JPH02301208A - Gain adjustment circuit - Google Patents

Abstract

PURPOSE:To adjust the gain with a feedback resistor of an operational amplifier only by connecting two output current paths of a 2R ladder resistor to each inverting input terminal of 1st and 2nd operational amplifiers connected in series. CONSTITUTION:Each output current path of a 2R ladder resistor 3 connects to each inverting input terminal of two operational amplifiers 2, 8 connected in series. Since the two output current path currents in the complementary relation from a 2R ladder 3 are subject to voltage conversion to the inverting input terminal by a 1st feedback resistor 4, the inverting input terminal is an imaginary grounding point. Thus, the sum between the input voltage and a voltage across the 1st feedback resistor 4 connecting to the inverting input terminal of the 1st operational amplifier 2 is equal to the sum between the output voltage and a voltage across a 2nd feedback resistor 5 connecting to the inverting input terminal of the 2nd operational amplifier 8. Thus, the factor in the 2R ladder resistor is cancelled and the gain adjustment is attained with the feedback resistor only.

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Industrial Application] The present invention relates to a gain adjustment circuit, and particularly to a gain adjustment circuit used for multiplication of analog signal amplitude and digital data, digital-to-analog conversion, and the like.

[Prior Art] FIG. 2 shows a conventional digital gain control circuit shown in Japanese Patent Application Laid-Open No. 63-102506, in which the signal K(1)?
, t2R ladder resistor (3), and is connected via a first resistor (4) to the inverting input terminal of a first operational amplifier (2) forming a feedback amplifier. Operational amplifier (2)
The output of is applied via a resistor (5) to the inverting input terminal of a second operational amplifier (8) for summing. The 2R ladder resistor (3) is a device for creating a multiplication current source. The first resistor (4) for feedback of the first operational amplifier (2)
) performs current-voltage conversion operation. A feedback resistor (6) for adding current to the voltage of the signal source (1) by the feedback resistor (7) of the second operational amplifier (8) is connected to the signal source (1) and the second operational amplifier (8). is connected between the inverting input terminal of A feedback resistor (7) which adds the voltage of the signal source (1) and the output voltage of the operational amplifier (2) is connected in parallel with a phase correction capacitor (9) for obtaining stability of the circuit.

Since the inverting input terminal of the first operational amplifier (2) is virtually grounded, the current flowing through the 2R ladder resistor (3), which is the feedback resistance of the feedback amplifier (2), is 3) is at the GND potential (Ov) regardless of the switching state of the semiconductor switch (10), which is equivalently the same as when the 2R ladder resistor (3) is connected to the same point. Therefore, if the output voltage of the first operational amplifier (2) is eo, the equivalent resistance value seen from the connection point of the 2R resistor to the GND side is R, so the control point D7
The current flowing through the 2R resistor (3) is (eo/2R), and the current flowing through D6, D5, . . . , Do is 1/2. Therefore, the 2R resistor at Dk (
3) The current flowing in It − (eo /R) (1/2)
Therefore, to simplify the explanation, consider the case where only the 2R resistor (3) at Dk is connected to the inverting input terminal of the first operational resistor (2), and the signal source voltage VIN
, the resistance values of the resistors (4), (5), (6), and (7) are R4, R5, R6, and R7, respectively, then (VIN
From O)-R4It, eo = (2) 8-k (R/R4)VIN. However, the semiconductor switch (10) is a 2R resistor (3)
A state in which all the current flowing in the
7 = 0), the first operational amplifier (2) does not form a negative feedback amplifier and is in an open loop state, so eo = AOVIN (AO; open loop gain of the first operational amplifier (2)). . On the other hand, since the signal source voltage VIN is applied to the inverting input of the second operational amplifier (8) via the feedback resistor (6), its output is Vout = -[(Ry /R6) (2)'' ( R/R4) (R7/R5)] VIN.Furthermore, in any control data, (R/R
4) (Rt /R5) = (Rt /Rs), IXV
, N. When R5-R, -R and further R4-R are set, it becomes X (R7/R) VIN. And in the case of D0~D7-1, θ~0, Vout = (2' 1) (R7/R) VI
N.

Also, Do-D? -1 to 1, Vout -
[28(2'-1)] becomes VIN.

[Problems to be Solved by the Invention] Since the conventional gain adjustment circuit was configured as described above, the gain parameters RR7/R4R,
includes the resistance value R of the 2R ladder resistor.

This R value is made up of the on-resistance value of the semiconductor switch and the resistance value of the ladder resistor, so it improves the variation in the on-resistance value, nonlinearity, temperature characteristics, etc. of the semiconductor switch.
There were problems such as the need to select a certain number of resistors, and the need to use high-performance resistors for the 2R ladder resistor.

This invention was made with the aim of solving these problems, and allows gain adjustment using only the feedback resistor of an operational amplifier, and can be configured with an inexpensive semiconductor switch and an inexpensive 2R ladder resistor. The purpose is to obtain a digital gain adjustment circuit.

[Means for Solving the Problems] A gain adjustment circuit according to the present invention is characterized in that each output current path of a 2R ladder resistor is connected to each inverting input terminal of two directly connected operational amplifiers. do.

