JPH0818353A - Operational amplifier circuit - Google Patents

Abstract

PURPOSE:To reduce the error brought about in an amplified signal value by compensating the offset generated in an operational amplifier without especially expending labor on adjustment of a correction circuit. CONSTITUTION:A main amplifier 10, a main switch means 20 which designates the amplification state of giving an input signal Vi to the main amplifier and the reset state of connecting its two inputs to the same potential, an auxiliary amplifier 30 which receives the output signal of the main amplifier 10 to perform operational amplification in the opposite direction, a holding means 40 which holds its output signal value to feed back it to the main amplifier 10, and an auxiliary switch means 50 which connects or disconnects this means 40 and the auxiliary amplifier 30 are provided. After the auxiliary switch means 50 is set to the connected state in the reset state and the offset compensation signal value is held in the holding means 40, the auxiliary switch means 50 is set to the disconnected state in the next amplification state to give the compensation signal value to the main amplifier 10, and in this state, an amplified signal Vo is taken out from its output. Thus, the offset error is reduced to about 1/(the gain of the main amplifier 10).

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an operational amplifier circuit for reducing a so-called offset that is inevitably generated during operational amplification of a signal.

[0002]

2. Description of the Related Art As is well known, operational amplifiers are widely used for amplification of analog signals, and most of them are differential amplification. Recently, operational amplifiers are often incorporated in integrated circuits. When configured, M
It may be composed of an OS transistor, but in any case, a so-called offset, which is usually a constant error, is always peculiar to the signal value of the amplified output due to non-uniformity of the characteristics of the constituent transistors. . For this reason, it is necessary to add a correction circuit to the conventional case where the offset is unacceptable, and particularly in an integrated circuit, it is necessary to derive a correction terminal to externally connect the correction circuit. FIG. 5 schematically shows a typical example in which an external correction circuit is connected to the operational amplifier.

In FIG. 5, an operational amplifier 70 for amplifying an input signal Vi into an output signal Vo is for inverting amplification in the example of the figure,
As usual, the input signal Vi is applied to its inverting input via the input resistor 71, and the feedback resistor 72 is connected between this inverting input and the output to set the amplification degree by the resistance value ratio of both resistors. The non-inverting input receives the reference potential, for example, the ground potential via the resistor 73 in the example of the figure. The correction terminal Tc is derived from the inverting input of the operational amplifier 70 in order to connect the correction circuit 80 shown on the left side of the figure to this operational amplifier circuit. The correction circuit 80 generates a voltage or current for offset correction given to the inverting input. For example, as shown in FIG.
It is adapted to be given to the correction terminal Tc via 82.

[0004]

However, if the offset correction circuit is externally attached as described above, the effect of circuit integration is naturally reduced, and it is necessary to incorporate a large number of operational amplifier circuits that require offset correction. If there is, the purpose of integration itself becomes questionable. In addition, even if it is not integrated, it is necessary to set the voltage and current generated in the correction circuit for each operational amplifier circuit according to the actual offset value. is there.

In view of such problems, it is an object of the present invention to provide an operational amplifier circuit which does not require an offset correction circuit to be externally attached to an integrated circuit and which does not require any special labor for adjusting the correction circuit. To do.

[0006]

According to the present invention, there is provided main switch means for designating, for a main amplifier which performs operational amplification of an input signal, an amplification state in which the input signal is applied and a reset state in which two inputs thereof are connected to the same potential. An auxiliary amplifier that receives the output signal of the main amplifier and performs operational amplification in the opposite direction, holding means that holds the value of the output signal of the auxiliary amplifier and returns it to the input side of the main amplifier, and the output signal of the auxiliary amplifier to the holding means. By providing the auxiliary switch means for designating the applied state and the separated state of the main switch, the auxiliary switch means is placed in the applied state in the reset state, and in the amplified state, the output of the main amplifier is taken out as the amplified signal in the separated state of the auxiliary switch means. The above objective is achieved.

