NPN BJT Amplifier Calculators
The NPN bipolar junction transistor is the most common discrete amplifying device. These calculators analyze the three basic single-transistor NPN amplifier configurations with the bias networks taught in every electronics course: fixed bias with a base resistor, voltage-divider bias, and split-supply common-base bias.
Each calculator solves the DC operating point first, including saturation and cutoff, then builds the hybrid-π small-signal model to find voltage gain, current gain, input and output impedance. Optional frequency-response analysis adds the coupling and bypass capacitors plus the transistor’s internal capacitances Cπ and Cµ.
Choose a NPN BJT calculator
Fixed Bias CE
Inverting amplifier biased through a single base resistor.
Open calculatorVoltage Divider CE
Inverting amplifier with a stable four-resistor (divider) bias network.
Open calculatorCommon Base
Low input impedance, non-inverting stage with high voltage gain.
Open calculatorCommon Collector
Emitter follower — near-unity gain, high Zin, low Zout.
Open calculatorVoltage Divider CC
Emitter follower with divider bias for a stiff, stable Q-point.
Open calculatorComparing the CE, CB and CC configurations
Every single-transistor amplifier connects one terminal to AC ground and uses the other two as input and output. Which terminal is common decides the stage’s personality, as the table shows.
| Configuration | Voltage gain | Input impedance | Output impedance | Phase | Typical use |
|---|---|---|---|---|---|
| Common Emitter (CE) | High, inverting | Medium (≈ rπ ∥ bias resistors) | High (≈ RC) | 180° | General voltage amplification, audio pre-amplifiers, driver stages |
| Common Base (CB) | High, non-inverting | Very low (≈ re) | High (≈ RC) | 0° | RF and wideband amplifiers, current buffers, cascodes |
| Common Collector (CC) | ≈ 1, non-inverting | High (≈ (β + 1)·RL′ ∥ bias) | Very low | 0° | Buffers, output stages, impedance matching |
Common Emitter (CE)
In the common-emitter (CE) amplifier the input signal is applied to the base, the output is taken from the collector, and the emitter is the terminal shared by input and output. The emitter sits at AC ground, either directly or through a bypass capacitor. A small change in base–emitter voltage produces a large change in collector current through the transconductance gm = IC/VT, and that current flows through the collector resistor to create the output voltage.
Common Base (CB)
In the common-base (CB) amplifier the base is held at AC ground, the signal drives the emitter, and the output is taken from the collector. The input source has to supply the entire emitter current, so the input impedance is very low, about re = VT/IE. That is often only a few tens of ohms. The collector, by contrast, behaves like a high-impedance current source.
Common Collector (CC)
In the common-collector (CC) amplifier, better known as the emitter follower, the signal enters at the base and the output is taken from the emitter. The collector is at AC ground, connected to the supply directly or through a small resistor. The emitter voltage follows the base voltage one base–emitter drop away, so the voltage gain is slightly less than one and the output is in phase with the input.
BJT parameters used in these calculators
- β (hFE)
- DC current gain, IC/IB. Typically 50–300 for small-signal transistors and strongly dependent on the individual part and temperature.
- VBE(on)
- Base–emitter voltage of a conducting silicon transistor, about 0.6–0.7 V. Treated as constant for DC analysis.
- VCE(sat)
- Collector–emitter voltage when the transistor is saturated, typically 0.1–0.3 V.
- VT
- Thermal voltage kT/q, about 25.9 mV at 300 K. It sets the transconductance.
- gm
- Transconductance, IC/VT. The change in collector current per volt of base–emitter signal.
- rπ
- Small-signal base–emitter resistance, β/gm.
- VA, ro
- The Early voltage VA models the slope of the output characteristics. It gives the output resistance ro = VA/IC.