Common-base amplifier small-signal analysis
A common-base BJT stage holds the base at AC ground, applies its signal at the emitter, and takes output from the collector. It has low input resistance, approximately unity current transfer, potentially substantial voltage gain, and useful high-frequency behavior.
The calculator applies a compact room-temperature small-signal model. It does not solve the bias network, voltage headroom, device limits, parasitic network, or source and load interactions in a complete circuit.
How to use the common-base amplifier calculator
- Enter the operating point: Provide supply voltage, collector current, current gain, and Early voltage.
- Enter circuit loading: Provide collector and load resistances plus capacitance estimates.
- Analyze: Generate the simplified midband gain, impedances, cutoff, and power.
- Verify the design: Check bias, headroom, source loading, stability, ratings, parasitics, and frequency response with device models.
Formula and variables
Collector current sets transconductance; the parallel collector, load, and transistor output resistances set the simplified loaded voltage gain.
gm = IC/VT; Zin ≈ 1/gm; Av ≈ gm(RC ∥ RL ∥ ro)- IC — Collector current
- DC operating collector current (A)
- VT — Thermal voltage
- Approximately 25.85 mV at room temperature (V)
- RC — Collector resistance
- External collector resistor (Ω)
- RL — Load resistance
- External output load (Ω)
- ro — Output resistance
- Estimated as Early voltage divided by collector current (Ω)
One-milliamp common-base stage
A 12 V stage operates at 1 mA with RC = 4.7 kΩ, RL = 10 kΩ, and Early voltage 100 V.
- Supply and current
- 12 V, 1 mA
- Resistors
- 4.7 kΩ and 10 kΩ
- Early voltage
- 100 V
- gm = 0.001/0.02585 ≈ 38.68 mS
- Zin ≈ 1/gm ≈ 25.85 Ω
- Av uses gm times the parallel loaded output resistance
Result: The calculator displays the resulting gain and related small-signal estimates.
The low emitter input resistance can strongly load a high-resistance source.
Understanding your results
Use the output as a first-pass estimate
Voltage gain is not the same as current gain; a common-base stage can have voltage gain greater than one while current gain remains near unity.
- Input impedance is intrinsically low in this simplified model.
- External collector and load resistors usually limit output impedance and gain.
- The cutoff estimate includes only the entered output capacitance and loaded resistance.
Assumptions
- The transistor remains forward-active and operates at room temperature.
- Signals are small enough for linear small-signal analysis.
- Early voltage and current gain are constant estimates.
Limitations
- Does not solve bias, source resistance, emitter degeneration, voltage swing, distortion, noise, stability, or device breakdown.
- The cutoff estimate uses separate first-order input and output RC poles and reports the lower one; it does not model the complete frequency response.
- Does not replace SPICE analysis or measured device parameters.
Common mistakes
- Confusing voltage gain with the near-unity current gain.
- Entering milliamps as amps or kilohms as ohms.
- Ignoring severe loading of the signal source.
- Assuming calculated gain is available across all frequencies and output swings.
Practical use cases
Analog design screening
Compare current, resistance, and loading choices before detailed simulation.
RF and electronics education
Study low input resistance and the absence of strong common-emitter Miller multiplication.
Frequently asked questions
Is common-base voltage gain always near one?
No. Its current gain is near unity, but small-signal voltage gain can be much larger depending on transconductance and output loading.
Why is input impedance low?
Looking into the emitter with the base at AC ground gives an approximate resistance of 1/gm before other circuit elements are included.
Does this calculator verify transistor ratings?
No. Verify voltage, current, power, safe operating area, bias, and thermal limits separately.
Sources and review
- Microelectronic Circuits — Oxford University Press. Accessed 2026-07-13.
- The Art of Electronics — Cambridge University Press. Accessed 2026-07-13.
Reviewed 2026-07-13.