The directions of operational amplification of the main amplifier and the auxiliary amplifier in the above configuration are such that, for example, when the former is inverting amplification, the latter is operated as non-inverting amplification. Further, it is desirable to connect the inputs other than the input signals of both and the input signal for receiving the output signal of the main amplifier to each other so as to commonly apply a constant potential in order to make the related operation of both of them accurate. Further, the holding means may be a capacitor in the simplest case, but in order to ensure the holding operation, the holding means outputs the same voltage value as the capacitor receiving the output signal of the auxiliary amplifier and the charging voltage thereof. A hold circuit using an operational amplifier is preferable. In any case, it is very advantageous to use the main switch to repeatedly specify the reset state and the amplification state of the circuit of the present invention at a predetermined short cycle in order to make the holding value of the holding means accurate during the operating state.

A preferred embodiment of the invention uses a single capacitor for the holding means, which is connected between one input and the output of the auxiliary amplifier and which outputs the main amplifier and the other input of the auxiliary amplifier. By inserting an auxiliary switch means between and, the overall configuration of the circuit is simplified. In another advantageous embodiment of the invention, sampling switch means is inserted on the input side of the main amplifier which is to receive the input signal and one sample and hold circuit is used for the main amplifier, for example for its input and output. A capacitor is connected between the sampling switch means and the sampling signal is sampled and held by the main amplifier itself by switching the sampling switch means from the sampling designation state of ON to the hold designation state of OFF during the amplification state of the circuit of the present invention. In advance, this can be taken out at an appropriate timing as needed.

In a further advantageous embodiment of the invention, the input terminal of the main amplifier receives the input signal from the input terminal,
The output terminals are derived from the outputs, and the input resistors for receiving the input signals are externally connected to the input terminals, and the feedback resistors are externally connected between the input terminals and the output terminals. Designate the rate to a desired value or change it at any time. The potential terminals are also derived from the above-mentioned interconnection points of the inputs other than the input signal of the main amplifier and the input other than the input of the auxiliary amplifier for receiving the output signal of the main amplifier, and the constant potential suitable for the designated amplification factor is obtained. It is convenient to be able to apply the electric potential from the outside. In such a mode, a resistor is fixedly connected between a pair of inputs of the main amplifier, and the main switch means is made into a simple on / off operation type, and the amplification state is reset by turning on the circuit of the present invention and reset by turning off. It is advantageous to be able to specify each state.

[0010]

According to the present invention, the operation state of the operational amplifier circuit is divided into an amplification state for an input signal and, so to speak, a reset state for its preparation. First, in the reset state, a signal value for compensating the offset of the main amplifier is supplied to the auxiliary amplifier. After being generated and held by the holding means, in the subsequent amplification state, the main amplifier is caused to perform a normal amplification operation while compensating the offset by this signal holding value for compensation, and then an amplified signal with a very small offset error is generated. It should be taken out.

Therefore, in the operational amplifier circuit of the present invention as described in the above paragraph, the main switch means is first provided in association with the main amplifier for operational amplification of the input signal, and the main switch means is set to the amplification state and the reset state. While designating, the input signal is supplied to the main amplifier in the amplification state, but in the reset state, the two inputs are connected to the same potential to completely eliminate the signal input. Also, an auxiliary amplifier that receives the output signal of the main amplifier and performs operational amplification in the opposite direction, holding means that holds the signal value of the output signal, and the state of giving and separating the output signal of the auxiliary amplifier to it are specified. Auxiliary switch means is provided.

In the circuit of the present invention having such a configuration, in the reset state, the auxiliary switch means is set to the applied state, the output signal value of the auxiliary amplifier is given to the holding means, and the held value is fed back to the input side of the main amplifier to control the principle. Above is the state in which the offset of the main amplifier is compensated by performing negative feedback, and in the next amplification state, the auxiliary switch means is separated and the holding means keeps the held value as it is to compensate the offset of the main amplifier. Then, the output is taken out as an amplified signal.

