Audio Engineering

306 Chapter 9

each of which will contribute its own element of capacitance, it is understandable that
these low channel resistance types will have a larger input capacitance. Also, in general,
P-channel devices will have a somewhat larger input capacitance than an N-channel one.
The drain/gate capacitance—a factor that is very important if the MOSFET is used as a
voltage amplifi er—is usually in the range of 50–250 pF. The turn-on and turn-off times
are about the same (in the range 30–100 nS) for both N-channel and P-channel types,
mainly determined by the ease of applying or removing a charge from the gate electrode.
If gate-stopper resistors are used—helpful in avoiding UHF parasitic oscillation and
avoiding latch-up in audio amplifi er output source followers—these will form a simple
low-pass fi lter in conjunction with the device input capacitance and will slow down the
operation of the MOSFET.

PP

N N N

N

N N

Drain

T or D MOS

Source

SiO 2

Gate

PP

N N

N

N N

Drain

V or U MOS

Source Gate

SiO 2

Figure 9.22 : MOSFET design styles.