JPH11225028A - Variable gain amplifier - Google Patents

Abstract

PROBLEM TO BE SOLVED: To enhance the linearity of an output characteristic of the variable gain amplifier. SOLUTION: An operational amplifier AMP2 is used for a feedback circuit of an operational amplifier AMP1. A variable voltage division circuit configured by connecting resistors R1, R2,... Rn and analog switches SW1, SW2, SW3,... SWn in a lattice shape is used as a feedback circuit of the operational amplifier AMP2 is employed. Since a voltage gain Vo/Vi (Vo is an output voltage and Vin is an input voltage) is nearly equal to a voltage division ratio of the variable voltage division circuit that is the feedback circuit of the operational amplifier AMP2, the output linearity is enhanced. Since the analog switches SW1, SW2, SW3, ...SWn connect to terminals different from a terminal of the operational amplifier AMP2 to which an output voltage Vo is applied among input terminals of the operational amplifier AMP2, the error due to the on- resistance of the analog switches SW1, SW2, SW3,...SWn and the occurrence of the temperature dependence are prevented.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

[0001] 1. Field of the Invention [0002] The present invention relates to a variable gain amplifier which can be configured using an operational amplifier, a resistor, an analog switch, and the like.

[0002]

2. Description of the Related Art Various configurations of a variable gain amplifier using an operational amplifier have been proposed (for example, see Japanese Patent Application Laid-Open No. 61-91506). FIG. 5 shows an example of a variable gain amplifier using an operational amplifier. In the variable gain amplifier shown in FIG.
The input voltage V is applied to the inverting input terminal of P via an input resistor Rs.
While applying in, the bias voltage from the bias power supply BIAS is applied to the non-inverting input terminal. The output voltage Vo obtained from the output terminal of the operational amplifier AMP is supplied to the operational amplifier via a feedback circuit including resistors R1, R2, R3,... Rn and analog switches (FETs in the figure) SW1, SW2, SW3,. It is applied to the inverting input terminal of the AMP (n: natural number of 2 or more).

The resistors R1, R2, R3,... Rn are connected in series with corresponding ones (same suffixes) of the n analog switches SW1, SW2, SW3,. Are connected in parallel with each other. Each of the analog switches SW1, SW2, SW3,.
Wn is selectively turned on / off by an on / off control signal provided from a gain control unit (not shown). Accordingly, when the combined resistance of the n resistors R1, R2, R3,... Rn whose corresponding analog switch is on is represented as Rx, the voltage gain A of the variable gain amplifier shown in FIG. The following expression

A = Vo / Vin = − (Rx / Rs) A0 >> Rs / Rx

In the variable gain amplifier shown in FIG. 5, analog switches SW1, SW2, SW3,.
Since the ON resistance of n causes an error in the combined resistance Rx, an error also occurs in the gain A. In addition, the analog switches SW1,
When the on-resistance of SW2, SW3,... SWn has a relatively large temperature coefficient, the temperature dependency of the gain A becomes large and also causes the temperature dependency of the offset voltage.

FIG. 6 shows an example of a variable gain amplifier which is hardly affected by the on-resistance of the analog switch and its temperature coefficient. In the variable gain amplifier shown in this figure, an input voltage Vin is applied to a non-inverting input terminal of an operational amplifier AMP, and an inverting input terminal is used for a feedback input. The feedback circuit from the output terminal of the operational amplifier AMP to the inverting input terminal has n resistors R1, R2,... Rn and n analog switches SW1, SW2, SW3,. It is a variable voltage dividing circuit. That is, n resistors R1, R2,... Are connected between the output terminal of the operational amplifier AMP and the ground.
Rn are sequentially connected in series, and furthermore, resistors R1, R
, Rn and an inverting input terminal of the operational amplifier AMP, analog switches SW1, SW2, SW3,
... SWn are provided.

Here, in the variable gain amplifier shown in FIG. 5, from the output terminal side of the operational amplifier AMP to the input resistor Rs via the feedback circuit (and further to the supply source of the input voltage Vin).
And analog switches SW1 and SW forming a feedback circuit
Current flows through SW2, SW3,... SWn. In contrast, in the variable gain amplifier shown in FIG. 6, almost no current flows through the analog switches SW1, SW2, SW3,. This is because the input impedance of the operational amplifier AMP is generally extremely large. As described above, in the variable gain amplifier shown in FIG.
, SW2, SW3,... SWn, because almost no current flows through them, the analog switches SW1, SW2, SW3,.
The ON resistance of SWn does not cause an error in the feedback voltage and thus the gain A.