Therefore, in the present invention, the offset of the main amplifier can be completely compensated in principle, but in reality, the auxiliary amplifier also has an offset, so that a slight error occurs. Now, let the offsets of the main amplifier and the auxiliary amplifier be δm and δa, respectively, and let the amplification factor or gain of the main amplifier be Gm.
Then, the reduction rate of the offset error of the amplified signal according to the present invention becomes approximately δa / Gmδm as described later, and δm and δm
If a is the same, this reduction rate is 1 / Gm. As can be seen, in the present invention, the gain of the main amplifier is
Offset error can be reduced using Gm,
For example, when the gain Gm is 100, the offset error of the amplified signal is reduced to about 1% of the conventional value.

[0014]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Some embodiments of the present invention will be described below with reference to the drawings. 1 to 4 are circuit diagrams of operational amplifier circuits each showing a different embodiment of the present invention, and the same reference numerals are given to corresponding portions. In any of these embodiments, the main amplifier is an inverting amplification type and the auxiliary amplifier is a non-inverting amplification type for the sake of convenience, but in the present invention, it suffices to have both amplifiers perform operational amplification in opposite directions.
Therefore, even if the main amplifier is of the non-inverting amplification type and the auxiliary amplifier is of the inverting amplification type, no problem will occur as long as the operating potentials of both are appropriately selected.

In FIG. 1, a main amplifier 10 is an inverting amplification type which receives an input signal Vi toward its inverting input, and amplifies it with an amplification factor set by a resistance value ratio of a feedback resistor 12 to an input resistor 11 as usual. This is an operational amplifier that operates, and a predetermined reference voltage Vr is applied to its non-inverting input.
The main switch means 20 inserted on the input side is an electronic switch for switching operation, and specifies the amplification state in which the input signal Vi is given to the main amplifier 10 in the state shown, and the main amplifier 10 in the state opposite to that shown in the figure. A reset state is specified in which both inputs are connected to the same potential via input resistor 11.

The auxiliary amplifier 30 is an operational amplifier which performs a non-inverting amplification operation in a direction opposite to that of the main amplifier 10.
At its non-inverting input, whose inverting input is connected to the non-inverting input of the main amplifier 10
We receive Vr in common. In this embodiment, the holding means 40 shown in the upper left part of the figure by the one-dot chain line is a capacitor 41 for receiving the output signal of the auxiliary amplifier 30 and an operational amplifier for holding operation for receiving the charging voltage and outputting the same voltage value. 42, and supplies the held signal value to the inverting input of the main amplifier 10 via another feedback resistor 13. The hold circuit by the operational amplifier 42 serves to maintain the charge state of the capacitor 41 when the main amplifier 10 is a current operation type using a bipolar transistor. The auxiliary switch means 50 inserted between the holding means 40 and the output side of the auxiliary amplifier 30 is an electronic switch for ON / OFF operation, and when the auxiliary switch means 50 is turned on, the state of giving the output signal of the auxiliary amplifier 30 to the holding means 40 is designated. Then, the off state designates the separation state of the holding means 40 from the auxiliary amplifier 30.

In the operational amplifier circuit of the present invention constructed as described above, it is preferable that the reset state and the amplified state are alternately and repeatedly designated by the main switch means 20 in a predetermined cycle, and the auxiliary switch means 50 is designated in the reset state. In the giving state, in the amplifying state, the amplified signal Vo is taken out from the output of the main amplifier 10 in the separating state. First, the reset state will be explained. Since there is no input in the main amplifier 10, its output signal value should be the reference voltage Vr if there is no offset. Is on the input side, and this is amplified by the gain Gm of the main amplifier 10, so the output voltage value is the reference voltage Vr.
Deviates by Gmδm from. This deviation is amplified in the opposite direction by the gain Ga by the auxiliary amplifier 30, and the holding means 40
Is negatively fed back to the input side of the main amplifier 10 to compensate the offset Δm.

