This invention relates to cascode amplifiers, and in particular what is termed herein an extended cascode amplifier.
Some early types (Circa 1961) of silicon transistors, e.g. General Electric type 4JX 10B513 NPN, exhibited, in common emitter configuration, unacceptably high saturation voltage (6 V at 0.5 A Ic). This was due to the use of base semiconductor material having a resistivity that was chosen to be much too high in aid of the primary objective of making transistors that would be much more thermally stable than previously used germanium transistors in contemporary (circa 1961) design circuits (common emitter).
The use of the common base configuration was the only obvious expedient to overcome the saturation voltage. In this mode of operation the swing of collector voltage along a power load line is able to reach all the way down to zero volts V.sub.c in contrast to saturation near five or six volts in the common emitter configuration. However, common base operation involves very low input impedance to the common base stage as well as an absence of current gain in the output stage.
The present invention enables the current gain to be augmented in the driver stage of a cascode by a step-down transformer, while preserving essentially the voltages at the input collector, the input stage base and the output collector substantially the same magnitudes they would have in a conventional cascode circuit, i.e. one having an interstage transformer with a turns ratio not substantially different from one-to-one.