5.3 - Transistor

Terminals and Types

• A transistor is an electronic component with three terminals.

Terminal	Function
Emitter, E	Supplies charge carriers to the collector.
Base, B	A thin middle layer that controls charge-carrier flow from emitter to collector.
Collector, C	Receives charge carriers from the emitter.

Type	Structure and arrow direction
npn	A p-type base between n-type collector and emitter regions. The symbol’s emitter arrow points outwards, showing current from B towards E.
pnp	An n-type base between p-type collector and emitter regions. The symbol’s emitter arrow points inwards, showing current from E towards B.

Base Circuit and Collector Circuit

• A transistor circuit has a base circuit and a collector circuit. A small base current $I_B$ controls a much larger collector current $I_C$.
• In a transistor circuit with bulb $L_1$ and resistor $R_B$ in the base circuit, and bulb $L_2$ in the collector circuit:
  • Switch open: The base circuit is incomplete, $I_B=0$, and bulb $L_1$ is off. The transistor is off, so $I_C=0$ and $L_2$ remains off even though the collector circuit is complete.
  • Switch closed: The large base resistance $R_B$ limits $I_B$, so $L_1$ is dim. The transistor turns on and a larger $I_C$ makes $L_2$ bright.
• Base resistor $R_B$ limits the base current to prevent overheating and damage to the transistor.
• Base–emitter voltage $V_{BE}$ must reach the minimum needed to turn on the transistor: 0.7 V for silicon and 0.3 V for germanium.
• Replacing an npn transistor with a pnp transistor requires reversing battery polarity in both circuits; $I_B$, $I_C$ and emitter current $I_E$ reverse direction.
• For these operating circuits, $I_B<I_C<I_E$; when $I_B=0$, $I_C=0$.

Transistor as a Current Amplifier

• A small change in base current produces a much larger change in collector current. This allows amplification, such as in a singer’s loudspeaker system.
• The amplification factor, $\beta$, is:

• Use the same current unit for $I_C$ and $I_B$; $\beta$ has no unit.
• The example graph of $I_C$ against $I_B$ is a straight line through the origin with positive gradient. Its gradient equals $\beta$.
• With $I_C=78.0\mathrm{mA}$ at $I_B=0.5\mathrm{mA}$:

• A base-current change of 1 mA corresponds to a collector-current change of 156 mA for this graph.

Potential Divider and Automatic Switching

• A potential divider gives a smaller output potential difference from a fixed supply. Connect $R_1$ and $R_2$ in series across input voltage $V_{\text{in}}$; take $V_{\text{out}}$ across $R_2$.
• The same current flows through the two resistors in the divider calculation:

• The divider controls the base voltage and hence the base current. Turning the base current on or off allows the transistor to act as an automatic switch in the collector circuit.

Light-controlled Switch

• A light-dependent resistor (LDR) has high resistance in dim light and low resistance in bright light.
• In a 6 V light-controlled switch circuit, the 10 kΩ resistor is above the LDR in the potential divider. The base connects to their junction through a 1 kΩ resistor; the controlling voltage is across the LDR.

Lighting	Circuit response
Dark	LDR resistance rises, so $V_{\text{LDR}}$ rises. When it exceeds the minimum base–emitter voltage, $I_B$ flows and turns on the transistor. A large $I_C$ flows and the bulb lights.
Bright	LDR resistance falls, so $V_{\text{LDR}}$ falls. Below the minimum base–emitter voltage, no base current turns on the transistor; the collector current and bulb remain off.

• Application: Automatic street lights.
• In a 6 V d.c. light-controlled transistor kit, covering the LDR reduces the light intensity and turns on the transistor and LED.

Temperature-controlled Alarm

• A thermistor changes resistance with surrounding temperature. The thermistor shown has high resistance at low temperature and lower resistance when heated.
• In a 6 V temperature-controlled alarm circuit, the thermistor is above the 10 kΩ resistor $R$ in the potential divider. The base connects to their junction through 1 kΩ; the controlling voltage is across $R$.

1. The surrounding temperature rises and the thermistor resistance falls.
2. Voltage across the thermistor falls, while voltage $V_R$ across $R$ rises.
3. When $V_R$ exceeds the minimum base–emitter voltage, base current flows and turns on the transistor.
4. A large collector current flows and the bulb lights.

• Application: Automatic switching in a temperature-controlled alarm system.
• In a 6 V d.c. heat-controlled transistor kit, warming the thermistor turns on the transistor and LED.