A gain control circuit having a pair of first and second NPN-type transistors whose emitters are grounded and whose collectors are connected with each other and through a constant current source to a DC source. A third NPN-type transistor has its collector connected to the DC source, its emitter connected to the bases of the first and second transistors through separate resistors and also grounded through a resistor connected in parallel with a capacitor, and its base connected to the collector of the first transistor. A PNP-type transistor has its emitter connected to the base of the first transistor and its collector grounded. An input signal is applied to the base of the first transistor and a control signal is applied to the base of the PNP-type transistor.
A D.C. gain controlled differential amplifier exhibiting balanced operation and a linear gain control response. The amplifier includes input and output emitter coupled transistors and a current source transistor. Input signals are supplied to the base of the input transistor, and a negative feedback network is D.C. coupled from the collector to base of the output transistor. A D.C. gain control voltage varies the conduction of the current source transistor to thereby vary the gain of the emitter coupled transistors. The control voltage is also D.C. coupled to a collector signal output of the output transistor to maintain the output quiescent level substantially constant as the amplifier is gain controlled.
A signal to be controlled is applied to the base of an emitter-follower input transistor, the emitter of which is connected to an input terminal of a differential amplifier. This input terminal of the amplifier is connected by a diode to a controllable current source so that the base current of the differential amplifier input transistor is controlled. A constant current source is connected to the common emitter circuit of the differential amplifier, and the gain of the circuit is determined by the ratio of current in the latter source to current in the controllable source. A current conducting circuit, such as, an emitter load resistor, is connected to the emitter of the input transistor to hold the output impedance thereof at a substantially constant low value inspite of changes in the current of the controllable current source, Maximum gain of the circuit may be determined by separating the controllable current source into a controllable portion and a fixed portion in parallel so that the minimum current is at least as great as the current of the fixed portion. This limits the maximum circuit gain by limiting the maximum ratio of fixed current to controllable current. A fixed voltage source connected to the common circuit point between the controllable current source and the diode assures that the gain of the circuit will not fall below a selected minimum value.
In an amplifier stage a first FET receives the input signal at its gate electrode. The drain of the first FET is connected to the drain of a second FET, and both drains are connected to the same drain voltage supply. The amplifier stage output is from the drain of the second FET. A third FET has its drain connected to the drain voltage supply and its source connected to the reference voltage supply. The physical characteristics of the third transistor are selected to produce a voltage of a fixed value that is applied as a bias to the source of the second transistor to compensate for an offset of the amplifier stage input signal voltage. A fourth FET has its drain connected to the drain voltage supply and its source connected to the drain of the third FET. The drain of the fourth FET is connected to the gate of the second FET. The physical characteristics of the fourth FET and the current flowing through it determine the gain of the amplifier stage. The third and fourth FETs are not in the signal path. Several stages can be connected in cascade with only one pair of said third and fourth transistors connected to the source and gate of the second transistor of each stage.
In an amplifying circuit having a plurality of amplifier stages which are DC-coupled, and in which an AGC voltage is applied to one of the earlier amplifier stages for providing an AGC operation; changes in the DC level of a later amplifier stage resulting from the AGC operation are detected, and such detected changes in the DC level are cancelled at the DC-coupling between the earlier and later amplifier stages.
The disclosed amplifier has a simple structure which is particularly suited for integrated circuit applications. The amplifier includes a differential amplifier having first and second input terminals across which an input signal is developed. Diodes are connected from each of the input terminals to a circuit node. A reference signal is coupled to the input terminals of the differential amplifier and hence to the diodes. A gain control transistor is connected between the circuit node and ground. The conductance of the gain control transistor varies in response to a gain control signal to thereby control the conductance of the diodes. As the conductances of the diodes increase, less of the input signal voltage is developed across the diodes and hence the gain is decreased and vice versa. The d.c. output voltage remains constant even though the gain is changed.
