Single-transistor amplifier configurations

Once you have a small-signal model of a three-terminal transistor, there are exactly three useful single-transistor amplifier topologies, distinguished by which terminal is held common (AC-grounded) to both input and output. Each has a characteristic combination of voltage gain, input resistance, and output resistance, and the choice of topology is the design decision. The MOSFET and BJT versions are direct analogues; they differ in one detail traceable to a single physical fact.

The three MOSFET configurations: common-source, common-gate, common-drain.

The three patterns

Common-source / common-emitter — input at gate/base, output at drain/collector, source/emitter common. High voltage gain, inverting ($180^{\circ }$), moderate input and output resistance. This is the default voltage amplifier; reach for it first. See Common-source amplifier and Common-emitter amplifier. Gain $\approx -g_m(R_D\Vert r_o)$ for the MOSFET.

Common-gate / common-base — input at source/emitter, output at drain/collector, gate/base common. Voltage gain similar in magnitude to CS/CE but non-inverting; low input resistance; high output resistance. It does not give you more gain — its value is the low input resistance (it acts as a current buffer / current follower: current in at the source comes out at the drain almost unchanged) and excellent high-frequency behaviour (it sidesteps the Miller effect). Used at high frequency and as a current buffer. See Common-gate amplifier and Common-base amplifier.

Common-drain / common-collector — input at gate/base, output at source/emitter, drain/collector common. These are the Source follower and Emitter follower. Voltage gain $\approx 1$ (just under), high input resistance, low output resistance. No amplification, but it transforms impedance — the classic Buffer amplifier that goes at the output of a chain to drive a heavy load without the gain stage sagging.

The three BJT configurations: common-emitter, common-base, common-collector — direct counterparts.

Why MOSFET and BJT versions differ at all

Every difference between the MOSFET and BJT families traces to one fact: the MOSFET gate draws no DC current (the gate is insulated by the Gate oxide; $G$ is an open circuit at DC), whereas the BJT base draws $i_B=i_C/\beta$. That single difference makes the BJT’s small-signal input resistance finite ($r_{\pi }$, looking into the base) while the MOSFET’s is effectively infinite (looking into the gate). Everything else — bias-network design, the exact input-resistance formulas, why BJT stages load preceding stages and MOSFET stages do not — is a downstream consequence of this. Learn the three patterns once and the device just sets whether $r_{\pi }$ is finite or infinite. See Input and output resistance (amplifier) for why $R_{in}$ and $R_{out}$ are the quantities that decide which topology a given job needs.