However, the offset δ also exists on the auxiliary amplifier 30 side.
There is a, and this also adds to the signal value that is negatively fed back after being amplified by that gain Ga. Therefore, assuming that the deviation of the output signal value of the main amplifier 10 from the reference voltage Vr in this reset state is e, this is offset by the offset δa of the auxiliary amplifier 30.
The sum e + δa of the above is amplified by the gain Ga and given to the holding means 40, and this is fed back to the input side of the main amplifier 10 and amplified by the gain Gm together with the offset δm to become the deviation e, so that the following equation holds. To do.

E = -GmGa (e + δa) -Gmδm By solving this equation for e, the following equation is obtained. e = −δa G / (1 + G) −δm Gm / (1 + G) However, G = GmGa, the gain G of the whole system is very large, and the second term in the above equation is much smaller than the first term. In practice, it is very easy to set e = -δa.

Next, in the amplification state, the auxiliary switch means 50 is placed in the separation state, and the input signal Vi is given to the main amplifier 10 to amplify the difference from the reference voltage Vr. At this time, the reset state is continued. Since the signal value held in the holding means 40 is given to the input side of the main amplifier 10, the shift e corresponding to the above equation overlaps with the output signal of the main amplifier 10 even in this amplification state. e remains as an offset error generated in the amplified signal Vo by the operational amplifier circuit of the present invention.

Comparing the values of the offset errors e to δa of the circuit of the present invention with the case without offset compensation,
When there is no compensation, the offset δm of the main amplifier 10 should be amplified by the gain Gm as described above and a deviation of Gmδm should occur in the output signal value. The reduction rate r of the offset error of the amplified signal Vo due to is approximately expressed by the following equation.

R = δa / Gm δm = (δa / δm) 1 / Gm As can be seen, the offset δ on the auxiliary amplifier 30 side
If a is not present, the error will be 0, but since δa is almost the same as δm on the main amplifier 10 side, r = 1 / Gm may be set. It is one of the advantages of the present invention that the offset error of the amplified signal Vo can be reduced as the gain Gm of the main amplifier 10 is higher.

As described above, the reason why the offset error e generated in the amplified signal Vo is almost equal to the offset δa on the auxiliary amplifier 30 side is that the offset δm on the main amplifier 10 side is almost perfect by the auxiliary amplifier 30 and the holding means 40. It is considered that the offset Δa on the auxiliary amplifier 30 side has almost no compensation means, and the offset Δa on the auxiliary amplifier 30 side remains almost as it is. Further, as in the embodiment of FIG. 1, the holding means 40
If the operational amplifier 42 for hold operation is installed in the, the offset is actually added, but this operational amplifier 42
The amplification factor is 1 and the offset is added to the input side of the main amplifier 10 as it is, so that it may be almost completely compensated by the auxiliary amplifier 30 together with the offset Δm of the main amplifier 10.

In the embodiment shown in FIG. 2, a capacitor 41 as a holding means is connected, for example, between the inverting input and the output of the auxiliary amplifier 30 as shown in the figure, so that the auxiliary amplifier 30 has a function of holding the output value. Simplify the circuit configuration. Also,
Correspondingly, the auxiliary switch means 50 is inserted between the output side of the main amplifier 10 and the input side of the auxiliary amplifier 30, that is, its non-inverting input in the illustrated example. The circuit configuration of the other portions is the same as that of FIG. 1, and the circuit operation and the effect of reducing the offset error of the amplified signal Vo of this embodiment are almost the same, and therefore the description thereof will be omitted to avoid duplication.

In the next embodiment shown in FIG. 3, the main amplifier 10 constitutes a sample and hold circuit, and the amplified signal Vo can be read out at any time or at an appropriate timing.
Therefore, instead of the feedback resistor 12 for the main amplifier 10 of FIG. 1, a capacitor 14 is connected for holding the amplified signal Vo, and is connected to the input side of the main amplifier 10, for example, in series with the input resistor 60 as shown in the figure. The sampling switch means 60 is inserted.
The circuit configuration of the other parts is the same as that of FIG.

