Audio Engineering

Audio Amplifi ers 299

arrangement is to allow a JFET amplifi er stage to operate at a much higher rail voltage
than would be allowable to the FET on its own; this layout is often found as the input stage
of high-quality audio amps.

A feature that is very characteristic of the JFET is that for drain potentials above about
3 V, the drain current for a given gate voltage is almost independent of the drain voltage,
as shown in Figure 9.16. BJTs have a high characteristic collector impedance, but their
Ic / Vc curve for a fi xed base voltage, also shown, for comparison, in Figure 9.16 , is not as
fl at as that of the JFET. The very high dynamic impedance of the JFET resulting from
this very fl at Id / Vd relationship encourages the use of these devices as constant current
sources, shown in Figure 9.17. In this form the JFET can be treated as a true two-terminal
device, from which the output current can be adjusted, with a suitable JFET, over the
range of several milliamperes down to a few microamperes by means of RV1.

9.8 Insulated Gate FETs (MOSFETs) .........................................................................

Insulated gate FET devices, usually called MOSFETs, are by far the most widely
available, and most widely used, of all the fi eld effect transistors. They normally have a

4

I (d

I) mAc

3

2

1

0

0 10203040

Vd (Vc)

BJT

0.5 V

1.0 V

1.5 V

2.5 V

2.0 V

Figure 9.16 : Drain current characteristics of junction FET.