For the cascade amplifier of Fig. 93, using JFETs with...

For the cascade amplifier of Fig. 93, using JFETs with specifications $I_{D S S}=12 \mathrm{~mA}$, $V_{P}=-3 \mathrm{~V}$, and $g_{a s}=25 \mu \mathrm{S}$, calculate the circuit input impedance $\left(Z_{i}\right)$ and output impedance $\left(Z_{\circ}\right)$.

Key Concepts

Cascade Amplifier Configuration

A cascade amplifier is built by connecting two or more amplifier stages so that the output of one stage feeds the input of the next. This configuration increases overall gain but requires careful consideration of how the input and output impedances of each stage interact, as they affect signal transfer and loading between stages.

Input Impedance

The input impedance of an amplifier is the equivalent resistance seen by the source at the input terminal. In JFET amplifiers, this is largely determined by the high impedance of the gate and any biasing network resistors connected to it. A high input impedance minimizes loading on the preceding circuit or signal source.

JFET Small-Signal Model

This concept involves representing a JFET with its essential parameters for AC analysis, including the transconductance (g?) that describes the control of the drain current by the gate voltage and the drain-source output resistance (r?), often related to the channel?length modulation parameter (g??) via r? = 1/g??. The model also highlights that the gate presents a very high input impedance due to its reverse-biased pn junction.

Output Impedance

The output impedance is the resistance seen by the load connected to the amplifier's output. It is influenced by the intrinsic properties of the JFET (notably the drain-source resistance) as well as any external circuit elements. In a cascade configuration, ensuring a low output impedance can be important to drive subsequent stages or loads effectively.

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