JPH0318766B2 - - Google Patents

Description

[Detailed description of the invention] [Technical field of invention] The present invention relates to a variable gain control circuit.

[Technical background of the invention and its problems]

Various types of circuits have been considered as variable gain control circuits.

FIG. 3 shows an example. In Figure 3, Q ₁₁ and Q ₁₂ are gain control transistors,
V ₁₁ is a constant voltage source, and V ₁₂ is a variable voltage source for gain control. I ₁₁ is the signal source. _D11 is a diode,
Q ₁₃ is a transistor, and these represent a current mirror circuit on the input side. _D12 is a diode,
_Q14 is a transistor, and these form a current mirror circuit on the output side. OUT is the output terminal, V _CC
is the power supply.

The above circuit is configured to change the magnitude of the signal current flowing through the transistor _Q14 by changing the gain control voltage output from the variable voltage source _V12 . Although this circuit has small output distortion, it has the problem that it is difficult to drive at a low voltage because a large number of elements are inserted in series between the power supply V _CC and ground. In particular, in recent years, low-voltage driven integrated circuits have become desirable, and under such circumstances, the above-mentioned circuits have become extremely difficult to use.

FIG. 4 is a circuit diagram showing another example of a conventional variable gain control circuit.

In Figure 4, Q ₁₄ and Q ₁₅ are transistors,
R ₁₁ and R ₁₂ are resistors, which form a current mirror circuit. _I12 is a variable current source for gain control.

The above circuit changes the signal current flowing through the transistor _Q15 by changing the magnitude of the gain control current output from the variable current source _I12 . This circuit can be driven at relatively low voltages. However, the gain control current
When the direct current flowing through _Q15 is controlled, the transfer characteristics of the current mirror circuit become non-linear, causing output distortion.

[Purpose of the invention]

The present invention has been made to address the above-mentioned circumstances, and an object of the present invention is to provide a variable gain control circuit that can be driven at a low voltage and has low output distortion.

[Summary of the invention]

In this invention, a resistor is inserted between the base and collector of a diode-connected transistor (that is, an input transistor) in a current mirror circuit. Further, a gain control signal to be subjected to gain control is input to a common connection point between the collector of the input side transistor and the base of the output side transistor of the current mirror circuit, and the gain control output is obtained from the collector of the output side transistor.

In such a configuration, a variable current source is connected to a common connection point between the resistor and the base of the input transistor. By changing the value of the output current of this variable current source as a gain control current, the voltage across the resistor is changed and the current mirror ratio (gain) is changed.

According to such a configuration, if the base currents of the two transistors are ignored, the only current flowing through the resistor is the gain control current. Therefore,
The above gain can be made constant, and output distortion can be prevented from occurring.

[Embodiments of the invention]

Embodiments of the present invention will be described in detail below with reference to the drawings.

FIG. 1 is a circuit diagram showing one embodiment of the present invention.

In the figure, the emitters of transistors Q ₂₁ and Q ₂₂ are commonly connected to the power supply V _CC . The base of transistor Q ₂₂ is connected to the collector of transistor Q ₂₁ . A resistor _R21 is inserted between the collector and base of the transistor _Q21 . A variable current source I ₂₁ for gain control is connected to a common connection point between the base of the transistor Q ₂₁ and the resistor R ₂₁ . The base of the transistor Q ₂₂ and the transistor
A signal current source I ₂₂ is connected to the common connection point of the collectors of Q ₂₁ . An output terminal OUT is provided at the collector of the transistor _Q22 .

The operation in the above configuration will be explained. In the following description, it is assumed that the symbol of the resistor R ₂₁ is the current source I ₂₁ and the symbol of the current source I ₂₂ is the resistance value or current value.

A gain-controlled current I ₂₂ that is output from the signal current source I ₂₂ and is to be gain-controlled is input to a common connection point between the collector of the transistor Q ₂₁ and the base of the transistor Q ₂₂ . The gain control output is then led to the output terminal OUT from the collector of transistor _Q22 .

Transistors Q ₂₁ and Q ₂₂ constitute a current mirror circuit. Transistor Q ₂₁ is a transistor on its input side, that is, a diode-connected transistor, and transistor Q ₂₂ is a transistor on its output side.

The current mirror ratio of this current mirror circuit is
In other words, the gain G of the variable gain control circuit is the resistance
It can be changed by changing the voltage across R ₂₁ . The base currents of transistors Q ₂₁ and Q ₂₂ are small and can be ignored. Therefore, ideally, the current flowing through the resistor R ₂₁ is controlled by the variable current source I ₂₁
The output current is only _I21 . This makes the resistance R ₂₁
The voltage across is a constant. Therefore, the gain G also becomes a constant, and no output distortion occurs. The gain G can be changed by changing the value of the output current I ₂₁ of the variable current source I ₂₁ as a gain control current.

