Transkonduktans

Transkonduktans, or transconductance, is a measure of how effectively a device converts a change in input voltage into a change in output current, with other terminal voltages held fixed. The parameter is defined differently for different device families: for field-effect transistors (FETs) it is gm = ∂Id/∂Vgs, and for bipolar junction transistors (BJTs) it is gm = ∂Ic/∂Vbe. The unit is siemens (S), equivalent to amperes per volt.

In small-signal models, transkonduktans is represented as a dependent current source whose current equals gm times the small-signal input voltage. This makes gm a key factor in determining amplifier gain, input and output impedance, and overall dynamic performance. A higher gm generally means a larger current change for a given input voltage change, improving voltage-to-current conversion in the device.

For MOSFETs in saturation, a common long-channel square-law approximation gives gm ≈ 2Id/(Vgs − Vth) = 2Id/Vov, where Vov is the overdrive voltage. Thus gm increases with bias current and decreases with a higher threshold or lower overdrive. For BJTs, transconductance is typically gm ≈ Ic/Vt, with Vt ≈ 25 mV at room temperature, meaning gm scales with the quiescent collector current.

Transkonduktans is a fundamental parameter in the design and analysis of amplifiers and transistors, influencing small-signal gain, bandwidth, noise performance, and biasing sensitivity. It varies with temperature, device geometry, and operating point, and is a central concept in transconductance-based (voltage-to-current) amplification.