transistoroutput

Transistor output refers to the electrical signal that a transistor produces at its external terminals when used in a circuit as a switch or amplifier. The term encompasses both analog voltage or current that varies with an input signal and the binary levels used in digital logic, where a transistor is arranged to pull an output node toward a supply rail or toward ground.

In bipolar junction transistor (BJT) circuits, the most common output nodes are the collector (common-emitter configuration) or the emitter (emitter follower). In field-effect transistor (FET) circuits, outputs are taken at the drain (common-source) or the source (source follower). The output can be modeled as a source or sink with an output impedance, often described by a Thevenin equivalent: a voltage source in series with an output resistance that interacts with the load to shape the delivered signal.

Analog transistor outputs provide voltage or current gains. Common-emitter stages offer high voltage gain but relatively high output impedance and inverted phase; emitter followers provide low output impedance with near-unity gain but no phase inversion; common-base stages are fast with low input impedance. Output characteristics include saturation or cutoff limits, the maximum undistorted swing, and the load line determined by supply voltage and external resistors.

In digital logic applications, transistor outputs switch between supply and ground, producing logic-high and logic-low levels. Performance is described by VOH, VOL, propagation delay, and fan-out. Transistors must drive the input capacitances of subsequent stages, and the output impedance influences switching speed and power dissipation.

For MOSFETs, the same concepts apply with terminology like common-source and source follower. Key parameters include threshold voltage, transconductance, and on-resistance (Rds(on)) in the on state, which determine the available output swing and drive capability.

Transistor output is a fundamental concept in electronics, central to amplifier design, buffering, and digital logic, with practical considerations including linearity, bandwidth, temperature effects, and supply headroom.