Now let's find the gain G. transistor
The base potentials of Q ₂₁ and Q ₂₂ are as follows: V _B21 = V _{T lo} (I ₂₁ + I ₂₂ / I _S ) …(1) V _B22 = V _{T lo} (I _OUT / I _S )…(2) expressed. Furthermore, the base potential V _B22 can also be expressed as follows.

V _B22 = V _B21 − I ₂₁・R ₂₁ …(3) In the above equation, I _S is the reverse saturation current between the base and emitter of transistors Q ₂₁ and Q ₂₂ , and I _OUT is the current output from terminal OUT. It is. Also, V _T
is expressed by the following equation (4).

V _T =kT/q (4) where k: Boltzmann's constant T: absolute temperature q: electron charge The following relationship can be found from the result of substituting equation (1) into equation (3) and equation (2).

V _{T lo} (I _OUT / I _S ) = V _{T lo} (I ₂₁ + I ₂₂ / I _S ) −R ₂₁・I ₂₁ …(5) When formula (5) is transformed, R ₂₁・I ₂₁ /V _T = l _o (I ₂₁ + I ₂₂ /I _S ) - l _o (I _OUT /I _S
) = l _o (I ₂₁ + I ₂₂ /I _OUT ) …(6). When formula (6) is transformed, I ₂₁ + I ₂₂ /I _OUT =exp(R ₂₁ ·I ₂₁ /V _T )...(8). Therefore, the gain G is expressed by the following equation.

If the AC signal components in the controlled current I ₂₂ and the output current I _OUT are respectively ΔI ₂₂ and ΔI _OUT , then I _OUT + ΔI _OUT /I ₂₁ + I ₂₂ + ΔI ₂₂ = 1/exp(R ₂₁・I ₂₁
/V _T ). Focusing on the AC signal component, the AC gain G
is expressed by the following equation.

G=ΔI _OUT /ΔI ₂₂ =1/exp(R ₂₁・I ₂₁ /V _T )...(9) As is clear from equation (9), the gain G does not include the gain-controlled current I ₂₂ and the resistance R ₂₁ is a constant determined by the gain control current I ₂₁ and the constant V _T . Therefore, the transfer characteristic of the current mirror circuit exhibits linearity, and no output distortion occurs. Gain G is gain control current
The gain control current is varied by I ₂₁ as an example.
When I ₂₁ is set to 0, the gain G becomes 1 and the output current
I _OUT will be the same magnitude as the gain controlled current.

Furthermore, in the case of the configuration shown in FIG. 1, the number of elements connected and inserted in series between the power supply V _CC and the ground is small, so low voltage driving is possible. Specifically, V _CC
It has been confirmed through experiments that it can be driven even at ≒0.8V.

In the above embodiment, the case where PNP transistors are used as the transistors Q ₂₁ and Q ₂₂ was explained.
Of course, an NPN transistor may be used as shown in FIG. In FIG. 2, elements having the same functions as those shown in FIG. 1 are given the same reference numerals. Further, in FIG. 2, a plurality of transistors are provided on the output side, so that a plurality of outputs can be obtained. The figure shows a typical case in which three transistors Q ₂₂ to Q ₂₄ having a common base are provided.
_OUT1 to _OUT2 are output terminals provided for each transistor _Q22 to _Q24 .

〔Effect of the invention〕

As described above, according to the present invention, it is possible to provide a variable gain control circuit that can be driven at a low voltage and has low output distortion.

[Brief explanation of the drawing]

Fig. 1 is a circuit diagram showing one embodiment of the present invention, Fig. 2 is a circuit diagram showing another embodiment, Fig. 3 is a circuit diagram showing an example of a conventional variable gain control circuit, and Fig. 4 is a circuit diagram showing an example of a conventional variable gain control circuit. It is a circuit diagram showing another example. Q ₂₁ ~ Q ₂₄ ... Transistor, R ₂₁ ... Resistor, I ₂₁
...Variable current source, I ₂₂ ...Signal current source, OUT...
Output terminal, V _CC ... power supply.

Claims (1)

[Claims] 1. A first transistor whose emitter is connected to the reference potential point side; and a second transistor whose emitter is connected to the reference potential point side and whose base is connected to the collector of the first transistor. a transistor, a resistor inserted between the base and collector of the first transistor, and a variable current that is connected to a common connection point between the resistor and the base of the first transistor and outputs a current for gain control. a collector of the first transistor and a collector of the second transistor;
A variable gain control circuit, characterized in that it is configured to input a gain control signal from a common connection point with the base of the second transistor, and take out the gain control output from the collector of the second transistor.