The sampling switch means 60 may be a simple on-off type electronic switch, and is always turned on at the insertion position as shown in the figure, and the main switch means 20 is in the switching position in the figure and within the amplification state. Then, it is operated so as to switch from the sampling designation state of ON to the hold designation state of OFF. In this hold designation state, the state of the main switch means 20 may be arbitrary, but the auxiliary switch means 50
It is desirable to keep them separated. The amplified signal Vo can be read at an appropriate timing after the main amplifier 10 is put in the sample hold state.

Further, in the embodiment shown in FIG. 4, the amplification factor of the main amplifier 10 incorporated in the integrated circuit can be externally set to a desired value. Therefore, the input signal of the main amplifier 10
The input terminal Ti is derived from the inverting input that receives Vi, and the output terminal To is derived from that output.As shown by the thin line, the input resistor 11 is the input terminal Ti, and the feedback resistor 12 is the input terminal Ti and the output terminal To. Externally connected to each other so that the amplification factor of the main amplifier 10 can be set to a desired value by the resistance value ratio of both resistors. Further, in the example of the figure, the potential terminal Tp is also derived from the interconnection point of the non-inverting input of the main amplifier 10 and the inverting input of the auxiliary amplifier 30, and a value suitable for the set value of the amplification factor of the main amplifier 10 is derived from this. The potential of the reference voltage Vr can be externally applied.

In this embodiment as well, the same switching type main switching means 20 as in FIG. 1 may be used, but in the illustrated example, a resistor 15 is connected between both inputs of the main amplifier 10 and a simple on / off type is used. The main switch means 21 is used to specify the amplification state when turned on and the reset state when turned off. The circuit configuration of the other parts is the same as that of FIG. In the reset state in which the main switch means 21 is off, both inputs of the main amplifier 10 are connected to the same potential via the resistor 15, so that the operation of the embodiment of FIG.
There is no particular difference from the embodiment of FIG. 1 except that the amplification factor of 10 is set by an external circuit.

In any of the embodiments described above, it is advantageous to maintain the signal holding value of the holding means 40 accurately by repeatedly designating the reset state and the operating state by the main switch means 20 and 21 in a predetermined short cycle. Yes, this repetition frequency is
It is better to set it to about 100 kHz. If the repetition frequency is set high in this way, the capacitor 41 in the holding means 40 can have a small capacitance of several to several tens pF.

[0030]

As described above, in the operational amplifier circuit of the present invention, the main switch means attached to the main amplifier designates the amplification state and the reset state of the circuit, and in the reset state, the auxiliary signal receiving the output signal of the main amplifier is received. The amplifier is made to generate the offset compensation signal, and the holding means holds the signal value. In the amplification state, the holding means value is fed back to the input side of the main amplifier after the holding means is separated from the auxiliary amplifier by the auxiliary switch means. By doing so, the following effects can be achieved.

(A) In principle, the offset error of the main amplifier can be completely eliminated by compensating the offset of the main amplifier with the auxiliary amplifier and the holding means. In reality, the auxiliary amplifier also has an offset and its effect remains uncompensated, but the offset error of the amplified signal is reduced to about 1 / gain of the main amplifier, for example, to about 1% compared to the conventional case. be able to.

(B) Since the auxiliary amplifier and the holding means can automatically compensate for the offset of the main amplifier according to the magnitude of the offset, it is unnecessary to adjust the offset compensation circuit one by one as in the conventional case. It can be omitted altogether. (c) Even when an operational amplifier circuit is incorporated in an integrated circuit, it is sufficient to simply incorporate the circuit of the present invention with offset compensation, so it is necessary to mount the offset compensation circuit externally as in the conventional case. You can eliminate any space or extra effort.

(D) Since the offset error of the amplified signal decreases as the gain of the main amplifier increases, an operational amplifier circuit with a small offset error and a high gain can be easily constructed by implementing the present invention. The embodiment of the present invention in which a capacitor is connected between the input and output of the auxiliary amplifier for the holding means and the auxiliary switch means is inserted between the main amplifier and the auxiliary amplifier has the effect that the circuit configuration can be most simplified. In the embodiment in which the sampling switch means is inserted on the input side of the main amplifier and the sample-hold circuit is constituted by the main amplifier, the sample-and-hold amplification is performed after the sampling switch means moves from the sampling state to the holding state during the amplification state. It has the advantage that the signal can be extracted from the main amplifier at any desired timing.

