Transistors are typically denoted by the letter "Q" followed by a number in circuit diagrams. For instance, Q17 refers to transistor number 17.
1. Characteristics: A bipolar junction transistor (BJT), also known as a triode, is a device that contains two PN junctions and possesses amplification capabilities. 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 an OTL (Output Transformerless) amplifier circuit, for example, PNP and NPN transistors are paired to work in tandem.
Commonly used PNP transistors in telephone circuits include models like A92 and 9015, while common NPN transistors include A42, 9014, 9018, 9013, and 9012.
2. The primary function of a BJT in an amplifier circuit is to amplify signals. There are three basic connection configurations: common emitter, common collector (emitter follower), and common base. Each has distinct input and output impedance, gain characteristics, and frequency response. Below is a comparison of their key features for reference.
| Configuration | Common Emitter | Common Collector | Common Base |
|---|---|---|---|
| Input Impedance | Several hundred ohms to several thousand ohms | High (tens of thousands of ohms or more) | Low (a few ohms to tens of ohms) |
| Output Impedance | Several thousand ohms to tens of thousands of ohms | Low (a few ohms to tens of ohms) | High (tens of thousands of ohms to hundreds of thousands of ohms) |
| Voltage Gain | Greater than 1 | Less than 1, close to 1 | Greater than 1 |
| Current Gain | High (tens to hundreds) | High (tens to hundreds) | Low (less than 1) |
| Power Gain | High (about 30–40 dB) | Low (about 10 dB) | Moderate (about 15–20 dB) |
| Frequency Response | Good at low frequencies, limited at high frequencies | Good across a wide range | Excellent at high frequencies |
Each configuration has its own advantages and is selected based on the specific requirements of the circuit design. Understanding these differences helps in choosing the right transistor configuration for various applications, such as audio amplification, signal processing, or high-frequency operations.
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