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- Release Time： 2026-01-19 01:31 Source： This site
- The core component of an amplifier circuit is the transistor, so it is necessary to have a certain understanding of it. There are many types of amplifier circuits composed of transistors; let's explain using a few commonly used ones (as shown in Figure 1). Figure 1 is a basic Common Emitter (CE) amplifier circuit. Generally, what content should we master regarding amplifier circuits?
(1) Analyze the role of each component in the circuit;
(2) Understand the amplification principle of the amplifier circuit;
(3) Be able to analyze and calculate the static operating point of the circuit;
(4) Understand the purpose and methods of setting the static operating point;
Understanding the Purpose and Methods of Setting the Static Operating Point
An amplifier circuit outputs an input signal after amplifying it (generally including voltage amplification, current amplification, and power amplification, which are not discussed here). Let's start with the signal we want to amplify, taking a sinusoidal AC signal as an example. During the analysis, we only consider that the signal changes in magnitude, both positively and negatively. As mentioned above, in the amplifier circuit of Figure 1, the static operating point is set such that is close to half of the supply voltage. Why? This is to provide symmetrical variation space for both the positive and negative parts of the signal. When there is no signal input (input is 0), assuming is half of the supply voltage, we treat it as a horizontal line serving as a reference point. When the input signal increases, increases, and the current increases. Consequently, the voltage across resistor () increases, and decreases. Theoretically, can reach a maximum equal to , meaning can reach a minimum of . This means that when the input signal increases, the maximum change in is from to .
Similarly, when the input signal decreases, decreases, and the current decreases. The voltage across resistor () decreases, and increases. When the input signal decreases, the maximum change in is from to . Thus, when the input signal fluctuates positively and negatively within a certain range, using as a standard allows for a symmetrical range of positive and negative changes. Therefore, the static operating point in Figure 1 is generally set so that is close to half of the supply voltage.
Designing to be close to half of the supply voltage is our objective, but how can we achieve this? That is the method.
First, you need to know a few things. The first are the and we often mention, which are the collector current and base current of the transistor. They have the relationship . However, when we first start learning, teachers often do not explicitly tell us what values of and are appropriate. This is a difficult question to answer because it involves many factors. Generally speaking, for low-power transistors, is usually set between a few tenths of a milliampere to several milliamperes; for medium-power transistors, it is between several milliamperes to tens of milliamperes; and for high-power transistors, it ranges from tens of milliamperes to several amperes.
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