Doherty Amplifier: Advantages and Disadvantages

This page explores the advantages and disadvantages of the Doherty Amplifier architecture.

Doherty Amplifier Architecture

The Doherty amplifier employs two amplifier devices within a single unit to achieve a high PAPR (Peak-to-Average Power Ratio) without sacrificing power efficiency. It’s primarily used with signals that have a higher PAPR, commonly found in both FM (Frequency Modulation) and AM (Amplitude Modulation) schemes.

Doherty Amplifier Architecture

The diagram above illustrates the block diagram of a Doherty amplifier.

As shown, the Doherty amplifier comprises a main amplifier and an auxiliary amplifier. The main amplifier is typically a Class AB type, while the auxiliary amplifier is usually a Class C type. The architecture utilizes an RF splitter (a quadrature generator) at the input, which creates two signals: a -90-degree shifted signal (top) and a -180-degree phase-shifted signal (bottom).

These two signal outputs from the amplifiers are 90 degrees out of phase, which can introduce issues. To address this, a quarter-wave impedance transformer is used in the top path, as shown at the output, to bring both signals into the same phase. Subsequently, both of these signals are combined at the output. The combined amplified RF output signal then passes through another quarter-wave impedance transformer to restore it to its normal phase.

The combined use of a Class AB amplifier and a Class C amplifier in the Doherty architecture helps to restore signal fidelity at the output.

Benefits or Advantages of Doherty Amplifier

Here are the benefits and advantages of using a Doherty Amplifier:

• Maximizes Power Efficiency: It maximizes power efficiency while maintaining the linearity of the amplifier, which is crucial for high PAPR signals commonly seen in OFDM modulated signals.
• Reduces Signal Regrowth: It reduces signal regrowth in high PAPR signals compared to a Class AB amplifier operating near its compression region.
• Versatile Application: It is applicable for both low power and high power amplifiers.
• Optimization Potential: There are many ways to optimize the Doherty architecture for different applications, such as adjusting biasing and phasing.
• Balanced Input: The balanced input circuit in the Doherty architecture reduces variations in return loss and magnitude over the power range.

Drawbacks or Disadvantages of Doherty Amplifier

Here are the drawbacks and disadvantages of using a Doherty Amplifier:

• Complex Circuit Topology: The Doherty circuit topology is more complex compared to a classical Class AC amplifier.
• Unsuitable for Constant Carrier Modulation: Doherty architecture is not suitable for applications requiring constant carrier modulation types, such as FM, FSK, and PSK.
• Difficult Tuning: It’s very difficult to tune all the parameters in order to find the best operating point within the Doherty architecture.
• Sensitivity to Input Levels: Input signal levels affect the operating characteristics of the circuit.
• Lower Gain: Doherty amplifier gain is lower (around 3 dB) than a Class AB amplifier due to the power splitter at the input.
• Device Parasitics: Device parasitics complicate the design of real-world amplifiers.