Common Emitter Amplifier Calculator
Compute voltage gain, input/output impedance, and bandwidth of a common-emitter BJT amplifier with emitter degeneration.
How to Use
- Enter Ic bias point, β, collector resistor Rc, and emitter resistor Re.
- Enter source impedance Rs for bandwidth estimate (via Miller effect).
- Tool computes Av, Zin, Zout.
Show Work
Formulas
History of the Common-Emitter Amplifier
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.
About This Calculator
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.
Frequently Asked Questions
Why CE?
Highest voltage gain topology (factors of 50-500). Inverts signal. Moderate Zin, high Zout. The standard signal-amplifier building block.
Emitter bypass?
Re adds DC bias stability but limits gain. Adding a bypass cap (Ce) across Re gives high AC gain while preserving DC bias — the classic CE+bypass configuration.
Miller effect?
Collector-base capacitance Ccb appears at input multiplied by (1+Av), limiting bandwidth. High-gain CE amps are bandwidth-limited; use cascode topology to eliminate Miller.
Common Use Cases
Microphone Preamp
Av = -50, Zin = 1 kΩ, Zout = 10 kΩ. Classic preamp stage.
Video Buffer
Low-gain, wide-band CE with small Rc and no Re bypass.
Classic Radio IF
1950s-60s superheterodyne IF stages used cascaded CEs.
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