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.


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