Amplifier Gain

How to use this calculator

Pick the amplifier configuration (common-emitter/base/collector for BJT, common-source/drain for MOSFET) and the device. Enter the bias current (which sets the transconductance gm) and the resistors. The calculator computes the midband voltage gain including emitter/source degeneration: Av = −gm(RC‖RL)/(1 + gm·RE) for CE/CS, Av = gm(RC‖RL) for CB, and Av ≈ gm·RE/(1 + gm·RE) (≈1) for CC. For a BJT, gm ≈ IC/VT (VT ≈ 25.85 mV); for a MOSFET, gm = 2·ID/(VGS − VT).

What it means in a real circuit

A common-emitter stage with IC = 1 mA (gm ≈ 38.7 mA/V), RC = 2 kΩ and RL = 10 kΩ gives Av = −38.7 mA/V × 1.67 kΩ ≈ −65 (inverting, ~65×). Adding RE = 100 Ω reduces the gain to ≈ −5.5 but makes it far more stable and linear — that's the trade-off of degeneration. A common-collector (emitter follower) has ≈1 voltage gain but high input and low output impedance, ideal as a buffer. Use this to size an amplifier's gain, to decide whether you need degeneration for stability, or to pick the right configuration for your application.