5.3 Transistor
A transistor is an electronic component with three terminals: emitter (E), base (B), and collector (C). It plays a vital role in modern electronics, serving as a current amplifier to boost small signals and as an automatic switch in sensor-controlled circuits such as light-controlled street lights and temperature-controlled alarms.
Learning Objectives
- Describe the terminals and functions of emitter (\(E\)), base (\(B\)), and collector (\(C\)) in a transistor.
- Differentiate between \(npn\) and \(pnp\) transistors based on their physical structures, circuit symbols, and current directions.
- Explain the operation of a transistor circuit comprising a base circuit and a collector circuit.
- Explain the function of a transistor as a current amplifier and calculate the amplification factor (\(\beta\)).
- Explain the use of a potential divider circuit to switch on a transistor.
- Analyze the use of a transistor as an automatic switch controlled by a light-dependent resistor (LDR) or a thermistor.
Transistor Types, Switching Actions, and Potential Divider Operations
Compare \(npn\) and \(pnp\) transistor symbols and current paths, understand the potential divider formula, and trace the dual sensor switching mechanisms using an LDR and a thermistor.
\(npn\) and \(pnp\) Transistor Symbols
Potential Divider and Automatic Switching
Short Explanation
What is a Transistor?
A transistor is an electronic component that has three terminals: emitter (\(E\)), base (\(B\)), and collector (\(C\)).
Terminal Functions
Emitter (\(E\)): Supplies charge carriers to the collector. Base (\(B\)): A thin middle layer that controls the flow of charge carriers from emitter to collector. Collector (\(C\)): Receives charge carriers from the emitter.
\(npn\) Transistor
Formed by a p-type semiconductor sandwiched between two n-type semiconductors. The arrow on the emitter in the circuit symbol points outwards, from base to emitter (\(B \rightarrow E\)).
\(pnp\) Transistor
Formed by an n-type semiconductor sandwiched between two p-type semiconductors. The arrow on the emitter points inwards, from emitter to base (\(E \rightarrow B\)).
Transistor Circuit Connections
A transistor circuit consists of two main parts: the base circuit and the collector circuit. In an \(npn\) transistor circuit, the positive terminal of the base battery connects to the base, and the positive terminal of the collector battery connects to the collector. Current relationships: \(I_B < I_C < I_E\) and \(I_E = I_B + I_C\). If the base circuit is open (\(I_B = 0\)), the collector current is zero (\(I_C = 0\)), so a bulb in the collector circuit does not light up even if that circuit is complete. When the base circuit is closed, a small base current \(I_B\) flows. This allows a large collector current \(I_C\) to flow. Resistor \(R_B\) in the base circuit has a large resistance to limit the base current \(I_B\), preventing the transistor from overheating and burning.
Minimum Base-Emitter Voltage
A minimum base-emitter voltage, \(V_{BE}\), is required to turn on the transistor. For a silicon transistor, the minimum \(V_{BE} = 0.7\text{ V}\). For a germanium transistor, the minimum \(V_{BE} = 0.3\text{ V}\).
Amplification Factor
\(\beta = \dfrac{I_C}{I_B}\)Where \(I_C\) is collector current and \(I_B\) is base current. A small increase in base current produces a large increase in collector current.
Potential Divider Output Voltage
\(V_{\text{out}} = \left(\dfrac{R_2}{R_1 + R_2}\right) V_{\text{in}}\)Where \(R_1\) and \(R_2\) are two series resistors connected across an input voltage \(V_{\text{in}}\).
| Sensor | Condition | Resistance Change | Effect on Transistor |
|---|---|---|---|
| LDR | Dark | High resistance | \(V_{\text{LDR}}\) increases; transistor turns on |
| LDR | Bright light | Low resistance | \(V_{\text{LDR}}\) falls below \(V_{BE}\); transistor turns off |
| Thermistor (NTC) | Temperature rises | Resistance decreases | \(V_R\) increases; transistor turns on |
| Thermistor (NTC) | Room temperature | High resistance | \(V_R\) below \(V_{BE}\); transistor stays off |
Try to Answer First
Answer in your mind, then press “Check Answer”.
What is the main function of the base (\(B\)) terminal in a transistor?
Check Answer
State the formula for the amplification factor, \(\beta\), of a transistor amplifier.
Check Answer
In a temperature-controlled switch using an \(npn\) transistor where the thermistor is connected at the top and a fixed resistor \(R\) is connected across base-emitter at the bottom, why does the transistor turn on when temperature rises?
Check Answer
Common Mistakes
- Confusing the emitter arrow direction between \(npn\) and \(pnp\) transistors. Remember: \(npn\) arrow points outwards (\(B \rightarrow E\)); \(pnp\) arrow points inwards (\(E \rightarrow B\)).
- Forgetting that current cannot flow in the collector circuit even if the collector battery is connected, unless base current \(I_B\) is actively flowing.
- Neglecting the role of the base limiting resistor \(R_B\) and assuming it is unnecessary. It is essential to limit \(I_B\) and prevent the transistor from overheating and burning.
- Applying the potential divider formula upside down by taking the resistance of the wrong resistor in the numerator.
Concept Misunderstandings
The transistor creates energy or current out of nothing to achieve amplification.
The transistor does not generate energy; the large collector current is supplied by the external collector power supply/battery, controlled and modulated by the tiny base current.
An LDR has high resistance under bright light.
An LDR has high resistance in the dark and low resistance under bright light.
The collector current and base current are equal.
The base current \(I_B\) is very small (often in microamperes or low milliamperes), while the collector current \(I_C\) is much larger (\(I_B < I_C < I_E\)).
Summary
- A transistor consists of three terminals: Emitter (\(E\)), Base (\(B\)), and Collector (\(C\)).
- In an \(npn\) transistor, current flows from \(B\) to \(E\); in a \(pnp\) transistor, current flows from \(E\) to \(B\).
- \(I_E = I_B + I_C\), where \(I_B \ll I_C\).
- Amplification factor: \(\beta = \dfrac{I_C}{I_B}\).
- Potential divider relation: \(V_{\text{out}} = \left(\dfrac{R_2}{R_1 + R_2}\right) V_{\text{in}}\).
- A silicon transistor requires a minimum \(V_{BE}\) of \(0.7\text{ V}\) to turn on (\(0.3\text{ V}\) for germanium).
- LDR resistance drops in light and rises in the dark; thermistor resistance drops when heated and rises when cold.
Short Activity
Objective: Test your understanding of transistor terminals, amplification, and automatic switching.
A. Objective Quiz
1 Which statement correctly explains why a transistor can function as a current amplifier?
2 What is the minimum base-emitter voltage (\(V_{BE}\)) required to turn on a silicon transistor?
B. Fill in the Blanks
3 In a transistor circuit, the resistor \(R_B\) is connected to the base circuit to limit the base current so that the transistor will not become too hot and .
4 The ratio of collector current to base current, given by the formula \(I_C / I_B\), is known as the factor of the transistor.
C. Matching / Drag and Drop
Match each transistor component or terminal to its correct textbook description. If using a phone, tap the answer first, then tap the matching box.
