Discrete-circuit MOSFET amplifier

The discrete-circuit MOSFET amplifier is the standard board-level realisation of a Common-source amplifier: a voltage-divider-biased MOSFET wrapped in the resistors and capacitors needed to make it bias reliably and amplify AC signals. It is where biasing, gain, and the eventual Amplifier frequency response all come together.

Voltage-divider bias, source resistor, coupling caps, optional source bypass.

The components and what each one does

• Voltage-divider gate bias ($R_{G1},R_{G2}$): sets the fixed gate reference $V_G$. Because the gate draws no current (Gate oxide), these can be megohms, keeping input resistance high. See Voltage-divider bias.
• Source resistor $R_S$: provides DC Negative feedback so the operating point is set by resistors, not the spread-prone $V_t$/$k_n$ ($I_D\approx V_G/R_S$). The MOSFET biasing backbone.
• Drain resistor $R_D$: converts the signal drain current into the output voltage swing. The gain is $-g_m{R}_D$ (see Common-source amplifier); $R_D$ also sets how far the output can swing.
• Input coupling capacitor $C_{C1}$ (signal source → gate): blocks DC, passes AC. It stops the source’s DC level from disturbing the carefully set $V_G$ while letting the AC signal through. See Coupling capacitor.
• Output coupling capacitor $C_{C2}$ (drain → load): blocks the drain’s DC bias from reaching the load, passes the amplified AC.
• Source bypass capacitor $C_S$ (across $R_S$, optional): shorts $R_S$ for AC so the full gain $-g_m{R}_D$ is recovered, while leaving $R_S$ in place for DC so the bias stabilisation survives. See Bypass capacitor. Without it the stage has Source degeneration (lower but more robust gain); with it, full gain.

Why this is the canonical circuit

It cleanly separates two jobs that conflict. DC: you want $R_S$ in the source for a stable operating point. AC: you want $R_S$ gone, because it kills gain. The bypass capacitor resolves the conflict — DC-open (keeps $R_S$), AC-short (removes it). The coupling capacitors similarly isolate the DC bias of each stage from its neighbours so each can be biased independently. Together these reactive elements introduce frequency dependence: at low frequencies the coupling and bypass caps stop being short circuits and the gain falls off. That is precisely the low-frequency end of the Amplifier frequency response, and sizing these capacitors so their corners fall well below the band of interest (e.g. ≪ 20 Hz for audio) is the next design step.