Bias Circuit

Unit 10. Common-Source Amplifier with
Current-Source Load
The amplifier in this part is the same, in principle, as the basic common-source amplifiers of
Figs. 2.4 and 5.1. However, now the actual resistor RD is replaced with a current-source load
as shown in the circuit of Fig. 10.1. As noted in Unit 9, a benefit of replacing bias resistors with
current sources is a reduced requirement for resistors in the circuit; the reference voltage
associated with M3 can be used elsewhere in a typical circuit. Additionally, as will be shown,
the gain of the common-source stage is substantially improved over that with actual bias
resistor RD. The specific circuit of Fig. 10.1 is that for the project amplifier. The bias and gain
of the amplifier are evaluated in the following.
Figure 10.1. Common-source amplifier with driver transistor M1 and
current-source load from drain of M2. In the amplifier project, an
output channel sets VG1 for a given current and another output channel
will search for the VSS to set up bias VO = VDD/2. For the signal
measurements, a signal voltage is superimposed on the dc VG1.