JPH0354887B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION (Technical Field of the Invention) The present invention relates to a gain control amplifier circuit for high frequencies.

(Prior Art) A gain control amplifier circuit that has been commonly used in the past will be explained with reference to FIG.

The bases of transistors 5 and 8 are connected to input terminals 3 and 4.
From each collector to the high potential end 1 via resistors 10 and 12, and from each emitter to resistor 11,
13 to the low potential end 2, and the emitter of a bidirectional transistor 6 between each emitter was connected to the high potential end 1 from the base through a variable current source 9. In the gain control amplifier circuit, input terminal 3,
The input signal inputted from 4 is transmitted through resistors 11 and 13.
(resistance values are R ₁ and R ₂ respectively) and the saturation resistance (resistance value R _S ) of the bidirectional transistor 6 and the load resistance 12 (resistance value R _L ). Determined gain A _V ≒ R _L /
The signal is amplified by a factor of (R ₂ R _S /R ₁ +R ₂ +R _S ) and outputted from the output terminal 7 and the inverting output terminal 14. The saturation resistance R _S of the bidirectional transistor 6 can be easily changed by changing the variable current source 9, and the gain of the gain control amplifier circuit can be controlled.

The high frequency characteristics of such a gain control amplifier circuit are
It was determined based on the device characteristics of transistors 5 and 8. Therefore, in order to improve high-frequency characteristics from a circuit perspective, some devices use peaking to widen the band. The gain control amplifier circuit shown in FIG. 2 is a circuit that uses this peaking to improve broadband characteristics.

A capacitor 100 connected in parallel with the bidirectional transistor 6 is added to the gain control amplifier circuit of FIG. Saturation resistance R _S of bidirectional transistor 6
By the time constant of the capacitor 100 and the capacitor 100, an output signal having a conking characteristic can be obtained. However, the saturation resistance of the bidirectional transistor 6
Since R _S changes with the current supplied from the variable current source 9, the time constant with the capacitor 100 changes during gain control, causing a change in the peaking level, and as a result, it functions as a gain control amplifier circuit. There was a serious drawback that there was no.

(Object of the Invention) An object of the present invention is to obtain a gain control amplifier circuit that exhibits stable high frequency characteristics even when the gain is changed.

(Structure of the Invention) According to the present invention, the bases of the first and second transistors are used as input terminals, the collectors are used as output terminals, and a parallel circuit of a resistor and a constant current source is connected to a low potential terminal at each emitter. The emitters of the bidirectional transistors are connected between the respective emitters, and the bases and collectors of these bidirectional transistors are connected to each other to connect a variable current source.

On top of that, the constant current source is set to produce an emitter current that makes the emitter diffusion capacitance of the bidirectional transistor the required value, and the variable current source is set to produce a current that is twice the current value of the constant current source and a gain control component. It is controlled to supply the sum of the current and the current.

(Example) Next, the present invention will be described in more detail with reference to FIG. Note that the same parts as those in the gain control amplifier circuits in FIGS. 1 and 2 are given the same numbers. In FIG. 3, constant current sources 101 and 102 are connected between the emitters of transistors 5 and 8 shown in FIG. Connect to each emitter of 5 and 8. The base and collector of the bidirectional transistor 6 are connected to each other and to a variable current source 9.

That is, the emitter diffusion capacitance of each transistor forming the bidirectional transistor 6 is made to function similarly to the capacitor 100 in FIG. 2. The emitter diffusion capacitance (C _DE ) of each transistor in the bidirectional transistor 6 is C _DE = q/kT・I _E /2D _P W ² [F] W: Effective base width I _E : Emitter current; Therefore, the emitter diffusion capacitance _CDE can be determined. The emitter current that determines this emitter diffusion capacitance C _DE is controlled by constant current sources 101 and 102. Further, the variable current source 9 for gain control includes constant current sources 101 and 102.
The sum of the current twice the current value and the current for gain control is supplied. Also, in order to have the same capacitance value (C ₁₀₀ ) as the capacitor 100 in Figure 2, C ₁₀₀
Emitter diffusion capacitance C _DE (or I _E ) so that ≒2C _DE
Set.

The saturation resistance R _S of the bidirectional transistor 6 decreases as the supply current of _the variable current source 9 increases, but the emitter diffusion capacitance C _DE increases as the supply current increases. The time constant of the time constant circuit composed of _DE remains constant even if the current of the variable current source 9 is changed for gain control.

Therefore, stable peaking can be obtained at the output terminal 7 and the inverting output terminal 14 even with wide gain changes, and a gain control amplifier circuit that is more stable and has a wider band than conventional circuits can be achieved.

The present invention is a circuit particularly useful in integrated circuits as well as discrete circuits.

[Brief explanation of drawings]

FIGS. 1 and 2 are circuit diagrams showing conventionally used gain control amplifier circuits. FIG. 3 is a circuit diagram showing a gain control amplifier circuit according to an embodiment of the present invention. 1...High potential end, 2...Low potential end, 3, 4...
Input terminal, 5, 8...Transistor, 6...Bidirectional transistor, 7...Output terminal, 9...Variable current source, 10, 11, 12, 13...Resistor, 100...
...Capacitor, 101,102... Constant current source.

Claims (1)

[Claims] 1 first and second transistors each having a signal input terminal connected to each base, a signal output terminal connected to each collector, and whose collectors are connected to a high potential end via a resistor; A parallel circuit of a resistor and a constant current source each connected between each emitter of a second transistor and a low potential end, and a parallel circuit each having an emitter connected to each emitter of the first and second transistors. and a variable current source connected to each interconnected base and collector of the bidirectional transistor, the constant current source being configured to provide a desired value for the emitter diffusion capacitance of the bidirectional transistor. The amplifier circuit is set to a current value, and the variable current source is controlled to supply a sum of a current twice the current value of the constant current source and a current for gain control.