Purpose and composition of amplifier circuit: low frequency voltage amplifier amplifier circuit

Knowledge about the purpose and composition of amplifier circuits, the characteristics of amplifier circuits, and several common amplifier circuits: common emitter amplifier circuit, voltage divider bias common emitter amplifier circuit, emitter follower, coupling of low-frequency amplifiers, etc.

Purpose and composition of amplifier circuit

Amplifiers are divided into AC amplifiers and DC amplifiers.

AC amplifiers can be divided into low frequency, medium source and high frequency according to frequency; and divided into voltage amplification, power amplification, etc. according to the strength of the output signal.

In addition, there are amplifiers that use integrated operational amplifiers and special transistors as devices. It is the most complex and changeable circuit in electronic circuits.

But beginners often encounter only a few typical amplifier circuits.

When reading the amplifier circuit diagram, we should still follow the principle and steps of "step-by-step decomposition, grasping the key points, careful analysis, and comprehensive integration". First, separate the entire amplifier circuit step by step according to input and output, and then grasp the key points step by step to analyze and understand the principle.

The amplifier circuit has its own characteristics :

First, there are two working states: static and dynamic. Therefore, it is sometimes necessary to draw its DC path and AC path for analysis.

Secondly, circuits often have negative feedback, which is sometimes within the same level and sometimes from the later level to the previous level, so when analyzing this level, we must be able to "look ahead and look behind."

After understanding the principles of each level, the entire circuit can be connected together for comprehensive integration.

A low-frequency voltage amplifier refers to an amplifier whose operating frequency is between 20 Hz and 20 kHz and whose output requires a certain voltage value but not a very strong current.

1. Common emitter amplifier circuit

Figure 1 (a) is a common emitter amplifier circuit. C1 is the input capacitor, C2 is the output capacitor, the transistor VT is the device that performs the amplification function, RB is the base bias resistor, and RC is the collector load resistor. Terminals 1 and 3 are inputs, and terminals 2 and 3 are outputs. Terminal 3 is a common point, usually grounded, also known as the "ground" terminal. The DC path in static state is shown in Figure 1 (b), and the AC path in dynamic state is shown in Figure 1 (c). The circuit is characterized by a voltage amplification factor ranging from a dozen to more than a hundred, the phase of the output voltage is opposite to the input voltage, the performance is not stable enough, and it can be used in general occasions.

2. Voltage-divided bias common emitter amplifier circuit

Figure 2 uses 3 more components than Figure 1. The base voltage is obtained by dividing the voltage between RB1 and RB2, so it is called voltage division bias.

Add resistor RE and capacitor CE to the emitter. CE is called AC bypass capacitor, which is a short circuit for AC.

RE has a DC negative feedback effect. Feedback means sending the output change to the input end in some way as a part of the input. If the returned part is subtracted from the original input part, it is negative feedback.

The actual input voltage of the base in the figure is the difference between the voltage on RB2 and the voltage on RE, so it is negative feedback.

Due to the adoption of the above two measures, the circuit's working stability is improved, and it is the most widely used amplifier circuit.

3. Emitter follower

Figure 3 (a) is an emitter follower. Its output voltage is output from the emitter. Figure 3 (b) is its AC path diagram, and it can be seen that it is a common collector amplifier circuit.

In this figure, the actual input of the transistor is the difference between Vi and Vo, so this is a circuit with deep AC negative feedback.

Due to the deep negative feedback, the characteristics of this circuit are:

The voltage amplification factor is less than 1 but close to 1, the output voltage is in phase with the input voltage, the input impedance is high and the output impedance is low, the distortion is small, the frequency band is wide, and the operation is stable.

It is often used as the input stage, output stage of an amplifier or for impedance matching.

4. Coupling of low frequency amplifier

An amplifier usually has several stages, and the connection between the stages is called coupling. There are three types of inter-stage coupling in amplifiers:

①RC coupling, see Figure 4 (a). The advantages are simplicity and low cost. But the performance is not optimal.

② Transformer coupling, see Figure 4 (b). The advantages are good impedance matching, high output power and efficiency, but the transformer is more difficult to make.

③ Direct coupling, see Figure 4 (c). The advantage is that the frequency bandwidth is wide and it can be used as a DC amplifier, but the operation of the previous and next stages is restrained, the stability is poor, and the design and production are more complicated.