Amplifiers using bipolar and unipolar transistors with limited drain-source voltage

An important class of amplifiers uses a field-effect device in the first stage and a bipolar transistor in the second. If the drain-source voltage of the former is limited to the base-emitter voltage of the latter, a particularly simple circuit arrangement results. Other advantages of operation in this mode include reduction in the power dissipated in the field-effect device, improvement in high-frequency response and, when circuit constants are suitably chosen, some degree of inherent compensation for change of working conditions due to thermal effects. Of the four possible configurations which give high input impedance and voltage gain, it is shown that only two can easily be stabilised against working-point changes, and of these one [in which a common-drain (c.d.) amplifier precedes a common-emitter (c.e.) stage] is preferred, because it exhibits higher gain and lower output impedance and needs only a single polarity of supply voltage. The transfer conductance, input and output impedances and thermal drift of the c.d.-c.e. amplifier are calculated, using an equivalent circuit in which the drain-source channel is represented by a variable resistor.