The transistor is typically denoted by a "Q" followed by a number in circuit diagrams. For instance, Q17 represents transistor number 17.
1. **Features**: A bipolar junction transistor (BJT), often referred to as a triode, is a semiconductor device that contains two PN junctions and has the ability to amplify signals. It comes in two main types: NPN and PNP. These two types have complementary working characteristics and are often used together in certain circuits. In OTL (Output Transformerless) amplifier circuits, for example, a PNP transistor is paired with an NPN transistor to achieve balanced operation.
Commonly used PNP transistors in telephone circuits include models like A92 and 9015, while typical NPN transistors include A42, 9014, 9018, 9013, and 9012.
2. **Applications**: The BJT is primarily used for amplification in electronic circuits. There are three common configurations: common emitter, common collector (emitter follower), and common base. Each configuration has different input and output impedance characteristics, as well as varying levels of voltage and current gain.
Here’s a comparison of the three configurations:
| Configuration | Input Impedance | Output Impedance | Voltage Gain | Current Gain | Power Gain |
|---------------------|----------------------|----------------------|--------------|--------------|------------|
| Common Emitter | Hundreds to thousands of ohms | Thousands to tens of thousands of ohms | Greater than 1 | High (tens) | High (30–40 dB) |
| Common Collector | Tens of thousands of ohms or more | Few ohms to tens of ohms | Less than 1 | High (tens) | Low (10 dB) |
| Common Base | Few ohms to tens of ohms | Tens of thousands to hundreds of thousands of ohms | Greater than 1 | Low (less than 1) | Moderate (15–20 dB) |
Each configuration has its own advantages and is chosen based on the specific requirements of the circuit. The common emitter configuration offers high voltage and current gain, making it ideal for general-purpose amplification. The common collector, also known as an emitter follower, provides high input impedance and low output impedance, which is useful for impedance matching. The common base configuration, although less commonly used, is suitable for high-frequency applications due to its low input impedance and good frequency response.
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