A transistor has h_FE = 100 and a base current of 20\ μA. Find the collector current. [2 marks]

EnglandElectronicsSyllabus dot point

How do bipolar and MOSFET transistors act as current amplifiers and voltage-controlled switches?

Transistors: the bipolar junction transistor as a current amplifier with current gain, the MOSFET as a voltage-controlled device, the common-emitter amplifier, and biasing.

An Eduqas A-Level Electronics answer on transistors: the bipolar junction transistor as a current amplifier with current gain, the MOSFET as a voltage-controlled device, the common-emitter amplifier and its voltage gain, and the biasing that sets the operating point.

Generated by Claude Opus 4.814 min answerUpdated 2026-06-02

Reviewed by: AI editorial process; not yet individually human-reviewed

Have a quick question? Jump to the Q&A page

Quick answer

A bipolar junction transistor (BJT) is a current-controlled device: a small base current $I_B$ controls a much larger collector current $I_C$ , with current gain $h_{FE} = \frac{I_C}{I_B}$ . Kirchhoff gives $I_E = I_B + I_C$ . A MOSFET is a voltage-controlled device: the voltage on an insulated gate controls the drain current, so it draws essentially no steady gate current (very high input resistance). In a common-emitter amplifier the transistor amplifies a small input voltage; the small-signal voltage gain is roughly the collector resistance divided by the emitter resistance, and the output is inverted. Biasing (usually a base potential divider) sets the no-signal operating point so the output can swing both ways without distortion.

Jump to a section

What this dot point is asking
The answer
Examples in context
Try this

What this dot point is asking

Eduqas wants you to describe the bipolar junction transistor as a current amplifier with current gain, the MOSFET as a voltage-controlled device, the common-emitter amplifier and its gain, and the biasing that sets the operating point. The transistor is the active device behind every amplifier and electronic switch.

The answer

The bipolar junction transistor as a current amplifier

The MOSFET as a voltage-controlled device

The common-emitter amplifier

Biasing

Designing the bias current for a common-emitter stage

A common-emitter amplifier is to run with a collector current of $2.0\ \text{mA}$ using a transistor with $h_{FE} = 200$ . Find the required base current and the base resistor if the base sits $0.7\ \text{V}$ above a $0\ \text{V}$ emitter on a $9.0\ \text{V}$ supply (simple base bias).

step 1: Find the base current

$I_B = \frac{I_C}{h_{FE}} = \frac{2.0 \times 10^{-3}}{200} = 1.0 \times 10^{-5}\ \text{A} = 10\ \mu\text{A}$ .

step 2: Find the voltage across the base resistor

With the base at $0.7\ \text{V}$ and the supply at $9.0\ \text{V}$ , the base resistor drops $9.0 - 0.7 = 8.3\ \text{V}$ .

step 3: Apply Ohm's law for the base resistor

$R_B = \frac{8.3}{10 \times 10^{-6}} = 8.3 \times 10^{5}\ \Omega = 830\ \text{k}\Omega$ . (A divider bias would be used in practice for stability, but this fixes the order of magnitude.)

Examples in context

Transistors are the active heart of electronics: the common-emitter amplifier is the building block of audio pre-amplifiers, MOSFETs switch motors, heaters and LED strings under logic control, and millions of transistors make up the logic inside a microcontroller. The current gain decides how much base drive a switch needs, and the voltage-controlled MOSFET is why a $5\ \text{V}$ logic output can switch a large current with almost no loading.

Try this

Q1. A transistor has $h_{FE} = 100$ and a base current of $20\ \mu\text{A}$ . Find the collector current. [2 marks]

Cue. $I_C = 100 \times 20 \times 10^{-6} = 2.0\ \text{mA}$ .

Q2. State whether a MOSFET is current-controlled or voltage-controlled. [1 mark]

Cue. Voltage-controlled (by the gate-source voltage).

Q3. State the phase relationship between input and output of a common-emitter amplifier. [1 mark]

Cue. The output is inverted (180 degrees out of phase) relative to the input.

Exam-style practice questions

Practice questions written in the style of WJEC Eduqas exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.

Eduqas 20195 marksA bipolar transistor has a current gain

h_{FE} = 150

. A base current of

40\ \mu\text{A}

flows. Calculate the collector current and the emitter current, and state the relationship between them.

Show worked answer →

Collector current (up to 2 marks): $I_C = h_{FE}\, I_B = 150 \times 40 \times 10^{-6} = 6.0 \times 10^{-3}\ \text{A} = 6.0\ \text{mA}$ .

Emitter current (up to 2 marks): the emitter current is the sum of the base and collector currents, $I_E = I_B + I_C = 0.04 + 6.0 = 6.04\ \text{mA} \approx 6.0\ \text{mA}$ .

Relationship (up to 1 mark): $I_E = I_B + I_C$ (Kirchhoff's current law at the transistor), and $I_C = h_{FE}\, I_B$ , so the small base current controls a much larger collector current.

Markers reward $I_C = 6.0\ \text{mA}$ , $I_E \approx 6.0\ \text{mA}$ , and the relationship $I_E = I_B + I_C$ with $I_C = h_{FE} I_B$ .

Eduqas 20215 marksExplain the main difference between a bipolar junction transistor and a MOSFET, and give one advantage of the MOSFET when used as a switch.

Show worked answer →

Difference (up to 3 marks): a bipolar junction transistor is current-controlled, a small base current controls a larger collector current, so it draws a continuous input current. A MOSFET is voltage-controlled, the voltage on the insulated gate controls the drain current, and because the gate is insulated it draws essentially no steady input current.

Advantage as a switch (up to 2 marks): the MOSFET needs almost no drive current (high input resistance), so it loads the controlling circuit far less, and a logic output can drive it directly; it also has a very low on-resistance, so it dissipates little power when fully on.

Markers reward current-controlled versus voltage-controlled, the insulated gate drawing no steady current, and a valid switching advantage (negligible drive current or low on-resistance).

Related dot points

Sources & how we know this

Eduqas GCE AS/A Level Electronics specification (A410QS) — WJEC Eduqas (2017)