3.
2 FET biasing
Unlike BJTs, thermal runaway does not occur with FETs, as already discussed in our blog.
However, the wide differences in maximum and minimum transfer characteristics make I
D
levels unpredictable with simple fixed-gate bias voltage. To obtain reasonable limits on
quiescent drain currents I
D
and drain-source voltage V
DS
, source resistor and potential divider
bias techniques must be used. With few exceptions, MOSFET bias circuits are similar to those
used for JFETs. Various FET biasing circuits are discussed below:
Fixed Bias.
Fixed bias-FET
DC bias of a FET device needs setting of gate-source voltage V
GS
to give desired drain current I
D
. For a JFET drain current is limited by the saturation current I
DS
. Since the FET has such a high
input impedance that no gate current flows and the dc voltage of the gate set by a voltage divider
or a fixed battery voltage is not affected or loaded by the FET.
Fixed dc bias is obtained using a battery V
QG
. This battery ensures that the gate is always
negative with respect to source and no current flows through resistor R
G
and gate terminal that is
I
G
=0. The battery provides a voltage V
GS
to bias the N-channel JFET, but no resulting current is
drawn from the battery V
GG
. Resistor R
G
is included to allow any ac signal applied through
capacitor C to develop across R
G
. While any ac signal will develop across R
G
, the dc voltage
drop across R
G
is equal to I
G
R
G
i.e. 0 volt.
The gate-source voltage V
GS
is then
V
GS
= - v
G
v
s
= v
GG
0 = V
GG
The drain -source current I
D
is then fixed by the gate-source voltage as determined by equation.
This current then causes a voltage drop across the drain resistor R
D
and is given as V
RD
= I
D
R
D and output voltage, Vout = VDD ID RD
Self-Bias.
FET-Self Bias circuit
This is the most common method for biasing a JFET. Self-bias circuit for N-channel JFET is
shown in figure.
Since no gate current flows through the reverse-biased gate-source, the gate current I
G
= 0 and,
therefore,v
G
= i
G
R
G
= 0
With a drain current I
D
the voltage at the S is
V
s
= I
D
R
s
The gate-source voltage is then
V
Gs
= V
G
- V
s
= 0 I
D
R
s
= I
D
R
s
So voltage drop across resistance R
s
provides the biasing voltage V
Gg
and no external source is
required for biasing and this is the reason that it is called self-biasing.
The operating point (that is zero signal I
D
and V
DS
) can easily be determined from equation and
equation given below :
V
DS =
V
DD
I
D
(R
D +
R
S
)
Thus dc conditions of JFET amplifier are fully specified. Self biasing of a JFET stabilizes its
quiescent operating point against any change in its parameters like transconductance. Let the
given JFET be replaced by another JFET having the double conductance then drain current will
also try to be double but since any increase in voltage drop across R
s
, therefore, gate-source
voltage, V
GS
becomes more negative and thus increase in drain current is reduced.
Potential-Divider Biasing.
fet-potential-divider-biasing
A slightly modified form of dc bias is provided by the circuit shown in figure. The resistors R
Gl
and R
G2
form a potential divider across drain supply V
DD
. The voltage V
2
across R
G2
provides
the necessary bias. The additional gate resistor R
Gl
from gate to supply voltage facilitates in
larger adjustment of the dc bias point and permits use of larger valued R
S
.
The gate is reverse biased so that I
G
= 0 and gate voltage
V
G
=V
2
=
(V
DD
/R
G1 +
R
G2
) *R
G2
And
V
GS
= v
G
v
s
= V
G
- I
D
R
s
The circuit is so designed that ID Rs is greater than VG so that VGS is negative. This provides correct bias voltage.
The operating point can be determined as
I
D
= (V
2
V
GS
)/ R
S
And
V
DS
= V
DD
I
D
(R
D
+ R
S
)
