transconductance
The ratio of the change in output current to the change in input voltage across a circuit
transconductance: how much current swings per volt change
Transconductance is the ratio of a change in output current to a change in input voltage, expressed in siemens (S) or millisiemens (mS). A vacuum tube, field-effect transistor, or operational amplifier with high transconductance will produce a large swing in output current for a small shift in input voltage. Low transconductance means you need a bigger voltage nudge to get the same current response. It appears in every analog circuit where a voltage signal needs to control current flow.
The term breaks down logically: "trans" meaning across or through, "conductance" meaning the ability to conduct current. Conductance itself is the reciprocal of resistance, measured in siemens. Transconductance therefore measures how effectively a voltage input translates into current output, which is why it carries units of current per voltage: amps per volt, or siemens.
In vacuum tubes, transconductance (often written as gm) is a printed specification. A typical small-signal triode might have a transconductance of 2 to 8 millisiemens. Field-effect transistors publish transconductance as well, though it varies with gate-source voltage and drain current. The relationship is not always linear across the full operating range. In BJT bipolar transistors, transconductance depends on collector current and is smaller, typically in the range of tens of millisiemens for modest currents.
Where transconductance matters
High transconductance makes a stage more sensitive and gives higher voltage gain when feeding a resistive load. This is why low-noise preamplifiers and RF circuits favor tubes and FETs with strong gm. Conversely, a stage with weak transconductance requires larger input signals to achieve the same output swing, which can degrade noise performance. In transimpedance amplifiers (photo-detector circuits), transconductance of the input stage directly affects bandwidth and noise characteristics.
Temperature changes, supply voltage variations, and component aging all shift transconductance. Tubes age and lose transconductance over thousands of hours. FETs drift with temperature. Circuit designers who depend on a fixed transconductance value build in compensation networks or use feedback to stabilize the stage. When a vintage amplifier loses brightness or appears to lose gain uniformly, reduced transconductance in output tubes is often the culprit.