296 Chapter 9
characteristic, which is, in the case of an N-channel JFET, very similar to that of a triode
valve, as shown in curve ‘ c ’ of Figure 9.3. Like a thermionic valve, the operation of the
device is limited to the range between drain (or anode) current cutoff and gate (or grid)
current. In the case of the JFET, this is because the gate-channel junction is effectively
a silicon junction diode—normally operated under reverse bias conditions. If the gate
source voltage exceeds some 0.6 V in the forward direction, it will conduct, which will
prevent gate voltage control of the channel current.
P-channel JFETs are also made, although in a more limited range of types, and these have
what is virtually a mirror image of the characteristics of their N-channel equivalents,
although in this case the gate-source forward conduction voltage will be of the order
of 0.6 V, and drain current cutoff will occur in the gate voltage range of 3 to 8 V.
Although Sony did introduce a range of junction FETs for power applications, these are
no longer available, and typical contemporary JFETs cover the voltage range (maximum)
from 15 to 50 V, mainly limited by the gate-drain reverse breakdown potential, and with
permitted dissipations in the range 250–400 mW. Typical values of gm (usually called gfs
in the case of JFETs) fall in the range of 2–6 mS.
JFETs mainly have good high-frequency characteristics, particularly the N-channel types,
of which there are some designs capable of use up to 500 MHz. Modern types can also
offer very low noise characteristics, although their very high input impedance will lead to
high values of thermal noise if their input circuitry is also of high impedance; however,
this is within the control of the circuit designer. The internal noise resistance of a JFET,
R (n), is related to the gfs of the device by
Rg(n) ohms≈067./ fsand the value of gfs can be made higher by paralleling a number of channels within the
chip. The Hitachi 2SK389 dual matched-pair JFET achieves a gfs value of 20 mS by this
technique, with an equivalent channel thermal noise resistance of 33 ohms.^2
Although JFETs will work in most of the circuit layouts shown for junction transistors,
the most signifi cant difference in the circuit structure is due to their different biasing
needs. In the case of a depletion mode device it is possible to use a simple source bias
arrangement, similar to the cathode bias used with an indirectly heated valve, of the kind
shown in Figure 9.13. As before, the source resistor, R 3 , will need to be bypassed with
a capacitor, C 2 , if the loss of stage gain due to local NFB is to be avoided. As with a