RF Amplifier Basics and Power Amplifier Design

RF amplifiers are essential for boosting signal strength in communication systems. Power amplifiers, a specific type of RF amplifier, are engineered to deliver high power efficiently. This guide covers the fundamentals of RF amplifiers and offers insights into designing power amplifiers for optimal performance. It also delves into RF amplifier stages, power amplifier classes, specifications, applications, and manufacturers. We’ll also touch upon a solid-state RF power amplifier design example.

RF Amplifier Definition

An amplifier is a device or module that boosts (i.e., increases the power of) a radio frequency signal. It’s often referred to as an RF power amplifier due to its use in amplifying radio frequency signals or increasing the power at the input to achieve higher power at the output.

RF Power Amplifier Stages

Here are the typical stages or modules found in an RF power amplifier:

1. Input Matching Network: Used to match the amplifier device with the input 50 Ohm impedance line.
2. Amplifiers (One or in Stages): Depending on the required gain, one or more amplifier devices are used. If multiple devices are used, they are connected either in parallel or in cascade, based on the P1dB (output power at 1 dB compression) requirement.
3. Biasing Network: Used to provide bias/voltage to the device.
4. Accessories Network: These include various methods and facilities for improving the amplifier’s stability and linearity characteristics.
5. Output Matching Network: Used to match the amplifier device with the output 50 Ohm impedance line.

RF Power Amplifier Classes

RF power amplifiers are categorized into different classes based on their operating characteristics. To operate a transistor in a particular class, the gate and drain DC voltages must be carefully biased to a specific operating point (quiescent point or Q-point).

1. Class A: The Q-point is approximately 0.5, biased close to half of its saturated current. The conduction angle is $2\pi$ (360 degrees).
2. Class B: The Q-point is 0, biased at a point where it draws nearly zero DC current. The conduction angle is $\pi$ (180 degrees).
3. Class AB: The Q-point is between 0 and 0.5. The conduction angle is between $\pi$ and $2\pi$.
4. Class C: The Q-point is below 0. The conduction angle is between 0 and $\pi$.

RF Power Amplifier Technical Specifications

Before purchasing an RF power amplifier, consider the following typical specifications:

• Frequency Range: Example: 5925 to 6425 MHz
• 1dB Gain Compression Point: Example: 5 Watt
• Power Gain: Example: 40dB
• Ripple/Response Over the Frequency Band: Example: 2dB
• Input and Output VSWR: Example: 1.25:1 (19dB return loss)

Example Solid State RF Power Amplifier Design

The following figure illustrates the devices used in a 5 Watt Low-Cost Feed Mount Solid State Power Amplifier (SSPA) for C-band applications.

The SSPA design consists of three stages: input, output, and intermediate. The figure shows discrete amplifier MMICs (Monolithic Microwave Integrated Circuits) used in the design.

The power supply section should be designed to convert -48V DC to the appropriate device voltages. Furthermore, an appropriate heatsink should be designed to dissipate heat effectively.

RF Amplifier Applications

RF amplifiers are used in various functions throughout the communication chain.

• When providing gain to the signal before transmission, it’s known as a power amplifier and is connected to the transmitting antenna.
• When connected immediately after signal reception, it’s called a Low Noise Amplifier (LNA). The LNA amplifies the received, often distorted signal without significantly amplifying noise.
• They can also be used as Variable Gain Amplifiers in mobile channels, helping to recover transmitted information in the receiver, even under changing conditions.

RF Power Amplifier Manufacturers

• Anadigics
• Agile Microwave Technology Inc
• Analog Devices
• empower RF
• Skyworksinc

Conclusion

Mastering RF amplifier basics and power amplifier design is crucial for developing efficient communication systems. These concepts are fundamental to wireless and RF applications.