Common Emitter Amplifier Calculator

The common-emitter (grounded-emitter) topology dominated signal amplification from the 1950s through the op-amp revolution of the 1970s. Every 1960s ham radio, transistor radio, and Hi-Fi preamp had cascaded CE stages as their voltage-gain core. The Miller-effect bandwidth limitation (collector-base capacitance multiplied by gain) became a key constraint that drove the development of cascode topologies and integrated op-amps with internal compensation.

Enter DC bias Ic, β, Rc (collector-to-Vcc), Re (emitter-to-ground; 0 = bypassed), and load RL (∞ = open collector). The tool computes re = 26mV/Ic, voltage gain Av = -(Rc||RL)/(re+Re), input Zin = rπ + (β+1)Re, output Zout ≈ Rc.

For high-gain without DC instability: keep Re ≈ 0.1·Rc for bias, then add emitter bypass cap Ce sized for lowest passband frequency. Everything runs client-side.

The central tension in a common-emitter design is that the very thing giving you the most voltage gain — tying the emitter straight to ground — is also what makes the bias dangerously unstable. With no emitter resistor the gain becomes -gm·(Rc∥RL), which is large, but it now depends directly on the internal emitter resistance re = 26 mV/Ic. Since re moves with the collector current and with temperature, any drift in operating point changes the gain, and worse, the bias point itself can run away as the transistor heats up. That is why a bare grounded-emitter stage is rarely used on its own.

Adding an emitter resistor (emitter degeneration) fixes the stability at the cost of gain. The resistor introduces local negative feedback: if the current tries to rise, the voltage across Re rises and pushes back on the base-emitter junction, holding the operating point steady against temperature and device-to-device β spread. It also raises the input impedance to roughly rπ + (β+1)·Re and linearises the stage, lowering distortion. The classic compromise is to size Re for solid DC bias and then bridge it at signal frequencies with a bypass capacitor, so the stage runs the stable degenerated current but sees the high bypassed gain in its passband — you get DC stability and AC gain from the same circuit. Above a few hundred kilohertz the Miller effect (collector-base capacitance multiplied by the gain) eventually rolls the response off, which is the limitation that pushed designers toward cascode and op-amp topologies.