The most convenient small-signal model for a MOSFET, like that of a let, incorporates the trans conductance of the device. Recall that the characteristics of a depletion-type MOSFET are quite similar to those of a JFET, the only difference being that the depletion MOSFET can be operated in both the depletion and enhancement modes. As we might therefore expect, the small-signal model for the
depletion MOSFET is identical to the JFET model, and it is used in the same way to analyze a depletion-type MOSFET amplifier. The value of the trans conductance can be found graphically and algebraically in the same way it is found for a JFET. The trans conductance of the enhancement-type MOSFET can be found graphically using the transfer characteristic and the definition:
The JFET Chopper
A chopper is an analog switch that is turned on and off at a rapid rate by a periodic sequence of pulses, such as a square wave. It is used to convert a slowly varying signal into a series of pulses whose amplitudes vary -slowly in the same way as the signal. Figure 8-30 illustrates the concept. A chopper is an example of a modulator, in this case, a pulse-amplitude modulator. Figure 8-31 shows a JFET connected as a chopper. In this variation, the analog switch is in series with the load resistor, RI across which the chopped waveform is developed. When the switch is closed (JFET on), current flows from the analog signal source and into RL When the switch is open (JFET cut off), no current flows and the output voltage is O. Applying the voltage-divider rule to the circuit of Figure 8-31 when the JFET is on, we find If Rl. is much greater than RDION) + r.. then v/.. is approximately the same as Vd. Thus, the amplitude of the output (pulse) follows the analog input during each interval when the JFET is conducting.Figure 8-32 shows how gm is computed as (he slope of a line drawn tangent to the characteristic at the operating or Q-point. It is.c1ear from the figure that the slope of the characteristic, and hence the value of s-. changes as the Q-point is changed. Therefore small-signal analysis requires that t’ie signal variation around the Qpoint be confined to a limited range over which there is negligible change in gm, i.e., to an essentially linear segment of the characteristic. Also, to ensure that the device is operated within its active region, the variation must be such that the owing inequality is always satisfied:Figure 8-33 shows a common-source enhancement MOSFET amplifier and its small-signal equivalent circuit. The MOSFET is biased using the voltage-divider method discussed in Chapter 7 (Figure 7-40). Notice that the small-signal equivalent circuit of the NMOS amplifier is identical to that of the JFET amplifier (Figure 8-1 L(b». Consequently, all gain and impedance relations are the same as those derived for the JFET amplifier; Like Feisty, MOSFETs can be operated in common-drain and common-gate configurations. Since the JFET and MOSFET small-signal models me identical, the gain and impedance equations are also identical.