Furthermore, an embodiment in which an input terminal and an output terminal are derived from the input and output of the main amplifier, and an input input resistance is externally connected to the input terminal and a feedback resistance is externally connected between the input terminal and the output terminal is an integrated circuit. Has the effect that the amplification factor of the operational amplifier circuit incorporated in the external amplifier can be set to a desired value particularly accurately from the outside, or can be changed at any time as necessary, and a mode in which a resistor is fixedly connected between both inputs of the main amplifier is There is an advantage that the amplification state and the reset state can be designated for the operational amplification circuit by using a very simple on / off type main switch means.

[Brief description of drawings]

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

FIG. 2 is a circuit diagram showing a second embodiment of the operational amplifier circuit according to the present invention.

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

FIG. 4 is a circuit diagram showing a fourth embodiment of the operational amplifier circuit according to the present invention.

FIG. 5 is a circuit diagram showing an example of a conventional operational amplifier circuit.

[Explanation of symbols]

10 Main amplifier 20, 21 Main switch means 30 Auxiliary amplifier 40 Holding means 41 Capacitor holding the signal value for offset compensation 50 Auxiliary switch means Vi Input signal to be amplified Vo Amplified signal

Claims (7)

[Claims] 1. A main amplifier for operational amplification of an input signal, main switch means for designating and controlling an amplification state for applying the input signal and a reset state for connecting the two inputs to the same potential, and an output signal from the main amplifier. An auxiliary amplifier for performing operational amplification in a direction opposite to that of the main amplifier, holding means for holding the output signal value of the auxiliary amplifier and feeding it back to the input side of the main amplifier, and a state of giving the output signal of the auxiliary amplifier to the holding means. Auxiliary switch means for specifying the separated state is provided, and the auxiliary switch means is set in the applied state in the reset state, and the auxiliary switch means is set in the separated state in the amplification state to extract the amplified signal from the output of the main amplifier. And an operational amplifier circuit. 2. The circuit according to claim 1, wherein a capacitor connected between one input and the output of the auxiliary amplifier is used as the holding means, and the capacitor is connected between the output of the main amplifier and the other input of the auxiliary amplifier. An operational amplifier circuit characterized in that an auxiliary switch means is inserted. 3. The circuit according to claim 1, wherein a sampling switch means is inserted on an input side for receiving an input signal of the main amplifier, and a sample hold circuit is constituted by the main amplifier, and the sampling switch means is provided during an amplification state. An operational amplifier circuit characterized in that an amplified signal sample-held by a main amplifier can be taken out from its output after switching from an on-sampling specified state to an off-hold specified state. 4. The circuit according to claim 1, wherein an input terminal for receiving an input signal of the main amplifier is derived from an input terminal, and an output terminal is derived from the output thereof, and an input resistor for receiving the input signal is input to the input terminal. An operational amplifier circuit characterized in that a feedback resistor can be externally connected between the output terminal and the output terminal. 5. The circuit according to claim 4, wherein a resistor is connected between a pair of inputs of the main amplifier, and the main switch means cuts off the input signal from the amplification state in which the input signal is applied to the main amplifier. An operational amplifier circuit characterized in that a reset state is designated and controlled. 6. The circuit according to claim 1, wherein the holding means comprises a capacitor for receiving the output signal of the auxiliary amplifier and an operational amplifier circuit for holding operation for receiving the charging voltage and outputting the same voltage value. An operational amplifier circuit characterized by. 7. The circuit according to claim 1, wherein the inputs of the main amplifier and the auxiliary amplifier other than the input signals of the main amplifier and the inputs other than the output signal of the main amplifier are connected to each other to provide a common constant potential. An operational amplifier circuit characterized by the above.