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
- 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
Astable oscillator
Transistors concept map
Transistors
Transistors quiz
Transistors, quiz
1. An amplifier has an input of 20 mV and an output of 4 V. Its voltage gain is:
200800.0052. In the NPN transistor symbol shown, terminal Z carries the arrow. Terminal Z is the:
EmitterCollectorBase3. In the amplifier shown, the resistor Rb is fitted to:
Set the base bias currentLimit the collector current onlyCouple the output4. An amplifier gives an output of 12 V for an input of 0.2 V. Its voltage gain is:
602.4125. A transistor has a collector current of 200 mA and a base current of 2 mA. Its current gain is:
1004000.016. A transistor carries a collector current of 200 mA and a base current of 2 mA. The emitter current is:
202 mA198 mA200 mA7. A common-emitter amplifier produces an output that is:
Inverted, 180° out of phaseIn phase with the inputAt half the input frequency8. A transistor with β = 100 and I_b = 50 µA has a collector current of:
5 mA0.5 mA50 mA9. On an NPN transistor symbol, the emitter arrow:
Points outwardsPoints inwardsIs omitted10. The three terminals of a bipolar transistor are:
Emitter, base and collectorAnode, cathode and gateSource, gate and drain11. A Class A amplifier conducts for:
The full 360° of the input cycle180° of the cycleLess than 180°12. An emitter follower is used mainly because it has:
High input impedance and low output impedanceVery high voltage gainA 180° phase inversion