JPH0744411B2 - Gain control circuit - Google Patents

Description

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a gain control circuit, and more particularly to a gain control circuit in a semiconductor integrated circuit.

2. Description of the Related Art In the conventional example shown in FIG. 2, among the transistors Q3 to Q8 constituting the double-balanced differential amplifier 1, the transistors Q4 and Q5 of the upper differential pair transistors Q3, Q4 and Q5, Q6 are used as the bases. The variable current source 3 is connected and the transistor Q2
The emitters of are connected. Meanwhile, transistors Q3 and Q6
The base of is connected to the constant current source 2 and the emitter of the transistor Q1. The bases of the transistors Q1 and Q2 are connected to the reference voltage source via the terminal 16,
The reference voltage Vref is applied. The input signal of the double balance type differential amplifier 1 enters terminals 5 and 6, is amplified by the differential amplifier 1, and is then output from the current mirror circuits 7 and 8 to the current mirror circuits 10 and Q10 as a current I _5. At the same time, the current is output from the current mirror circuit 9 and Q9 as a current I ₄ , and the current difference (I ₄ −I ₅ ) between them flows through the resistor 11 to extract the output voltage Vo. In the meantime, the input signal is subjected to gain control in the upper differential pair transistors Q3, Q4 and Q5, Q6.

Here, the gain control operation of the transistors Q3 and Q4 will be described. First, the current value I ₂ of the variable current source 3 is changed from I ₁ = I ₂ to I _2.
When it is smaller than ₁ , the ratio of I ₁ / I ₂ becomes large, the input signal level output to the current mirror circuits 7 and 8 through the transistor Q3 becomes small, and as a result, the gain decreases. On the other hand, the current value I ₂ of the variable current source 3 is changed from I ₁ = I ₂ to I
_When it is larger than ₁ , the ratio of I ₁ / I ₂ becomes small, the input signal level output to the current mirror circuits 7 and 8 through the transistor Q3 becomes large, and the gain becomes high.

Problems to be Solved by the Invention In the above-mentioned embodiment, the transistor Q 2 is also set when the current I ₂ is zero.
Must supply the base currents of transistors Q4 and Q5, so that current flows. Therefore transistors Q1 and Q2
There is a drawback that the voltage difference of V _BE is not large and the damping ratio cannot be taken sufficiently. To explain this point in more detail, generally, Vf of a transistor is generally expressed as follows: k is Boltzmann's constant, T is absolute temperature, q is electron charge, I is forward current of diode characteristic of transistor shown in FIG. The reverse saturation current is expressed as Vf = (kT / q) ln (I / Is). Even when the gain in Fig. 2 is reduced to the maximum, the current of transistor Q2 is the same as that of transistor Q2.
The current flows as much as the base current of 4, Q5 is supplied (for example, 0.2 μA in FIG. 3). At this time, if the current of the transistor Q1 is 100 μA in FIG. 3, the difference Vc between the voltages Vf1 and Vf2 of both is small as shown in (a). If the current of the transistor Q2 does not flow, the difference becomes large as shown in (b).

Therefore, the attenuation amount ATT in this circuit diagram is expressed by the following formula: ATT = 20log (Vo / Vi) = 20log [2 / {1 + exp (qVc / kT)}] ...... Vc in this formula is small Therefore, the attenuation amount ATT cannot be made sufficiently large. Considering this example, the Vf difference between the transistors Q1 and Q2 is about 162 mV when calculated with kT / q = 26 mV, so ATT ≈ -48 dB, and also factors such as element variations such as base current and temperature variations are included. It will be.

The present invention has been made in view of such points,
It is an object of the present invention to provide a gain control circuit capable of obtaining sufficient attenuation with a simple structure.

Means for Solving the Problems In order to achieve the above object, the present invention provides a gain control in which the gain of the differential amplifier is controlled by varying the potential difference between the bases of a differential pair transistor that constitutes the differential amplifier. In the circuit, a unidirectional transistor having first and second transistors whose bases are connected to a common voltage source, and between the base and emitter of the second transistor, the diode portion of the base and emitter being connected in a reverse conductivity type. A conductive element, means for connecting the emitters of the first and second transistors to the bases of the differential pair transistors, first constant current source means connected to the emitter of the first transistor, and the second transistor Second constant current source means connected to the emitter of the second transistor, and variable current source means connected to the emitter of the second transistor. We are trying to be.

