Since the different configurations only vary in their connections via the capacitors, and the capacitors are open circuits to dc voltages and currents, we can study the dc bias for the general case. This is sometimes called common drain (CD) and is analogous to the emitter follower configuration for the BJT.Įach of these configurations is studied in more detail in Section 9, “FET Amplifier analysis”. – Source Follower (SF) – the ac input is applied at C G, the ac output is taken at C S and the drain is either connected to a dc voltage supply directly or via C D. The CG is analogous to the common base configuration for the BJT, although it is seldom seen in circuits. Sometimes in the CG configuration, C G is omitted and the gate is connected directly to a dc voltage supply. – Common Gate (CG) – the ac input is applied at C S, the ac output is taken at C D and C G is connected to a dc voltage source or ground. This is analogous to the emitter-resistor configuration for the BJT. – Source Resistor (SR) – the ac input is applied at C G, the ac output is taken at C D and C S is omitted. This is analogous to the common-emitter configuration for the BJT. For depletion MOSFETs or JFETs, R 2 can either be finite or infinite, as shown in Figure 21(b).įigure 21 – Amplifier biasing configurationsĬommon Source (CS)– the ac input is applied at C G, the ac output is taken at C D, and C S is connected to a dc voltage source or ground. In the voltage division biasing, there will be an R 1 and R 2 in order to obtain the positive voltage. For an enhancement mode transistor, there will always be a need for a positive voltage at the gate. The gate-to-source voltage determines the type of circuit which may be required for that transistor configuration. 4.1 Discrete-Component MOSFET Biasingĭiscrete-component biasing for MOSFET amplifiers is accomplished with the circuits shown in Figure 21. The IC MOSFET amplifiers are usually biased using dc current sources that are analogous to those used for the BJT IC amplifiers. IC MOSFET amplifiers are generally direct coupled because large capacitors are not practical. Discrete component designs use the large coupling and bypass capacitors to isolate the dc bias for each amplifier stage, much like the discrete component BJT amplifiers. We can separate the approaches into those used for discrete component versus integrated circuit amplifiers. The approaches that are used for biasing of BJTs can also be used for biasing MOSFETS.
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