The ratio (Change in current at output/Change in voltage at input), with a constant supply voltage, is called the mutual conductance of the particular bipolar transistor or FET to which it applies. Its symbol is gm. The values of mutual conductance obtainable from bipolar transistors are much greater than are those from FETs. In the mutual conductance graph shown in Figure 34.1, for instance, a 30 mV input wave gives a 1 mA current ﬂ ow at the output. The mutual conductance, gm, is therefore 1/0.03 33.3 mA/V. Metal oxide semiconductor ﬁ eld-effect transistor (MOSFET) circuits are used for small-signal ampliﬁ ers mainly in integrated circuit (IC) form, although power MOSFETs are found in hi-ﬁ ampliﬁ ers. The examples here are mainly for BJT circuits.