Preamplifi ers and Input Signals 1937.10.2 Field Effect Devices
Other devices that may be used as amplifying components are fi eld effect transistors
and MOS devices. Both of these components are very much more linear in their transfer
characteristics but have a very much lower mutual conductance ( Gm ).
This is a measure of the rate of change of output current as a function of an applied
change in input voltage. For all bipolar devices, this is strongly dependent on collector
current and is, for a small signal silicon transistor, typically of the order of 45 mA/V per
mA collector current. Power transistors, operating at relatively high collector currents,
for which a similar relationship applies, may therefore offer mutual conductances in the
range of amperes/volt.
Because the output impedance of an emitter follower is approximately 1/ Gm , power
output transistors used in this confi guration can offer very low values of output
impedance, even without externally applied negative feedback.
All fi eld effect devices have very much lower values for Gm , which will lie, for small-
signal components, in the range 2–10 mA/V, not signifi cantly affected by drain currents.
This means that amplifi er stages employing fi eld-effect transistors, although much more
linear, offer much lower stage gains, with other things being equal.
The transfer characteristics of junction (bipolar) FETs, and enhancement and depletion
mode MOSFETS are shown in Figure 7.25.
dB Output signal voltage3dBFrequencyfoFigure 7.24 : Infl uence of circuit stray capacitances on stage gain.