable phase compensating capacitor with an amplifier, and decreasing a capacity according to the increase of the gain. SOLUTION: A phase compensating capacitance CPV in which plural phase compensating capacitances CP1-CP3 are connected in parallel is connected with an arithmetic amplifier AMP of a device for changing a gain according to the adjustment of the capacitive rate of an input capacitance CI and a feedback capacitance CF. Then, the opening and closing of phase compensating switches SWP1 and SWP2 is controlled by control signals CTP1 and DTP2 so that the phase compensating capacitances CP2 and CP3 can be opened and short-circuited. The synthetic capacitance of the capacitance CPV are set so as to be made in reverse proportion to the gain, and the switches SWP2 and SWP3 are sequentially opened by the control signals CTP according to the increase of the gain. Thus, in the case of the high gain, the CPV is decreased so that the increase of any settling time can be reduced, and the oscillation of the arithmetic amplifier can be suppressed.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a variable gain operational amplifier, and more particularly to a variable gain operational amplifier which changes a gain by adjusting a capacitance ratio between an input capacitance and a feedback capacitance.

[0002]

2. Description of the Related Art As shown in FIG. 5, in a conventional variable gain operational amplifier, an input capacitance CI is connected to an input voltage Vin via an input switch.
I, an operational amplifier AMP is connected to the input and output terminals of the operational amplifier AMP.
It is connected by Fq. A gain adjustment switch "SW" is provided on the input side of these feedback capacitances CF1 to CFq.
21 to SW2q are connected, and the feedback capacitance is connected to the operational amplifier input or the reference voltage by these gain adjustment switches. Here, as shown in equation (1),
The gain of the variable gain operational amplifier is given by the ratio between the input capacitance and the feedback capacitance combined capacitance, and the gain increases as the feedback capacitance connected to the reference voltage increases.

(Equation 1)

However, the conventional variable gain calculation has a problem that the settling time increases with an increase in gain.

[0004]

SUMMARY OF THE INVENTION An object of the present invention is to provide a variable gain operational amplifier having a good response at a high gain.

[0005]

SUMMARY OF THE INVENTION A variable gain operational amplifier according to the present invention has a variable capacitance for a phase compensation capacitance and decreases the capacitance as the gain increases.

[0006]

FIG. 1 is a circuit diagram showing a variable gain operational amplifier according to the present invention, in which input capacitances CI1 to CIp are connected in parallel to a non-inverting input of an operational amplifier AMP, and a plurality of operational amplifiers are connected between input and output. Feedback capacitance CF
1 to CFq are connected in parallel. The input side of the input capacitances CI1 to CIp is connected to input switches SW11 to SW1p of two inputs and one output, and both terminals of each input switch are connected to the input voltage Vin and the reference voltage Vref. The input switches SW11 to SW1p are controlled to open and close by control signals CTI1 to CTIp, respectively.

[0007] Two-input, one-output gain adjustment switches SW21 to SW2q are connected between the input sides of the feedback capacitances CF1 to CFq and the non-inverting input of the operational amplifier AMP. And the reference voltage is connected. These gain adjustment switches SW21 to SW2q are connected to a control signal CTF1.
To CTFq to control opening and closing, respectively.

[0008] A phase compensation capacitance CPV is connected to the operational amplifier AMP.
The PV includes a plurality of phase compensation capacitances CP1 to CP3 in parallel. Phase compensation capacitance CP2, CP3
Are connected to the input side of the phase compensation switch SWP1 and SWP2, and the phase compensation capacitance is short-circuited by the phase compensation switches SWP1 and SWP2. The phase compensation switches SWP1 and SWP2 are controlled by a control signal CTP.
1, the opening and closing are controlled by CTP2.

FIG. 2 shows details of the operational amplifier AMP. The input voltage Vi is applied to the inverting input and the non-inverting input of the differential amplifier A21.
And the output of the differential amplifier A21 is connected to the second-stage amplifier A22. The input of the amplifier A22 is connected to the output of the differential amplifier A21. After these differential amplifiers A21 and A22,
a CMOS inverter including a pMOS transistor Tr21 and an nMOS transistor Tr22 is provided;
The outputs of A21 and A22 are input to the gates of Tr21 and Tr22, respectively. The differential amplifier A21 has a fully differential configuration to improve the noise resistance of the AMP, and the CMOS inverter amplifies the outputs of A21 and A22.

A phase compensation resistance RP is connected in series with the phase compensation capacitance CPV. This series circuit is connected between the output of the differential amplifier A21 and the output of the CMOS inverter. Thus, the phase compensation of the operational amplifier AMP is performed, and the oscillation of the operational amplifier AMP is prevented.

In the variable gain operational amplifier described above, the gain is set by the combined capacitance of the input capacitances CI1 to CIp and the combined capacitance of the feedback capacitances CF1 to CFq, and the following equation (2) equivalent to the equation (1) is obtained. Given.

(Equation 2)

The phase compensation switches SWP1, SWP2
Are sequentially released as the gain increases, and the combined capacitance of the phase compensation capacitance CPV is set to be inversely proportional to the gain. Due to the decrease in the phase compensation capacitance, the settling time does not increase even at a high gain. Although various causes can be considered, it is considered that the feedback capacitance connected to the reference voltage becomes an output load of the operational amplifier AMP and compensates for the capacity of the phase compensation capacitance. Note that the phase compensation capacitance CP
No phase compensation switch is connected to 1 and the operational amplifier A
Since it is connected to the MP, the stability of the circuit can be assured even in the event of an unexpected situation such as the incorporation of noise into the control signal.

