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BJT and FET circuits: board exam questions with solutions

On the ECE exam under Electronics Engineering. Try each question before you open its solution, then practise with fresh numbers below: 3 computed question types on this topic.

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BJT current gain (β) · Common-emitter amplifier · Transistor as a switch · MOSFET low-side switch · BJT small-signal model (hybrid-π): the same circuits on a live bench, with meters you can move.

Questions

1. A BJT has β = 150. What is its common-base current gain α?

  1. 0.0067
  2. 1.0067
  3. 0.9850
  4. 0.9934
Show the answer and solution

Answer: D. 0.9934

α = β/(β + 1) = 150/151 = 0.9934.

2. A voltage-divider biased silicon NPN stage has VCC = 12 V, R1 = 47 kΩ, R2 = 10 kΩ, RC = 3.3 kΩ and RE = 1 kΩ. Assuming β is large (IC ≈ IE) and VBE = 0.7 V, what is VCE?

  1. 7.36 V
  2. 2.95 V
  3. 5.96 V
  4. 2.11 V
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Answer: C. 5.96 V

VB = 12 × 10/57 = 2.105 V, VE = 1.405 V, IE = 1.405 mA, VCE = 12 − 1.405 × (3.3 + 1) = 5.96 V.

3. A BJT is biased at IC = 2 mA. Taking VT = 25 mV, what is its transconductance gm?

  1. 8 mS
  2. 80 mS
  3. 0.8 mS
  4. 12.5 mS
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Answer: B. 80 mS

gm = IC/VT = 2 mA/25 mV = 0.080 S = 80 mS. (12.5 Ω is re, not gm.)

4. A common-emitter amplifier has IE = 1 mA, RC = 4.7 kΩ and a fully bypassed emitter resistor. With VT = 25 mV and no load, what is the voltage gain?

  1. −188
  2. −4.7
  3. −4.7 × 10³
  4. +188
Show the answer and solution

Answer: A. −188

re = 25 mV/1 mA = 25 Ω; Av = −RC/re = −4700/25 = −188 (inverted output).

5. An n-channel JFET has IDSS = 10 mA and VGS(off) = −4 V. What is the drain current at VGS = −2 V?

  1. 5.0 mA
  2. 7.5 mA
  3. 0.0 mA
  4. 2.5 mA
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Answer: D. 2.5 mA

Shockley: ID = IDSS(1 − VGS/VGS(off))² = 10(1 − 0.5)² = 2.5 mA.

6. When does an n-channel enhancement-mode MOSFET begin to conduct significant drain current?

  1. When VGS = 0 V
  2. When VGS is made negative
  3. When VGS exceeds the threshold voltage VGS(th)
  4. When VDS exceeds the gate voltage, regardless of VGS
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Answer: C. When VGS exceeds the threshold voltage VGS(th)

An enhancement MOSFET has no channel at VGS = 0; a positive gate voltage above VGS(th) induces the n-channel.

7. An n-channel enhancement MOSFET in saturation follows ID = k(VGS − Vt)² with k = 0.5 mA/V² and Vt = 1 V. At VGS = 3 V, what is gm?

  1. 1.0 mS
  2. 2.0 mS
  3. 4.0 mS
  4. 0.5 mS
Show the answer and solution

Answer: B. 2.0 mS

ID = 0.5(3 − 1)² = 2 mA; gm = dID/dVGS = 2k(VGS − Vt) = 2 × 0.5 × 2 = 2.0 mS.

8. Which BJT configuration has a voltage gain of about one, high input impedance and low output impedance?

  1. Common collector (emitter follower)
  2. Common emitter
  3. Common base
  4. Cascode
Show the answer and solution

Answer: A. Common collector (emitter follower)

The emitter follower's output tracks the input minus VBE, so Av ≈ 1; its input impedance is about β(re + RE) and output impedance is low, making it a buffer.

Worked examples with computed numbers

These numbers are fixed on this page; in practice they change every time.

9. A voltage-divider-biased NPN amplifier has VCC = 20 V, R1 = 33 kΩ, R2 = 6.8 kΩ and RE = 1.2 kΩ. With VBE = 0.7 V and β large, what is the collector current?

  1. 2.26 mA
  2. 1.13 mA
  3. 2.85 mA
  4. 564 µA
Show the answer and solution

Answer: A. 2.26 mA

Stiff divider: VB = 20 × 6.8k/39.8k = 3.42 V; VE = VB − 0.7 = 2.72 V; IC ≈ IE = VE/RE = 2.26 mA.

10. A BJT with β = 150 has a base current of 25 µA. What is the emitter current?

  1. 25 µA
  2. 24.8 µA
  3. 3.75 mA
  4. 3.78 mA
Show the answer and solution

Answer: D. 3.78 mA

IE = IB + IC = (β + 1)IB = 151 × 25 µA = 3.78 mA.

11. An enhancement NMOS with k = 2 mA/V² (ID = ½k(VGS − VT)²) and VT = 2 V is in saturation with VGS = 3 V. What is ID?

  1. 2 mA
  2. 1.5 mA
  3. 9 mA
  4. 1 mA
Show the answer and solution

Answer: D. 1 mA

In saturation ID = ½k(VGS − VT)² = ½ × 2 mA/V² × (3 − 2)² = 1 mA.

Other topics in the same subjects

DC circuits · AC circuits · Transients (RC, RL, RLC) · Semiconductors and diodes · Operational amplifiers · Oscillators and feedback · Power electronics · Digital electronics · Microprocessors and microcontrollers · Control systems · Instrumentation and measurement

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