The source follower amplifier provides lower output impedance and essentially unity gain. The circuit diagram is shown in Figure P2.19(a), and the small-signal model is shown in Figure P2.19(b). This circuit uses an FET and provides a gain of approximately unity.

Assume that R2 R1 for biasing purposes and that Rg R2.

(a) Solve for the amplifier gain.

(b) Solve for the gain when gm = 1000 μΩ and Rs = 25 kΩ

where Rs = R1 + R2.

(c) Sketch a block diagram that represents the circuit equations.

Vin(t) Vin (1) + (a) VDD G S www 0+ RG R Vo(t) R2 8mVgs iin G Vgs S 0+ RG R Vo(t) R FIGURE P2.19 (b) The source follower or common drain amplifier using an FET.