Transistors
Transistor symbols, construction and orientation; PNP and NPN devices; base, collector and emitter configurations; current gain; amplifier classes; oscillators, multivibrators and flip-flops.
Transistors — summary notes
Free- A bipolar junction transistor (BJT) has three terminals: emitter, base and collector, and comes as NPN or PNP.
- A small base current controls a much larger collector current. Current gain β (or h_FE) = I_c / I_b.
- On the symbol, the NPN arrow points OUT of the emitter — remember 'Not Pointing iN' for NPN.
- Common-emitter: high voltage and current gain, and the output is INVERTED — 180° out of phase with the input.
- Common-collector (emitter follower): voltage gain of about 1, high input impedance, low output impedance. Used as a buffer. Output is IN PHASE.
- Common-base: low input impedance, high voltage gain, used at high frequency.
- Amplifier classes: Class A conducts for the whole 360° (linear, low efficiency); Class B for 180° (push-pull); Class C for less than 180° (RF only).
- ⚠ Exam trap: common-emitter inverts by 180°; the emitter follower does not. The NPN arrow points OUT.
- Current gain
- β = I_c / I_b
- Emitter current
- I_e = I_b + I_c
- Alpha
- α = I_c / I_e (just under 1)
A transistor has β = 100 and a base current of 50 µA. Find the collector current.
I_c = β × I_b = 100 × 50 × 10⁻⁶ = 5 mA.
Which configuration would you choose to drive a low-impedance load from a high-impedance source, and why?
Common-collector (emitter follower). It has high input impedance and low output impedance with unity voltage gain, so it buffers the source without loading it.
Transistor as a switch & amplifier
FreeAstable oscillator
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Transistors concept map
FreeTransistors
Transistors quiz
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