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MOSFET Operating Point Calculator

Calculate MOSFET drain current, saturation voltage, transconductance, and operating region (cutoff, triode, saturation) for NMOS transistors

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Formula

ID=kn/2×(VGS−Vth)2(sat),ID=kn×[(VGS−Vth)VDS−VDS2/2](triode)I_D = k_n/2 × (V_GS−V_th)² (sat), I_D = k_n × [(V_GS−V_th)V_DS − V_DS²/2] (triode)
V_GS— Gate-source voltage (V)
V_th— Threshold voltage (V)
k_n— Process transconductance parameter (A/V²)
V_DS— Drain-source voltage (V)
g_m— Transconductance (A/V)

How It Works

MOSFET operating point calculator computes DC bias conditions (Vgs, Vds, Id) for linear amplifier and switching applications — essential for RF amplifiers, power stages, and analog switches. RF engineers, power electronics designers, and IC layout engineers use this to establish proper gate drive and ensure operation in the desired region (cutoff, linear, or saturation). Per Horowitz & Hill 'Art of Electronics' (3rd ed., Ch.3), MOSFETs operate as voltage-controlled current sources in saturation: Id = K(Vgs - Vth)² where K = μnCox(W/L)/2. Unlike BJTs, MOSFETs have virtually infinite input resistance (10¹²-10¹⁴ Ω), zero DC gate current, and threshold voltage Vth typically 1-4V for enhancement-mode devices. Temperature coefficient is +3mV/°C for Vth and -0.3%/°C for mobility — Id decreases with temperature, providing inherent thermal stability.

Worked Example

Design a MOSFET source follower buffer using 2N7000 (Vth = 2.1V, K ≈ 0.1 A/V²) for audio output stage with Id = 5mA and Vdd = 12V. Saturation requires Vds > Vgs - Vth. Calculate Vgs: Id = K(Vgs - Vth)², so 0.005 = 0.1×(Vgs - 2.1)². Vgs - Vth = √0.05 = 0.224V, Vgs = 2.32V. Set drain resistor Rd for Vds = 6V (50% headroom): Vds = Vdd - Id×Rd, Rd = (12V - 6V)/5mA = 1.2kΩ. Gate bias: simple resistor divider with R1 = 1MΩ, R2 = 240kΩ gives Vg = 12V × 240k/(1M+240k) = 2.32V. Source bypass capacitor 10μF maintains DC operating point while allowing AC signal coupling.

Practical Tips

  • ✓Use source degeneration resistor Rs for bias stability — temperature-induced Id changes create negative feedback through Vs = Id×Rs
  • ✓For switching applications, ensure Vgs > Vth + 5V for full enhancement — this achieves Rds(on) specified on datasheet (typically at Vgs = 10V)
  • ✓Power MOSFETs (IRFZ44N) have Vth = 2-4V; logic-level MOSFETs (IRLZ44N) have Vth = 1-2V for direct microcontroller drive

Common Mistakes

  • ✗Operating in linear region instead of saturation for amplification — linear region gives variable resistance behavior; saturation gives constant-current behavior essential for voltage gain
  • ✗Ignoring Vth variation — 2N7000 specifies Vth = 0.8-3V; always design for worst-case Vth_max when setting minimum gate drive
  • ✗Neglecting gate capacitance at high frequency — typical Ciss = 20-100pF limits bandwidth; calculate transition frequency ft = gm/(2πCiss)

Frequently Asked Questions

Linear region (Vds < Vgs - Vth): MOSFET acts as voltage-controlled resistor, Id proportional to Vds. Saturation region (Vds > Vgs - Vth): MOSFET acts as voltage-controlled current source, Id independent of Vds. Amplifiers operate in saturation; switches operate in linear (on) or cutoff (off).
Vth increases +3mV/°C (reduces Id); mobility decreases -0.3%/°C (reduces K, thus Id). Net effect: Id decreases with temperature, opposite to BJTs. This provides inherent thermal stability — paralleled MOSFETs share current naturally without ballast resistors per Infineon application note.
Yes, with polarity reversal: Vgs is negative (typically -3 to -10V), Id flows from source to drain, and Vds is negative. The same Id = K(Vgs - Vth)² equation applies with |Vgs|, |Vth|. P-channel has lower mobility (2-3× lower K) requiring larger device area for equivalent current.
For full enhancement: Vgs > Vth + 4-5V. Most power MOSFETs specify Rds(on) at Vgs = 10V. Logic-level MOSFETs achieve full Rds(on) at Vgs = 4.5V. Never exceed Vgs(max) — typically ±20V for Si MOSFETs, ±8V for SiC; exceeding this destroys gate oxide per JEDEC JEP122H.

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