Bias Circuit

7.2 Source-Follower Voltage Transfer Relation
A signal circuit is shown in Fig. 7.2, where the dc supplies have been replaced with ground.
Also, the internal resistance, rds = 1/gds, of the transistor has been included in the signal
circuit. Note that it is in parallel with RS since the drain is tied to signal ground. Assume for
now that source and body are connected as in Fig. 7.1.
Figure 7.2. Signal circuit of the source follower. Body effect is absent
since the source and body are assumed connected.
Regardless of the terminal configuration, for infinite output resistance and with the body and
source connected, the relationship between the drain current and gate – source control voltage
of the transistor is (4.1) with gds = gmb = 0, which is [(2.5)]
In the source-follower circuit, the output resistance of the transistor can be included in the
source-follower load and (2.5) effectively applies.
The transconductance for the circuit configuration with a source resistor was shown in Unit 4 to
be [(4.8)]
where, for this case, is the source-follower load, which includes the transistor output
resistance and is
Equation 7.3
Then using
Equation 7.4