Classification and Circuit Composition of Power Amplifier

Class a working state: the collector current of the transistor is always flowing throughout the working cycle, the efficiency of the amplifier is the lowest, and the nonlinear distortion is relatively small. It is generally used in situations sensitive to distortion, such as hi-fi sound.

Class B working state: it works for half a cycle and ends for the other half. Class B working state is also called class B working state. Two complementary transistors work in push-pull mode, and the efficiency is higher than that of class a power amplifier, but there is the problem of crossover distortion. This form is generally used in power amplifier.

Class A and class B working state: it is between Class A and class B, that is, the transistor working cycle is greater than half. The characteristics of this power amplifier are between Class A and class B.

Class C working state: in this state, the working time of the transistor is less than half a cycle. Class C working state is also called class C working state. Class C power amplifier is generally used for high-frequency resonant power amplifier.

Class D working state: the sound signal is modulated into PWM form, the transistor works in the switching state, and the output end recovers the signal waveform through LC filter. High efficiency and poor high-frequency characteristics. It is used for miniaturized battery power supply and occasions requiring high efficiency.

According to different working conditions, power amplifiers are classified as follows:

The working frequency of traditional linear power amplifiers is very high, but the relative frequency band is narrow. RF power amplifiers generally use frequency selection network as load loop. RF power amplifiers can be divided into three working states: a (a), B (b) and C (c) according to different current conduction angles. The conduction angle of class a amplifier current is 360 °, which is suitable for small signal and low power amplification. The conduction angle of class B amplifier current is equal to 180 °, and the conduction angle of class C amplifier current is less than 180 °. Both class B and class C are suitable for high-power working state. The output power and efficiency of class C working state are the highest among the three working states. Most RF power amplifiers work in class C, but the current waveform distortion of class C amplifier is too large, so it can only be used for load resonant power amplification with tuning loop. Because the tuning loop has filtering ability, the loop current and voltage are still close to sinusoidal waveform, and the distortion is very small.

Switching mode PA (SMPA) enables electronic devices to work in the switching state. The common ones are class D amplifier and class E amplifier. The efficiency of class D amplifier is higher than that of class C amplifier. SMPA drives the active transistor to the switching mode. The working state of the transistor is either on or off. There is no overlap between the time domain waveforms of voltage and current, so the DC power consumption is zero and the ideal efficiency can reach 100%.

Traditional linear power amplifier has high gain and linearity, but low efficiency, while switching power amplifier has high efficiency and high output power, but poor linearity. See the following table for details:

Circuit composition

There are different types of amplifiers. To simplify, the circuit of the amplifier can be composed of the following parts: transistor, bias and stability circuit, input and output matching circuit.

1. Transistors

There are many kinds of transistors, including the invention of transistors with various structures. In essence, the transistor works as a controlled current source or voltage source, and its working mechanism is to convert the DC energy without content into a "useful" output. DC energy is obtained from the outside, consumed by transistors and converted into useful components. Different transistors have different "capabilities", such as their ability to withstand power, which is also due to their ability to obtain DC energy; For example, its reaction speed is different, which determines how wide and high the frequency band it can work in; For example, its impedance facing the input and output terminals is different, and its external response ability is different, which determines the difficulty of matching it.

2. Bias circuit and stabilization circuit

Bias and stabilization circuits are two different circuits, but they are often difficult to distinguish, and the design objectives converge, so they can be discussed together.

The transistor needs to work under certain bias conditions, which we call the static operating point. This is the foundation of the transistor and its own "positioning". Each transistor has a certain positioning for itself. Its different positioning will determine its own working mode, and there are different performance in different positioning. Some positioning points have small fluctuation and are suitable for small signal operation; Some positioning points fluctuate greatly, which is suitable for high-power output; Some positioning points require less and release pure, which is suitable for low-noise work; At some fixed points, the transistor always hovers between saturation and cut-off and is in the on-off state. An appropriate offset point is the basis for normal operation. When designing broadband power amplifier, or when the working frequency is high, the bias circuit has a great impact on the circuit performance. At this time, the bias circuit should be considered as a part of the matching circuit.

There are two types of offset networks, passive networks and active networks. Passive networks (i.e. self biased networks) are usually composed of resistance networks to provide appropriate operating voltage and current for transistors. Its main defect is that it is very sensitive to the changes of transistor parameters and has poor temperature stability. Active bias network can improve the stability of static operating point and good temperature stability, but it also has some problems, such as increasing the circuit size, increasing the difficulty of circuit layout and increasing the power consumption. The stabilizing circuit must be before the matching circuit, because the transistor needs to exist the stabilizing circuit as a part of itself and then contact with the outside world. In the view of the outside world, the transistor with a stable circuit is a "new" transistor. It makes certain "sacrifices" and obtains stability. The mechanism of stabilizing the circuit can ensure the smooth and stable operation of the transistor.

3. Input output matching circuit

The purpose of matching circuit is to select an acceptable way. For transistors that want to provide greater gain, the way is overall acceptance and output. This means that through the interface of matching circuit, the communication between different transistors is more smooth. For different amplifier types, matching circuit is not only a design method of "overall acceptance". Some small tubes with small DC and shallow foundation are more willing to block when accepting to obtain better noise performance. However, they cannot block too much, otherwise their contribution will be affected. For some giant power tubes, they need to be cautious in output, because they are more unstable. At the same time, a certain reservation will help them play more "undistorted" energy.

Typical impedance matching networks include l-matching, π - matching and T-Matching. Among them, l matching is characterized by simple structure and only two degrees of freedom L and C. Once the impedance transformation ratio and resonant frequency are determined, the Q value (bandwidth) of the network is also determined. One advantage of π - shaped matching network is that no matter what kind of parasitic capacitance is connected to it, it can be absorbed into the network, which also leads to the universal application of π - shaped matching network, because in many practical situations, the dominant parasitic element is capacitance. T-shape matching, when the parasitic parameters at the power supply end and load end are mainly inductive, T-shape matching can be used to absorb these parasitic parameters into the network.