[0007]

However, the variable gain amplifier shown in FIG. 6 has a problem that the characteristic of the output voltage Vo with respect to the feedback gain, that is, the output characteristic becomes non-linear. That is, the analog switches SW1, SW2, SW3,
... When any one of SWn is selected and turned on and the rest are turned off, as shown in FIG. 7, a series of resistors on the output terminal side of the operational amplifier AMP and a grounded The output voltage Vo is divided by a series of resistors at a resistance value ratio α, and is fed back to the inverting input terminal of the operational amplifier AMP (0 <α <1). Since the gain relating to the feedback input, that is, the feedback gain, is the voltage division ratio α, when the circuit shown in FIG. 7 is represented by a block diagram, it becomes a circuit as shown in FIG. The gain A of the circuit shown in FIG.
As is well known

A = A0 / (1 + α · A0) = 1 / α... Α >> 1 / A0 Therefore, when the input voltage Vin is constant, the output voltage Vo has a hyperbolic characteristic with respect to the feedback gain α (see FIG. 9).

SUMMARY OF THE INVENTION The present invention has been made to solve such a problem, and a technique for suppressing the influence of the on-resistance of an analog switch by utilizing the high input impedance of an operational amplifier is shown in FIG. In addition, it is an object of the present invention to suppress the non-linearity of the output characteristic by modifying the configuration of the feedback circuit, thereby changing the feedback gain in a wider range than before.

[0009]

In order to achieve the above object, a variable gain amplifier according to the present invention comprises a first operational amplifier, a second operational amplifier and a variable voltage dividing circuit, and a second operational amplifier and a variable voltage dividing circuit. The voltage circuit is used as a feedback circuit for the first operational amplifier, and the variable voltage dividing circuit is used as a feedback circuit for the second operational amplifier.

First, the first and second operational amplifiers have first and second input terminals and output terminals, respectively. A first input terminal of the first operational amplifier is used for inputting a signal to be amplified, and an output terminal is used for outputting an amplified signal. The output terminal of the first operational amplifier is further connected to the first input terminal of the second operational amplifier so that the signal amplified by the first operational amplifier is fed back to the second input terminal of the first operational amplifier. Connected to the second input terminal of the first operational amplifier. Further, the output terminal of the second operational amplifier is connected to a variable voltage dividing circuit, and the variable voltage dividing circuit is further connected to a second input terminal of the second operational amplifier so that the signal amplified by the second operational amplifier is fed back to its second input terminal. Connected to. Further, the variable voltage dividing circuit includes a plurality of resistors connected in series between the output terminal of the second operational amplifier and the constant potential point, and a plurality of points including the series connection points of the resistors connected to the second operational amplifier. A predetermined number of analog switches selectively connected to the second input terminal. The variable voltage dividing circuit variably divides the voltage of the signal output from the second operational amplifier and feeds it back to the second input terminal of the second operational amplifier.

Therefore, the non-linear output characteristic which would appear when the variable voltage dividing circuit is used as the feedback circuit of the first operational amplifier is, in the present invention, the second operational amplifier having the variable voltage dividing circuit as the feedback circuit. It does not appear because it is used as a feedback circuit for one operational amplifier. As a result, the output characteristics of the variable gain amplifier according to the present invention have higher linearity than the output characteristics of the conventional circuit using the variable voltage dividing circuit as the feedback circuit of the operational amplifier. The signal amplified by the first operational amplifier, that is, the signal to be amplified by the second operational amplifier, is input to the first input terminal of the second operational amplifier, and the output of the feedback circuit related to the second operational amplifier, that is, the output of the variable voltage dividing circuit is The signal is input from the first input terminal to a second input terminal separated by a high input impedance. Therefore, the effect of the conventional circuit of suppressing the influence of the on-resistance of the analog switch using the high input impedance characteristic of the operational amplifier continues to occur.

[0012]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Preferred embodiments of the present invention will be described below with reference to the drawings.

FIG. 1 shows a configuration of a variable gain amplifier according to an embodiment of the present invention. In this embodiment, two operational amplifiers AMP1 and AMP2 are used. An input voltage Vin is applied to a non-inverting input terminal of the operational amplifier AMP1, and an amplified voltage, that is, an output voltage Vo is extracted from an output terminal of the operational amplifier AMP1. Further, the output terminal of the operational amplifier AMP1 is connected to the non-inverting input terminal of the operational amplifier AMP2, and the output terminal of the operational amplifier AMP2 is connected to the inverting input terminal of the operational amplifier AMP1. As a feedback circuit from the output terminal of the operational amplifier AMP2 to the inverting input terminal, n resistors R1, R2,... Rn and n analog switches SW1, SW2, SW3,.
Is provided. That is, the variable gain amplifier according to the present embodiment has a configuration in which a circuit similar to the conventional variable gain amplifier shown in FIG. 6 is arranged as a feedback circuit from the output terminal of the operational amplifier AMP1 to the inverting input terminal. I can say. Therefore, the operational amplifier A
The voltage dividing ratio in the variable voltage dividing circuit as the feedback circuit of MP2 is α
In this case, the circuit shown in FIG. 1 becomes a circuit as shown in FIG. 2, and further has a block configuration as shown in FIG.