The input capacitance and the feedback capacitance may cause residual charges or leakage of charges, and it is necessary to refresh these capacitances in order to ensure the accuracy of the output voltage Vout.
During this refresh, all input switches SW
11 to SW1p and gain adjustment switches SW21 to S
W2q is connected to a reference voltage.

FIG. 3 shows a second embodiment of the present invention. This embodiment differs from the first embodiment only in that a CMOS inverter is used as the operational amplifier AMP and only in control of the input switch.

In FIG. 3, input switches SW11-S
W1p is controlled by a common refresh signal, and is always connected to the input voltage Vin side except for refresh. By thus defining the input capacitance as a combined capacitance of a plurality of capacitances, it is possible to absorb variations in capacitance of each capacitance. In this case, the gain is set only by adjusting the gain adjustment switch. The circuit library is required to have higher versatility, and in this respect, the configuration of the input capacitance of this embodiment is advantageous.

As shown in FIG. 4, the operational amplifier AM shown in FIG.
P is an odd-numbered stage CMOS inverter I11, I12, I1
3 connected in series, and the phase compensation capacitance CPV
The phase compensation resistance RP is connected between the input and output of the second-stage CMOS inverter I12. With such a configuration, the same effect as in the first embodiment can be obtained.

As described above, in the variable gain operational amplifier according to the present invention, the phase compensation capacitance is made variable in capacitance, and the capacitance is reduced as the gain is increased. It has an excellent effect.

[Brief description of the drawings]

FIG. 1 is a circuit diagram showing a first embodiment of a variable gain operational amplifier according to the present invention.

FIG. 2 is a circuit diagram showing an operational amplifier used in the first embodiment.

FIG. 3 is a circuit diagram showing a second embodiment of the variable gain operational amplifier according to the present invention.

FIG. 4 is a circuit diagram showing an operational amplifier used in a second embodiment.

FIG. 5 is a circuit diagram showing a conventional variable gain operational amplifier.

[Explanation of symbols]

CI1-CIp Input capacitance CF1-CFq Feedback capacitance CPV, CP1-CP3 Phase compensation capacitance AMP Operational amplifier SW11-SW1p Input switch SW21-SW2q Gain adjustment switch SWP1, SWP2 Phase compensation switch CTI1-CTIp, CTF1-CTFq, CTP1,
CTP2 Control signal Vin Input voltage Vout Output voltage Vref, Vr Reference voltage

Claims (11)

[Claims] 1. an input capacitance having an input connected to an input voltage; an operational amplifier connected to an output of the input capacitance; a plurality of feedback capacitances connecting an output of the operational amplifier to its input; A plurality of gain adjustment switches provided corresponding to each of the gain adjustment switches, wherein each gain adjustment switch is connected to any one feedback capacitance, and disconnects the feedback capacitance from the operational amplifier as appropriate. A variable gain operational amplifier comprising: a switch; and a phase compensation capacitance for phase-compensating the operational amplifier, wherein the capacitance of the phase compensation capacitance is variable, and the capacitance of the phase compensation capacitance increases as the gain increases. Variable gain characterized by a reduction Calculation amplifier. 2. The phase compensating capacitance comprises a plurality of parallel phase compensating capacitances; and is provided corresponding to one or more of the phase compensating capacitances, and is connected to each corresponding one of the phase compensating capacitances. Phase compensating switches are provided; the phase compensating switches cut off the connection between each phase compensating capacitance and the operational amplifier, and reduce the combined capacitance of the phase compensating capacitances as the gain increases. The variable gain operational amplifier according to claim 1, wherein: 3. The variable gain operational amplifier according to claim 1, wherein the phase compensation capacitance is adjusted to a capacity inversely proportional to the gain. 4. The variable gain operational amplifier according to claim 1, wherein the operational amplifier is a differential amplifier. 5. An operational amplifier comprising a plurality of stages of amplifiers in series and a CM having outputs of these amplifiers input to respective gates.
A variable gain operational amplifier comprising an OS transistor and a drain connection point of the CMOS transistor serving as an output terminal. 6. The variable gain operational amplifier according to claim 1, wherein the operational amplifier comprises an odd-numbered series CMOS inverter. 7. The variable gain operational amplifier according to claim 1, wherein a phase compensation resistance is connected in series to the phase compensation capacitance. 8. A feedback capacitance having an output connected to the output of the operational amplifier and an input connected to a gain adjustment switch, the gain adjustment switch connecting the input of the feedback capacitance to an input of the operational amplifier or a reference voltage. 2. The variable gain operational amplifier according to claim 1, wherein: 9. The variable gain operational amplifier according to claim 1, wherein the phase compensation switch short-circuits the phase compensation capacitance. 10. The phase compensation capacitance may be any one of
5. The variable gain operational amplifier according to claim 4, wherein the variable gain operational amplifier is connected between inputs and outputs of the CMOS inverters. 11. The input capacitance comprises a plurality of parallel input capacitances; input switches provided corresponding to the input capacitances, each being connected to one input capacitance; The connection between the capacitance and the input voltage is broken; variable gain operational amplifier.