Bias Circuit

14.3 Current-Source Load for the Common-Source
Stage
We now add to the differential stage the common-source stage to obtain a two-stage amplifier
as in Unit 13 (Fig. 14.2). Transistor M12, which replaces bias resistor RD3, provides a currentsource
load as in the circuit of Fig. 10.1. Note that current ID12 mirrors ID10 and no additional
resistors are required with the addition of the common-source stage. That is, M11 and M12 both
use the reference voltage provided by the diode-connected M10.
Figure 14.2. Cascade of the differential amplifier stage and commonsource
stage. M12 provides a current-source load for the commonsource
stage. This circuit remains inadequate in terms of dc bias
stability of ID3. This is improved in the modification that follows.
The bias design for the common-source stage consists of picking W12 to obtain a specified ID12
relative to ID10 and making ID3 = ID12. Parameter W12 is determined from
Equation 14.6
The approximation is sufficient, as the current magnitude, again, is not critical.
The circuit is adjusted to make ID3 = ID12 by equating the relations for the two currents. This is
Equation 14.7
A much simpler equation replaces this rather complicated one when RD2 is finally replaced by a
transistor as well. Veff12 is known from Veff12 = Veff10. Thus, the current balance can be obtained
by a selection of the various remaining parameters.
The load on the common-source stage is now rds12 = 1/gds12. The gain is, including the output
resistance of M3.