Professional BJT & MOSFET circuit calculators for engineers
Calculate transistor amplifier parameters, analyze frequency response, and understand circuit behavior with detailed step-by-step solutions.
Read moreDC operating point
Forward-active, saturation, triode and cutoff are detected and solved properly, with a DC load line.
AC small-signal analysis
Voltage, overall, current and power gain plus input and output impedance and the output waveform.
Frequency response
Coupling, bypass and internal capacitances give fL, fH and a full Bode plot.
Step-by-step working
Every result shows the equation and the substituted values, ready for coursework and design reviews.
NPN BJT Configurations
Bipolar junction transistor stages with an NPN device.
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 calculatorPNP BJT Configurations
Complementary PNP stages — same analysis, mirrored polarities.
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 calculatorFixed Bias CC
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 calculatorNMOS Configurations
Enhancement-mode n-channel MOSFET amplifier stages.
Fixed Bias CS
Inverting amplifier with the gate held at VGG through RG.
Open calculatorVoltage Divider CS
Inverting amplifier with a resistive gate-divider bias.
Open calculatorFixed Bias CG
Current-buffer stage: low Zin, non-inverting voltage gain.
Open calculatorVoltage Divider CG
Current-buffer stage with divider-biased, AC-grounded gate.
Open calculatorFixed Bias CD
Source follower — near-unity gain with low output impedance.
Open calculatorVoltage Divider CD
Source follower with divider bias — low output impedance.
Open calculatorPMOS Configurations
Enhancement-mode p-channel MOSFET amplifier stages.
Fixed Bias CS
Inverting amplifier with the gate held at VGG through RG.
Open calculatorVoltage Divider CS
Inverting amplifier with a resistive gate-divider bias.
Open calculatorFixed Bias CG
Current-buffer stage: low Zin, non-inverting voltage gain.
Open calculatorVoltage Divider CG
Current-buffer stage with divider-biased, AC-grounded gate.
Open calculatorFixed Bias CD
Source follower — near-unity gain with low output impedance.
Open calculatorVoltage Divider CD
Source follower with divider bias — low output impedance.
Open calculatorChoosing an amplifier configuration
Every single-transistor amplifier grounds one of the device’s three terminals for AC signals and uses the other two as input and output. The common terminal decides the stage’s gain, impedance levels and phase. BJT and MOSFET stages behave alike, with the MOSFET versions offering much higher input impedance and the BJT versions more transconductance at the same current.
| 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 Source (CS) | Moderate to high, inverting | Very high (bias resistors) | High (≈ RD) | 180° | General voltage amplification, sensor front ends |
| Common Base (CB) | High, non-inverting | Very low (≈ re) | High (≈ RC) | 0° | RF and wideband amplifiers, current buffers, cascodes |
| Common Gate (CG) | Moderate to high, non-inverting | Low (≈ 1/gm ∥ RS) | High (≈ RD) | 0° | RF low-noise amplifiers, current buffers, cascodes |
| Common Collector (CC) | ≈ 1, non-inverting | High (≈ (β + 1)·RL′ ∥ bias) | Very low | 0° | Buffers, output stages, impedance matching |
| Common Drain (CD) | < 1, non-inverting | Very high (bias resistors) | Low (≈ 1/gm ∥ RS) | 0° | Buffers for high-impedance sources, level shifters |
Common-emitter and common-source stages are the general-purpose voltage amplifiers. Common-base and common-gate stages give the same voltage gain without inversion and with a wider bandwidth, at the cost of a low input impedance. Emitter and source followers give almost no voltage gain but act as buffers that let a weak source drive a heavy load.
How to use the calculators
Pick a circuit
Choose the transistor type, configuration and bias network that match your schematic.
Enter your values
Supplies, resistors and device parameters, with units or suffixes like 4.7k and 1M.
Read the results
The operating point, gains, impedances, waveform and power update as you type.
Check the working
Open “View Calculation Steps” under any result to see each equation with numbers.
Why these calculators are accurate
Many online transistor calculators only handle the textbook case and give meaningless numbers when the circuit leaves it. Circuits Math solves the bias circuit exactly, using the full Thevenin equation for divider bias instead of the stiff-divider shortcut, and re-solves the circuit when the transistor saturates or enters triode instead of reporting negative voltages. Small-signal results are shown only when the device is actually amplifying, and the output waveform warns you when the signal would clip or distort.
The models are the ones used in standard textbooks: constant VBE(on) and the hybrid-π model for BJTs, the square-law model with channel-length modulation for MOSFETs, and time-constant methods for bandwidth. That means you can reproduce every number by hand, which is the point of a learning tool. Read more about Circuits Math.