Action According to such a configuration, the current value of the variable current source 15 is I ₃ =
When 0, the current values of I ₁ and I ₂ are the same, and the _first and _second current values are the same.
Since it is Vf of the transistor of, the differential amplifier is in a balanced state. Next, when the current value of the variable current source 15 is increased, the gain decreases, and finally the unidirectional conductive element connected between the base and emitter of the second transistor becomes conductive. When the unidirectional conductive element conducts as described above, the first and second transistors are connected between the emitters of the first and second transistors.
A voltage difference occurs between Vf of the transistor Q1 and the voltage drop of the unidirectional conductive element, and the voltage difference is applied to the base of the transistor of the differential amplifier, so that the attenuation of the input signal passing through the differential amplifier becomes large. Now, if the unidirectional conductive element is a diode, there will be a gap between the emitters of the first and second transistors.
A voltage difference of 2Vf will occur.

Embodiments Embodiments of the present invention will be described below with reference to the drawings. In FIG. 1 in which the present invention is implemented, the same parts as those in the conventional example of FIG. In this embodiment, the circuit for supplying the base currents of the differential pair transistors Q3, Q4 and Q5, Q6 is the first and second transistors Q.
1, Q2, a diode D1 connected between the base and emitter of the second transistor Q2 in a conductivity type opposite to the diode portion of the base-emitter, and the emitters of the first and second transistors Q1 and Q2, respectively. A line connected to the bases of the pair of transistors Q3, Q4 and Q5, Q6 of the differential amplifier 1.
13, 14 and a variable current source 15 for supplying a current to the emitter side of the second transistor Q2, and transistors Q11, Q12 connected to the emitters of the first and second transistors Q1, Q2,
Resistors R1 and R2 connected between their emitters and ground
And a diode D2 for applying a constant bias to the transistors Q11 and Q12, a resistor R3, and a constant current source 12.

The gain of this differential amplifier is determined by the potential difference between points (a) and (b), that is, the difference between the emitter voltages of the first and second transistors Q1 and Q2 forming the emitter follower. Since the bases of these transistors Q1 and Q2 are held at the same potential, the difference in their emitter voltages is determined only by the ratio of the emitter currents. Here, the emitter current is determined by the constant current source transistors Q11 and Q12 and the variable current source 15 which are connected to the emitter. Since the transistors Q11 and Q12 are I ₁ = I ₂ if R1 = R2 the same shape, now, if it is I ₃ = 0, first, second transistor Q
The currents I ₆ and I ₇ output from 1 and Q2 are the same, and there is no potential difference between points (a) and (b).
And Q4 are in equilibrium, and transistors Q5 and Q6 are also in equilibrium. When I ₃ flows, the conductivities of the transistors Q4 and Q5 increase, while the conductivities of the transistors Q3 and Q6 are reduced, and the gain of the differential amplifier decreases, but I ₃ ≤ I ₂ + 2I _b , The diode D1 does not conduct, but when I ₃ > I ₂ + 2I _b , the diode D1 conducts and the potential difference between points (a) and (b) becomes 2Vf, and Vf = 0.7V Then, theoretically,
From the formula, a large attenuation of −461 dB can be obtained at room temperature. However, kT / q = 26 mV. The diode
There is also an advantage that the reverse bias of the transistor Q2 can be prevented by D1.

Although the diode D1 is used in this embodiment, it is needless to say that the diode D1 is not limited to the diode and may be another unidirectional conducting element.

EFFECTS OF THE INVENTION As described above, according to the present invention, there is an effect that a variation factor in the gain of the differential amplifier is extremely small and a large attenuation can be obtained with a simple configuration.

[Brief description of drawings]

FIG. 1 is a circuit diagram of a gain control circuit embodying the present invention. FIG. 2 is a circuit diagram of a conventional example, and FIG. 3 is an explanatory diagram thereof. 1 ... Differential amplifier, Q1 ... First transistor, Q2 ... Second transistor, D1 ... Diode (unidirectional conductive element), 13,14 ... Line, 15 ... Variable current source.

Claims (1)

[Claims] 1. A gain control circuit configured to control a gain of a differential amplifier by varying a potential difference between bases of a differential pair transistor constituting the differential amplifier, the base being connected to a common voltage source. A first and second transistor, a unidirectional conductive element connected between the base and emitter of the second transistor in a conductivity type opposite to that of the diode portion of the base and emitter, and the first and second transistors of the first and second transistors. Means for connecting respective emitters to the bases of the differential pair transistors, first constant current source means connected to the emitter of the first transistor, and second constant current source means connected to the emitter of the second transistor. And a variable current source means connected to the emitter of the second transistor, and a variable gain control circuit.