What is a transistor?

The transistor is commonly represented by "Q" and a number in circuits, for example, Q17 represents the transistor numbered 17.

1. Characteristics: A transistor (abbreviated as a transistor) is a special device that contains two PN junctions inside and has amplification capability. It is divided into two types: NPN type and PNP type. These two types of transistors can complement each other in terms of working characteristics. The so-called pair of transistors in OTL circuits are used in combination with PNP type and NPN type. The commonly used PNP transistors in telephones include models such as A92 and 9015; NPN transistors include models such as A42, 9014, 9018, 9013, 9012, etc.

2. The transistor is mainly used for amplifying in circuits, and there are three common connections in circuits. For the sake of comparison, the characteristics of the three transistor connection circuits are listed in the table below for your reference.

Name: Common emitter circuit, common collector circuit (emitter output), common base circuit

The input impedance is high (several hundred ohms to several thousand ohms), high (several tens of thousands of ohms or more), and low (several tens of ohms to several tens of ohms)transistor

Output impedance ranging from several thousand ohms to tens of thousands of ohms is small (several tens of ohms) and large (tens of thousands of ohms to hundreds of thousands of ohms)

The voltage amplification factor (less than 1 and close to 1) is large

The current amplification factor is large (several tens), large (several tens), and small (less than 1 and close to 1)

High power amplification factor (about 30-40 decibels), low power (about 10 decibels), medium power (about 15-20 decibels)

Good frequency characteristics and high frequency difference  transistor

Application of multi-stage amplifier intermediate stage, low-frequency amplifier input stage, output stage, or impedance matching for high-frequency or wideband circuits and constant current source circuits  transistor