NMOS Amplifier Calculators
Enhancement-mode n-channel MOSFETs are the building block of CMOS integrated circuits and a popular choice for discrete amplifiers because of their very high input impedance. These calculators cover the three single-transistor configurations, common source, common gate and common drain, each with fixed gate bias and voltage-divider bias.
The DC analysis uses the square-law model with the threshold voltage VTN and device constant kn, and it detects whether the transistor is in saturation, triode or cutoff. The small-signal analysis uses gm = 2·ID/VOV and ro = 1/(λ·ID).
Choose a NMOS calculator
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 calculatorComparing the CS, CG and CD 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 Source (CS) | Moderate to high, inverting | Very high (bias resistors) | High (≈ RD) | 180° | General voltage amplification, sensor front ends |
| Common Gate (CG) | Moderate to high, non-inverting | Low (≈ 1/gm ∥ RS) | High (≈ RD) | 0° | RF low-noise amplifiers, current buffers, cascodes |
| Common Drain (CD) | < 1, non-inverting | Very high (bias resistors) | Low (≈ 1/gm ∥ RS) | 0° | Buffers for high-impedance sources, level shifters |
Common Source (CS)
In the common-source (CS) amplifier the signal is applied to the gate, the output is taken from the drain, and the source is common to input and output. The source is either grounded directly or connected through a bypassed source resistor. A change in gate voltage produces a drain-current change gm·vgs, and that current develops the output voltage across RD ∥ RL. Like the BJT common-emitter stage, it is an inverting amplifier with a 180° phase shift.
Common Gate (CG)
In the common-gate (CG) amplifier the gate is held at AC ground, directly or through a bypass capacitor. The signal is applied to the source and the output is taken from the drain. It is the MOSFET counterpart of the BJT common-base stage.
Common Drain (CD)
In the common-drain (CD) amplifier, or source follower, the signal is applied to the gate and the output is taken from the source. The drain is at AC ground. The source follows the gate voltage offset by VGS, so the gain is a little below one and the output is in phase with the input.
MOSFET parameters used in these calculators
- Vt (VTN, VTP)
- Threshold voltage. The gate–source voltage at which the channel forms. Positive for NMOS, negative for PMOS enhancement devices.
- k (kn, kp)
- Device constant µCox·W/L in mA/V². It scales the drain current.
- VOV
- Overdrive voltage, |VGS| − |Vt|. It sets both the current and the transconductance.
- Saturation
- The amplifying region, reached when |VDS| ≥ VOV. Below that the device is in the triode (ohmic) region.
- gm
- Transconductance, k·VOV = 2·ID/VOV.
- λ, ro
- Channel-length modulation parameter λ gives the output resistance ro = 1/(λ·ID).