Audio Engineering

Audio Amplifi ers 307

Although circuit designers tend to be rather lazy about using the proper symbols for the
components in the designs they have drawn, enhancement-mode and depletion-mode
MOSFETs should be differentiated in their symbol layout, as shown in Figure 9.23. As a
personal idiosyncrasy, I also prefer to invert the symbol for P-channel fi eld effect devices,
as shown, to make this polarity distinction more obvious.

9.11 Useful Circuit Components ...................................................................................

By comparison with the situation that existed at the time when most of the pioneering
work was done on valve-operated audio amplifi ers, the design of solid-state amplifi er
systems has been facilitated greatly by the existence of a number of circuit artifi ces,
contrived with solid-state components, which perform useful functions in the design.
This section shows a selection of the more common ones.

9.11.1 Constant Current Sources

A simple two-terminal constant current (CC) source is shown in Figure 9.17 , and devices
of this kind are made as single ICs with specifi ed output currents. By comparison with
the discrete JFET/resistor version, the IC will usually have a higher dynamic impedance
and a rather higher maximum working voltage. In power amplifi er circuits it is more
common to use discrete component CC sources based on BJTs, as these are generally less
expensive than JFETs and provide higher working voltages. The most obvious of these

N-channel enhancement

MOSFET

P-channel enhancement

MOSFET

N-ch JFET P-ch JFET N-ch depletion

MOSFET

P-ch depletion

MOSFET

Figure 9.23 : MOSFET symbols.