General Problems of Power Amplifier Circuits

1. Definition of power amplifier circuit

A power amplifier circuit is an amplifier circuit that aims to output a large amount of power. It usually drives the load directly and has a strong load capacity.

We have introduced some electronic circuits before. The signals processed by these circuits are often sent to the load to drive certain devices. For example, these devices include the voice coil of the speaker in the radio, the motor control winding, the scanning deflection coil of the computer monitor or TV, etc. At this time, we have to consider not only the output voltage or current, but also a certain power output to make these loads work normally. This type of amplifier circuit that is mainly used to provide power to the load is often called a power amplifier circuit.

The main performance indicators of power amplifier circuits include: maximum output power, efficiency, etc.

2. The difference between power amplifier circuit and voltage amplifier circuit

The amplifier circuits discussed in the previous chapters are mainly used to enhance the voltage amplitude or current amplitude, and are therefore called voltage amplifier circuits or current amplifier circuits accordingly. However, no matter which amplifier circuit is used, the output voltage, current, and power are all present on the load at the same time. From the perspective of energy control, amplifier circuits are essentially energy conversion circuits, and there is no essential difference between power amplifier circuits and voltage amplifier circuits. The difference in the above names is just the emphasis on different output quantities.

However, the tasks to be completed by power amplifier circuits and voltage amplifier circuits are different. The main requirement for voltage amplifier circuits is to make the load obtain an undistorted voltage signal. The main indicators discussed are voltage gain, input and output impedance, etc. The output power is not necessarily large. The power amplifier circuit is different. It mainly requires a certain undistorted (or less distorted) output power, usually working under large signal conditions. Therefore, the power amplifier circuit contains a series of special problems that have never appeared in the voltage amplifier circuit.

3. Special issues of power amplifier circuits

(1) The output power is required to be as large as possible: In order to obtain a large power output, the voltage and current of the power amplifier tube are required to have a sufficiently large output amplitude, so the tube often operates at a state close to the limit.

(2) High efficiency: Since the output power is large, the power consumed by the DC power supply is also large, which leads to an efficiency problem. The so-called efficiency is the ratio of the useful signal power obtained by the load to the DC power supplied by the power supply. The larger this ratio is, the higher the efficiency is.

(3) Nonlinear distortion should be small: The power amplifier circuit works under large signals, so nonlinear distortion will inevitably occur. Moreover, the greater the output power of the same power amplifier tube, the more serious the nonlinear distortion is. This makes output power and nonlinear distortion a pair of major contradictions. However, in different occasions, the requirements for nonlinear distortion are different. For example, in measurement systems and electroacoustic equipment, this issue is important, while in industrial control systems and other occasions, output power is the main purpose, and the requirements for nonlinear distortion are reduced to secondary issues.

(4) Heat dissipation of BJT: In the power amplifier circuit, a considerable amount of power is consumed in the collector junction of the tube, which increases the junction temperature and the tube case temperature. In order to make full use of the allowable tube power consumption and make the tube output sufficiently large power, the heat dissipation of the amplifier device becomes an important issue.

(5) Parameter selection and protection of power tubes: In power amplifier circuits, in order to output a larger signal power, the voltage that the tubes withstand must be high and the current passing through them must be large. The possibility of damage to the power tubes is also relatively high, so the parameter selection and protection of the power tubes cannot be ignored.

(6) The analysis tasks of the power amplifier circuit are: maximum output power, maximum efficiency and safe operating parameters of the power transistor. In terms of analysis methods, since the tube works under large signals, the graphical method is usually used.

4. Working state of the amplifier circuit

According to the conduction status of the transistor in the amplifier circuit within one cycle of the input sinusoidal signal, the amplifier circuit can be divided into the following three working states:

(1) Class A magnification

During one cycle of the input sinusoidal signal, current flows through the transistor. This operating mode is usually called Class A amplification.

The typical working state of Class A amplification is shown in Figure 1. At this time, iC > 0 throughout the entire cycle, and the conduction angle of the power tube is q = 2p.

(2) Class A and B amplification

In one cycle of the input sinusoidal signal, there is more than half a cycle, and the iC of the triode is > 0, which is called Class AB amplification. Its typical working state is shown in Figure 1 (operate according to the instructions in the figure). At this time, the conduction angle q of the power tube satisfies: p < q < 2p.

(3) Class B amplification

In one cycle of the input sinusoidal signal, there is only half a cycle when the iC of the triode is > 0, which is called Class B amplification. Its typical working state is shown in Figure 1. At this time, the conduction angle q = p of the power tube.

5. Ways to improve efficiency

Efficiency h is the ratio of the useful signal power (i.e. output power Po) obtained by the load to the DC power (PV) supplied by the power supply. To improve efficiency, most of the power supplied by the power supply should be converted into useful signal output power.

In the Class A amplifier circuit, in order to prevent signal distortion, a suitable static operating point needs to be set to ensure that current flows through the triode within one cycle of the input sinusoidal signal. Therefore, when there is a signal input, part of the power supplied by the power supply is converted into useful output power, and the other part is consumed in the tube (and resistor) and dissipated in the form of heat, which is called tube consumption. When there is no signal input, all of this power is consumed in the tube (and resistor). The efficiency of the Class A amplifier circuit is relatively low. It can be proved that even under ideal conditions, the efficiency of the Class A amplifier circuit can only reach 50% at most.

Obviously, if the tube consumption can be reduced, the efficiency can be improved. Static current is the main factor causing tube consumption. Therefore, if the static operating point Q is moved downward, the power output of the power supply is also equal to zero (or very small) when the signal is zero, and the power supplied by the power supply increases when the signal increases. In this way, the power supply power and tube consumption change with the output power, which changes the low efficiency of Class A amplification. The circuits that realize the above idea are Class B and Class AB amplification.

Class B and Class AB amplification are mainly used in power amplifier circuits. Although the static power consumption is reduced and the efficiency is improved, serious waveform distortion occurs. Therefore, in order to keep the tube power consumption small during static state and make the distortion not too serious, measures need to be taken in the circuit structure.