What is RF Power Amplifier?

Ⅰ. What is RF Power Amplifier?

1. Definition

The radio frequency power amplifier (RF PA) is a critical component of many wireless transmitters, and its significance is self-evident. The power of the RF signal generated by the modulation oscillator circuit in the pre-stage circuit of the transmitter is very low, and it must go through a series of amplification stages (buffer stage, intermediate amplification stage, final power amplification stage) to obtain sufficient RF power before it can be fed Radiate to the antenna. A radio frequency power amplifier is required to obtain sufficient radio frequency output power. The radiofrequency modulated signal is amplified to adequate strength by the RF PA once it is generated by the modulator and then sent by the antenna through the matching network.

2. Principle

Take a close look at the green label. First, the RF signal enters from the left, goes through a gain, gains the "body signal," and then separates the "head signal" from the "body signal" and switches it through a switch. This is referred to as "first level amplification." It can be interpreted as "extensive" by anyone.

After that, we went to an RF power AMP amplifier. This mark is required knowledge for students who have studied analog circuits. The RF signal is amplified with a power of 10KW in this part, which we can call "secondary amplification."

Finally, we utilize a switch to separate the "BODY / HEAD" RF outputs. The primary function of this switch is to control when "head output" and "body output" occur. I may confidently state that the two channels will not operate at the same time.

3. Classification

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

Traditional linear power amplifiers have a very high operating frequency, but their frequency band is relatively restricted. A frequency selective network is commonly used as a load loop in radio frequency power amplifiers. According to the current conduction angle, radio frequency power amplifiers can be separated into three types of working states: A (A), B (B), and C (C). The class A amplifier current has a 360° conduction angle, which is ideal for low-power amplification of tiny signals. The class B amplifier current has a conduction angle of 180°, while the class C amplifier current has a conduction angle of less than 180°. Both Class B and Class C are ideal for high-power working circumstances, with Class C working conditions having the maximum output power and efficiency of the three. Most RF power amplifiers operate in Class C, but Class C amplifiers have too much current waveform distortion and can only be used to boost the resonant power of the load via a tuning loop. The loop current and voltage are still close to sinusoidal waveforms, and the distortion is low, thanks to the tuning loop's filtering capacity.

Changing the Mode Electronic gadgets work in a switching condition thanks to PA and SMPA. Class D (D) and class E (E) amplifiers are both often used. Class D amplifiers have a higher efficiency than class C amplifiers. In switch mode, SMPA drives active transistors. The transistors are either on or off when they are in use. Because the voltage and current time-domain waveforms do not overlap, the DC power consumption is nil, and the optimal efficiency can reach 100%.

Traditional linear power amplifiers have great gain and linearity but low efficiency, whereas switching power amplifiers offer excellent efficiency but poor linearity. Details can be found in the table below:

The amplifier's job is to amplify and output the stuff it receives. The content of input and output, which we refer to as "signal," is frequently expressed in terms of voltage or power. The "contribution" of a "system" like an amplifier is to boost what it "absorbs" to a specific level and "output" it to the outside world. If the amplifier can function well, it will be able to give more, reflecting its own "worth." If the amplifier has certain faults, it will no longer be able to offer any "contribution" after starting to function or working for a period of time, but there may be some unanticipated "oscillation," which is still the amplifier for the outside world. It's all a disaster in and of itself.

Ⅱ. Circuit composition

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

1-1. Transistor

Transistors come in a variety of shapes and sizes, as well as the invention of transistors with multiple architectures. A transistor's job is essentially that of a regulated current or voltage source, and its operating mechanism is to convert the direct current energy content into a "useful" output. DC energy is captured from the environment, consumed by transistors, and transformed into useable components. Different transistors have different "capabilities," such as their ability to tolerate power, which is owing to their different ability to obtain DC energy; for example, their response speed varies, determining how wide and high they can work. On the frequency range, for example, the input and output impedances are different, as is the ability to respond to the outside, which impacts the difficulty of matching it.

