Voltage Feedback

3.2.1.3. Summary

The common-source circuit can be operated without feedback, with current feedback or with voltage feedback. Figure 3.9 shows the three variations ofcircuits. The common-source circuit with voltage feedback is rarely used, since it is not possible to make use of the high input resistance of the FET.

In practice, the common-source circuit without feedback and the common-source cir­cuit with current feedback are used only if a high input resistance or a low noise figure with high-resistance sources is required. In all other cases, the common-emitter circuit is supe­rior due to the larger maximum gain, the much higher transconductance of the bipolar tran­sistor for the same operating current and the lower noise figure for low-resistance sources.

An important role is played by the common-source circuit in integrated CMOS circuits, because here no bipolar transistors are available.

Fig. 3.9. Variations of the common-sourse circuit

3.2.2. Common-Drain Circuit

Figure 3.10 shows the common-drain circuit, which consists of the MOSFET, the source resistance R_S, the supply voltage source V_b and the signal voltage source V_g with the internal resistance R_g.

Дата добавления: 2014-01-11