The gain A of the circuit shown in FIG.
A0 in Equation 2 above is the voltage gain A of the operational amplifier AMP1.
1 and α is replaced by A2 / (1 + α · A2) (where A2 is the operational amplifier AM
P2 voltage gain), a value represented by the following equation

A = A1 / (1 + A1 · A2 / (1 + α · A2)) = α... Α >> 1 / A1 and α >> 1 / A2. Therefore, the output characteristics of the variable gain amplifier according to this embodiment have high linearity as shown in FIG. 4, and the gain adjustment range by adjusting the voltage division ratio α is widened. Further, since the output terminal of the variable voltage dividing circuit is input to another input terminal (inverting input terminal) of the input terminal of the operational amplifier AMP2 which is different from the input terminal (non-inverting input terminal) to which the output voltage Vo is input, As in the prior art shown in FIG. 6, the influence of the on-resistance of the analog switches SW1, SW2, SW3,... SWn can be prevented.

The present invention can be embodied in other forms. For example, the non-inverting input terminal and the inverting input terminal can be interchanged. Further, analog switches other than FETs can be used as the analog switches SW1, SW2, SW3,... SWn. Further, the term "op amp" is used in this application.
As used herein, the term “op-amp” refers to an amplifier having at least two input terminals and an output terminal and having a sufficiently high input impedance (that is, one that is hardly affected by the on-resistance of the analog switch). Is included.

[0016]

As described above, according to the present invention,
Providing a first operational amplifier, a second operational amplifier, and a variable voltage dividing circuit, using the second operational amplifier and the variable voltage dividing circuit as a feedback circuit relating to the first operational amplifier, and using the variable voltage dividing circuit as a feedback circuit relating to the second operational amplifier; Therefore, the nonlinear output characteristic that would appear when the variable voltage dividing circuit is used as the feedback circuit of the first operational amplifier does not appear, and therefore,
Output characteristics with higher linearity than in the past can be obtained. Further, since the current flowing through the analog switch is suppressed by using the high input impedance characteristic of the operational amplifier, it is possible to suppress the influence of errors and the like caused by the on-resistance of the analog switch.

[Brief description of the drawings]

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

FIG. 2 is a circuit diagram showing an operation of a variable voltage dividing circuit according to the embodiment.

FIG. 3 is a block diagram showing a transfer function of the embodiment.

FIG. 4 is a diagram showing output characteristics of the embodiment.

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

FIG. 6 is a circuit diagram showing a configuration of a variable gain amplifier according to another related art.

FIG. 7 is a circuit diagram showing an operation of the variable voltage dividing circuit in the prior art of FIG. 6;

FIG. 8 is a block diagram showing the transfer function of the prior art of FIG. 6;

FIG. 9 is a diagram showing output characteristics of the prior art of FIG. 6;

[Explanation of symbols]

AMP1, AMP2 operational amplifier, R1, R2, ... Rn
Resistance, SW1, SW2, SW3,... SWn Analog switch, Vin input voltage, Vo output voltage, α voltage division ratio, A1, A2 AMP1, AMP2 voltage gain.

Claims (1)

[Claims] A first operational amplifier having first and second input terminals and an output terminal, wherein the first input terminal and the output terminal are used for inputting a signal to be amplified and outputting an amplified signal, respectively; A feedback circuit for feeding back a signal amplified by the first operational amplifier to a second input terminal of the first operational amplifier, wherein the feedback circuit has first and second input terminals and an output terminal. And the first
A second operational amplifier having an input terminal connected to the output terminal of the first operational amplifier, and an output terminal connected to a second input terminal of the first operational amplifier, respectively; and an output terminal of the second operational amplifier and a constant potential point. A predetermined number of analog switches for selectively connecting a plurality of resistors connected in series between the plurality of resistors and a plurality of points including a series connection point between the resistors to a second input terminal of the second operational amplifier; A variable voltage dividing circuit that variably divides a voltage of a signal output from the two operational amplifiers and feeds back to a second input terminal of the second operational amplifier.