1-2. Bias circuit and stabilization circuit

Although bias and stabilization circuits are two distinct circuits, they can be described together because they are often difficult to identify and have similar design aims.

The transistor must operate under specified bias circumstances, which we refer to as the static operating point. The transistor and its own "positioning" are built on this foundation. Each transistor has a unique location that determines its functioning mode. Varying positioning also results in different performances. Some positioning points have small fluctuations, which are suitable for small signal work; some positioning points have large fluctuations, which are suitable for high-power output; some positioning points require less demand, pure release, which is suitable for low-noise work; some positioning points, transistors have large fluctuations, which are suitable for high-power output; some positioning points, transistors have large fluctuations, which are suitable for high-power output; some positioning points, transistors have large fluctuations, which are suitable for high-power output; some positioning points, In the on-off stage, I'm always floating between saturation and cut-off. The foundation for normal operation is an adequate bias point. The bias circuit has a higher impact on the circuit performance when designing a broadband power amplifier or when the operating frequency is high. The bias circuit should now be regarded a component of the matching circuit.

Passive and active bias networks are the two types of bias networks. To supply sufficient operating voltage and current for the transistors, passive networks (also known as self-biased networks) are often made up of resistor networks. Its primary flaw is that it is extremely susceptible to changes in transistor settings and has poor temperature stability. The active bias network may improve the stability of the static operating point as well as temperature stability, but it comes with several drawbacks, including increased circuit size, increased circuit layout difficulties, and increased power consumption.

Because the transistor must have the stabilization circuit as part of itself before contacting the outside world, the stabilization circuit must come before the matching circuit. The transistor with the stabilizing circuit is a "new" transistor in the eyes of the outside world. It attained stability by making certain "sacrifices." The circuit stabilization mechanism can assure the transistor's smooth and steady operation.

1-3. input and output matching circuit

The matching circuit's objective is to select an appropriate approach. Accepting and outputting the entire signal is the way to go for transistors that want to deliver more gain. This means that communication between distinct transistors is smoother thanks to the matching circuit's interface. The matching circuit is not merely a "complete acceptance" design strategy for different types of amplifiers. Some short tubes with a shallow foundation and a small DC are more inclined to accept a certain block in order to improve noise performance. They cannot, however, block too much; else, their contribution will be harmed. You must be cautious while outputting from some huge power tubes because they are more unstable, although at the same time, a certain amount of retention allows them to exert more "undistorted" energy.

L matching, matching, and T matching are examples of common impedance matching networks. L matching is unique in that it has a simple structure and only two degrees of freedom, L and C. The network's Q value (bandwidth) is obtained once the impedance conversion ratio and resonance frequency have been measured. One of the benefits of the -shaped matching network is that it can absorb any parasitic capacitance that is linked to it. Because it is dominant in many practical circumstances, this leads to the general application of the -shaped matching network. The capacitance is the position's parasitic element. T-shaped symmetry. T-shaped matching can be used to absorb parasitic parameters into the network when the parasitic characteristics of the power supply and load terminals are primarily inductive.

Ⅲ.  How to choose the right RF amplifier?

When choosing an RF amplifier for a certain application, factors including gain, noise, bandwidth, and efficiency should be taken into account.

This article will go over the most popular RF amplifiers and discuss how gain, noise, bandwidth, efficiency, and other functional properties influence amplifier selection for diverse applications.

RF amplifiers come in a variety of shapes and sizes to suit a variety of applications. However, choosing the proper RF amplifier for the desired application is difficult due to the large number of RF amplifiers available. Although gain is a significant attribute of practically all RF amplifiers, it is not the only criterion to consider when selecting a device, and in many circumstances, it is not even the most essential.

