7

RF small-signal circuitry

The basic circuit arrangements for a single transistor amplifier stage were described in the last chapter, but there are many practical points of circuit design and these are illustrated below, starting with the common base (common gate) circuit. The low-frequency small-signal input impedance of a grounded base transistor is resistive and equal to 25/Ie in ohms, where Ie is in milliamperes. The reciprocal of this gives the mutual conductance gm, i.e. 40 mA per volt at 1 mA, and pro rata at other collector currents. So, for example, taking a collector current of say 2 mA, gives a grounded base input resistance of 12R5, and this may be taken as a starting point for circuit design even at higher frequencies, in the absence ...

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