Electrical
Transistor Amplifier
Small base/gate signal controlling large collector/drain current
A transistor amplifier uses a small input signal at the base (BJT) or gate (FET) to control a much larger current flowing between the collector/drain and emitter/source. Power for the output comes from the supply, not the input, so output power can far exceed input power. Common configurations include common emitter (voltage gain), common collector or emitter follower (current gain), and common base (high-frequency). Bias networks set the operating point in the linear region; coupling capacitors block DC and pass AC. Modern circuits combine many transistors into op-amps and integrated audio amplifiers, but discrete-transistor stages still teach the fundamentals.
- BJT controlBase current
- FET controlGate voltage
- Voltage gain (CE)A_v ≈ -g_m R_C
- ClassesA, B, AB, C, D
- BandwidthkHz to GHz
- Bias pointLinear region operation
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Why transistor amplifiers matter
- Audio. From microphone preamps to loudspeaker power stages.
- RF. Receivers, transmitters, low-noise front ends.
- Instrumentation. Sensor signal conditioning, op-amp internals.
- Power electronics. Motor drives, switching regulators.
- Communications. Telephone, broadcast, satellite uplinks.
- Medical. ECG, EEG, hearing aid amplification.
- Education. Foundation of analog electronics curricula.
Common misconceptions
- Power comes from input. Power comes from the supply; input only modulates flow.
- Higher gain is better. Stability, bandwidth, distortion all degrade with raw gain.
- Bias point doesn't matter. Wrong bias clips signals or wastes power.
- BJT and MOSFET are interchangeable. Different control variable, impedance, thermal behavior.
- Class A is always cleanest. Modern class AB and D rival class A with far better efficiency.
- Feedback always helps. Phase shifts at HF can turn negative feedback positive—oscillation.
Frequently asked questions
How does it amplify?
A transistor's output current responds to a small change in input current (BJT) or input voltage (FET). With a load resistor in the output path, that current change becomes a voltage change. If the controlled current is much larger than the controlling current, you get power gain. The supply rail provides the energy; the input signal merely modulates how much of it flows through the load.
What's the operating point?
The DC bias current and voltage on the transistor when no signal is present. It must sit in the active (linear) region, away from saturation and cutoff, so the device can swing both directions for AC signals. Bias is set by resistor networks and stabilized by emitter degeneration or feedback to handle temperature and device variation.
What are the amplifier classes?
Class A: transistor conducts 100% of the cycle, low distortion, max ~50% efficiency. Class B: each transistor conducts half the cycle, ~78% efficient, but crossover distortion. Class AB: small bias eliminates crossover, ~70% efficient, dominant in audio. Class C: conducts under half cycle, very efficient but only for tuned RF. Class D: PWM switching, over 90% efficient.
Why is feedback used?
Negative feedback subtracts a fraction of the output from the input, trading raw gain for stability, lower distortion, controlled bandwidth, and predictable input/output impedance. An op-amp with closed-loop gain of 10 may have open-loop gain of 100,000 and still sit on accurate feedback math. Feedback is the foundation of modern analog design.
What's a common-emitter stage?
Input on the base, output on the collector, emitter grounded (or with a degeneration resistor). Voltage gain is large and inverting (A_v = -g_m R_C). Input impedance is moderate, output impedance is set by the collector resistor. It's the workhorse single-transistor voltage amplifier, used in front-ends, intermediate stages, and discrete preamps.
What's a follower for?
A common-collector (emitter follower) or common-drain (source follower) provides voltage gain near 1 but high input impedance and low output impedance—an impedance buffer. It lets a high-impedance source drive a low-impedance load without losing signal. Followers appear at op-amp outputs, in cable drivers, and as the final stage of audio power amplifiers.
How are temperature effects managed?
BJTs change beta and V_BE with temperature; thermal runaway is a real risk in power stages. Solutions: emitter degeneration resistors (negative feedback), current-mirror biasing, thermistors thermally bonded to the output devices, V_BE multiplier biasing, and quasi-complementary topologies. Power amplifiers often use heat sinks plus electronic protection circuits to clamp current at safe levels.