Gain is the ratio of output power to input power that determines how much signal boost the amplifier can deliver (in dB). It is often specified for the linear mode of the amplifier (in which the change in output power is proportional to the change in input power) (see Figure 1). If you keep increasing the strength of the RF amplifier's input signal, the device will go into non-linear mode and emit spurious frequency components. Harmonics and intermodulation products (HD2, HD3, IMD2, and IMD3 in Figure 2), which reflect the intermodulation distortion (IMD) that emerges at the RF amplifier's output, are among the interference components. The RF amplifier's ability to withstand varying input power levels without severe distortion is reflected in its linearity performance, which can be expressed by a variety of characteristics (see Figure 1), including:

The output power when the system gain is lowered by 1 dB is known as the output 1 dB compression point (OP1dB).

Saturated output power (PSAT) is the output power at which changes in the input power have no effect on the output power.

The input (IIP2, IIP3) and output (OIP2, OIP3) signal power levels are represented by the second-order intermodulation point (IP2) and third-order intermodulation point (IP3), respectively. The relevant erroneous components are present at these locations. The strength will equal that of the fundamental wave component.

Figure 1. The Output Power Characteristics of the RF Power Amplifier and its Nonlinear Parameters

Figure 2. Harmonic and Intermodulation Products.

Ⅳ. What is the Difference Between Different RF Power Amplifiers?

Although the gain is the primary purpose of an RF amplifier, linearity and other properties play a significant role in RF amplifier selection. In fact, selecting an RF amplifier type always necessitates a compromise between many design criteria. Here's a quick guide on picking the proper type of RF amplifier for your application.

Amplifier with low noise

In receiver applications, low noise amplifiers (LNA) are frequently employed to amplify weak signals at the front end of the signal chain that interface with the antenna. When performing this duty, this sort of RF amplifier is designed to introduce the least amount of noise into the signal. Noise minimization is especially critical in the initial few stages of the signal chain, because these stages have the biggest impact on the overall noise figure of the system.

Low phase noise amplifier

Low phase noise amplifiers have very little excess phase noise, making them ideal for RF transmission chains that require great signal integrity. Close-in carrier noise exhibited as jitter is characterized by minor fluctuations in the phase of the signal in the time domain, and is referred to as phase noise. As a result, the low phase noise amplifier is well suited for usage in the LO network with high-speed clock and high-performance PLL frequency synthesizers.

Power amplifier

The power amplifier (PA) is designed for high-power applications, such as transmitter systems, and is optimized for power handling performance. These amplifiers typically have high OP1dB or PSAT characteristics and excellent efficiency, allowing for little heat dissipation.

High linearity amplifier

Over a large input power range, the high linearity amplifier is employed to give a high third-order intermodulation point with a very low spurious level. For communication applications that use complicated modulated signals, this type of device is a popular choice. RF amplifiers that can tolerate high crest factors with minimum signal distortion while retaining a low bit error rate are required for such applications.

Variable gain amplifier

Variable gain amplifiers (VGAs) are utilized in applications that demand variable gain modification to adapt to variations in signal level. VGA accomplishes this by allowing for customizable gain. With a digitally controlled VGA, the gain can be altered gradually, or it can be changed continuously with an analog controlled VGA. Automatic gain control (AGC) and compensation for gain drift induced by temperature or characteristic changes in other components are common applications for this type of amplifier.

Broadband amplifier

Broadband amplifiers may offer medium gain over a wide frequency range (typically many octaves), which is useful for a variety of broadband applications. Large gain-bandwidth products are provided at the expense of mediocre efficiency and noise performance in these amplifiers.

Obtain a block

Gain modules, a broad category of RF amplifiers that can encompass many frequencies, bandwidths, gains, and output power levels, can also be used in other general-purpose RF applications. The gain response of these amplifiers is usually flat, and the return loss is low. Its design frequently contains matching and bias circuits, allowing it to be integrated into the signal chain with few external components, reducing labor time.

The output power and efficiency of RF power amplifiers are the most important technical indicators. The fundamental design goals of RF power amplifiers are to improve output power and efficiency. An LC resonant circuit can be used to choose the fundamental frequency or a specific harmonic in a radio frequency power amplifier to provide distortionless amplification. In addition, to avoid interference with other channels, the harmonic components in the output should be as reduced as feasible.