Constant-resistance AGC attenuator for transistor amplifiers

IT IS well known that the variation of the transistor parameters with the dc bias conditions may be employed to control the gain of a transistor amplifier stage. There are, however, many applications that require a gain that is variable in accordance with an external voltage, with the added restraint that the input and output impedances shall be maintained approximately constant. One of these applications is the broad-band IF amplifier. The design of wide-band transistor amplifiers usually requires an effective load resistance of small value compared to the output resistance of the transistor. If this type of stage is operated at a fixed collector supply voltage and is gain-controlled by variation of the emitter current, then to a first approximation the output resistance is negligibly large