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 current-source 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 common-source stage. M12 provides a current-source load for the common-source 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.
Equation 14.8
The overall amplifier gain, av = Vo/Vi, is now
Equation 14.9
This can be evaluated with
Equation 14.10
Using the numbers from the previous calculation for the amplifier of Fig. 13.1 with RD3, and adding λn = λp, the gain magnitude for the circuit of Fig. 14.2 is 289. This compares with the value of 116 for the cascade amplifier of Fig. 13.1. In the next unit, the differential stage will be modified to include a current source load with a considerable additional improvement in gain.