[Function] According to the gain adjustment circuit according to the present invention, two output current path currents from the 2R ladder that are complementary (the sum is constant) are voltage-converted to the inverting input terminal by the first feedback resistor. Therefore, the inverting input terminal becomes virtual ground, and the sum of the input voltage and the voltage across the first feedback resistor connected to the inverting input terminal of the first operational amplifier, and the output voltage and the inverting voltage of the second operational amplifier. The sum of the voltages across the second feedback resistor connected to the input terminal becomes the same. Therefore, the elements of the 2R ladder resistor are canceled out, and the gain can be adjusted using only the feedback resistor.

[Example] Next, the present invention will be described in more detail based on an example shown in FIG.

In FIG. 1, the signal [(1) is connected to the first resistor (4
), one output current path of the 2R ladder resistor (3) and the inverting input terminal of the first operational amplifier (2) are connected to form a negative feedback amplifier.

The output point of the 2R ladder resistor (3) is switched by a semiconductor switch (10) according to digital control data, and its second output current path is connected to the second operational amplifier (
8) is connected to the inverting input terminal. This second operational amplifier (8) is connected in series with the first operational amplifier (2) via a second resistor (5), and both resistors (4) and (
5) respectively convert the current of the 2R ladder resistor (3) into voltage.

Here, the second operational amplifier (8) forms a negative feedback amplifier together with the 2R ladder resistor (3) and the second resistor (5). Further, (9) is a phase correction capacitor for obtaining stability of the circuit.

Since the 2R resistor of the 2R ladder resistor (3) is connected to the inverting input terminal which is the virtual ground of the first operational amplifier (2) and the second operational amplifier (8), the first operational Assuming that the output voltage of the amplifier (2) is eI ``'' OUT and the output voltage of the second operational amplifier (8) is e, ), the 2R resistor is connected to the semiconductor switch (1
0), the current flowing through the 2R resistor is (eo/R) (1/2), -, (eo/R) at each point D7 to D0, respectively. )(
1/2)', and the output voltage e0 when only the 2R resistor at Dk is connected to the inverting input terminal of the first operational amplifier (2) is VIN−-R4■, so eo ” (R /R4) (2)'-' VIN
So, for arbitrary control data, it becomes
(eo/R), e o "" (R/ Rs ) [1/ (1-(1/2)'-k)] eo+, and with arbitrary control data, X (R/ Rs) e or Become. Therefore, from both equations, the relationship between eol""VOut and VIN becomes X (R/Rs) eol, so lo (L)orsu, Z+-... 1υ7Z'ノX
(R5/R4) VIN Do +I), 2+...+D727X (R5
/R4) VIN.

Therefore, if Do=D7-1.0~0, eo+= (
2' 1) (R5/R4)VIN'': 28 (Rs
/R4)VIN Also, in the case of D0~D7-1~1, eo+-(1/ (281)) (R5/R4)VI
N”= 1/2” (R5/R4) Reversing the connection of the VIN2R resistor to the inverting input results in X (R5/R4
) VIN, and in each data, for D0~D7-θ~0, eo+W (1/ (2” 1) l (Rs
/Ra) VIND0~D7-0.1~1, eo+-2' (Rs/R4) VIN Therefore, according to the gain adjustment circuit according to the present invention, in either polarity, the 2R ladder resistor (3) does not include the resistance value R of the external first resistor R4 (4) and second resistor R6.
A gain adjustment resistor can be formed using only (5).

Thus, according to the invention, the output voltage eoI of the first operational amplifier (2), ie the output V of the circuit. 2R for LIT
Since the resistance value factor of the ladder resistor (3) is not included and only the first resistor (4) and the second resistor (5) are the gain parameters, gain adjustment is possible with only these two resistors. There is no need to make expensive semiconductor switches and 2R ladder resistors (3).

In the above embodiment, a digital gain adjustment circuit has been described, but the present invention can also be applied to a digital-to-analog conversion circuit using a DC voltage as the signal source voltage.

[Effects of the Invention] As explained above, the present invention enables gain adjustment using only two external resistors, the first resistor and the second resistor, and is suitable for use in high-precision semiconductor switches and resistors. There is no need to prepare a circuit, the circuit can be formed at low cost, and the gain can be adjusted with high precision.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a circuit diagram of a gain adjustment circuit according to an embodiment of the present invention, and FIG. 2 is a circuit diagram of a conventional gain adjustment circuit. In the figure, (1) is the signal source, (2) is the first operational amplifier, (3) is the 2R ladder resistor, (4) is the first resistor, (5) is the second resistor, ( 6) is the feedback resistor, (7)
is a feedback resistor, (8) is a second operational amplifier, (9) is a phase correction capacitor, and (10) is a semiconductor switch. In addition, in the figures, the same reference numerals indicate the same or corresponding parts.

Claims (1)

[Claims] A plurality of resistors connected to an input current path and sequentially branching into paths having a resistance value of 2R, and a plurality of semiconductor switches connecting the sequentially branched paths to either of two output current paths. a 2R ladder resistor whose resistance value ratio from the input current path to the two output current paths is changed according to the signal input to the semiconductor switch; and an inverting input terminal serving as a virtual ground. A first operational amplifier is connected to a signal source through a second resistor, and has an output terminal connected to a signal output terminal, and an inverting input terminal, which is a virtual ground, is connected to a first operational amplifier through a second resistor. and a second operational amplifier connected to the output terminal of the operational amplifier, the input current path of the 2R ladder resistor is connected to the output terminal of the second operational amplifier, and the input current path of the 2R ladder resistor is connected to the output terminal of the second operational amplifier. A gain adjustment circuit characterized in that an output current path is connected to an inverting input terminal of a first operational amplifier and an inverting input terminal of a second operational amplifier, respectively.