Bipolar Junction Transistors and Field-Effect Transistors
This domain tests the structure, bias conditions, characteristics and switching or amplifying behaviour of BJTs and FETs. Questions often ask you to identify the operating region or calculate terminal currents.
Core rules
- An NPN BJT conducts when the base-emitter junction is forward-biased and the base-collector junction is reverse-biased for active operation.
- Terminal currents: IE = IB + IC; IC = βIB; α = IC/IE = β/(β+1).
- Regions: cutoff (both junctions off), active (amplifier), saturation (both junctions on, VCE small).
- Common-emitter, common-base and common-collector configurations differ in gain, phase inversion and input or output resistance.
- A JFET is a voltage-controlled device; the gate-source junction is reverse-biased, so gate current is very small. ID is controlled by VGS.
- A MOSFET uses an insulated gate; enhancement mode needs VGS above threshold to form a channel.
Worked example
An NPN BJT has VBB = 5 V, RB = 430 kΩ, VBE = 0.7 V, β = 100, RC = 2.2 kΩ and VCC = 12 V. IB = (5 − 0.7)/430,000 = 10 μA. IC = βIB = 100 × 10 μA = 1 mA. VCE = VCC − ICRC = 12 − (1 mA × 2.2 kΩ) = 9.8 V. Since VCE is well above about 0.2 V, the transistor is in the active region.
Marks-losing traps
Do not assume β is constant; it varies with current, temperature and device. Always check whether the calculated IC is physically possible before saturation. In saturation, IC is limited by RC, not by βIB. For a JFET, do not assume gate current flows; for a MOSFET, never assume the gate is forward-biased like a BJT base.