Audio Engineering

Audio Amplifi ers 297

pentode valve, which the junction FET greatly resembles in its operational characteristics,
the simplest way of calculating stage gain is by the relationship:

A.≈gRfs L

The device manufacturers will frequently modify the structure of the JFET to linearize its
Vg / Id characteristics, but, in an ideal device, these will have a square-law relationship, as
defi ned by

gIVIfsd///.g ≈≈dss⎡ (^1) ()VVgs gc Vg
⎣⎢

⎤

⎦⎥

For a typical JFET operating at 2 mA drain current, the gfs value will be of the order of
1–4 mS, which would give a stage gain of up to 40 if R 2 , in Figure 9.13 , is 10 k Ω. This is
very much lower than would be given by a BJT and is the main reason why they are not
often used as voltage-amplifying devices in audio systems unless their very high input
impedance (typical values are of the order of 10^12 Ω ) or their high, and largely constant,
drain impedance characteristics are advantageous.

The real value of the JFET emerges in its use with other devices, such as the bipolar/FET
cascode shown in Figure 9.14 or the FET/FET cascode layout of Figure 9.15. In the fi rst of
these, use of the JFET in the cascode connection confers the very high output impedance
of the JFET and the high degree of output/input isolation characteristic of the cascode

0V 0V

Output

R 2

R 3

C (^1) Q 1
R 1
C 2
Input
Vcc
Figure 9.13 : Simple JFET biasing system.