A signal control circuit which includes an amplifying transistor connected to an input circuit to receive an input signal. A constant reference bias voltage is produced across a diode connected to the input circuit and signals are connected from the input circuit to the transistor by a resistor. A controllable impedance is connected at a point between the resistor and the transistor so that by controlling the impedance the current gain of the amplifier is linearly controlled over a wide range.
A common-emitter amplifier transistor has its gain controlled by change of its transconductance as a function of its quiescent emitter current. A first controlled current applied to a diode means develops a potential thereacross which is applied between the base and emitter electrodes of the amplifier transistor. This configuration causes the quiescent collector and emitter currents of the amplifier transistor to be in well-defined proportion to the first controlled current. This permits a second controlled current linearly related to the first to be used to counteract changes in the quiescent collector current of the common-base amplifier transistor, insofar as being coupled to the amplifier load.
The collector potential of a resistively loaded common base amplifier tends to change if its emitter current is altered to affect its emitter resistance and thereby control its gain. This tendency arises because of the change in transistor collector current, which is substantially the same value as the change in its emitter current, acting on the resistive load in accordance with Ohm's Law. A control current is applied to diode means to develop a potential thereacross, which is applied to the base emitter junction of the transistor to control its emitter current. This establishes a known proportion of control current to transistor emitter and collector currents. Changes in the transistor collector current are then compensated for by another current proportional to the control current applied at the transistor collector electrode, thereby to counteract the tendency for the transistor collector potential to change as the gain of the common-base transistor amplifier is changed.
A gain control circuit includes a first transistor with direct-coupled collector-to-base feedback converting current, applied to its collector, to logarithmically related voltage appearing between its emitter and base. This logarithmically related voltage is incremented or decremented by a voltage .DELTA.V.sub.1 proportional to T and applied to the emitter-base junction of a second transistor used as a logarithm of voltage to current converter, T being an absolute temperature close to that at which the first and second transistors are operated. .DELTA.V.sub.1 is developed as the potential drop across a resistive element R1, responsive to the output current of a current-to-logarithm-of-current converter circuit, making the conversion with a scaling factor proportional to T divided by the resistance R.sub.1 of resistive element R1. Accordingly, the gain setting of the gain control circuit is unaffected by change in T.
