Advantages of Doherty Amplifier | Disadvantages of Doherty Amplifier
This page covers advantages and disadvantages of Doherty Amplifier Architecture. It mentions benefits or advantages of Doherty Amplifier and drawbacks or disadvantages of Doherty Amplifier.
Doherty Amplifier Architecture
The doherty amplifier uses two amplifier devices in single box to achieve high PAPR (Peak to Average Power Ratio) without compromising power efficiency. It is used with higher PAPR signals having both FM (Frequency Modulation) and AM (Amplitude modulation) schemes.
The figure depicts block diagram of doherty amplifier. As shown in the figure, doherty amplifier consists of main amplifier and auxiliary amplifier. Main amplifier is usually class AB type where as auxiliary amplifier is usually class C type. As shown, it uses RF splitter (i.e. quadrature generator) at the input which creates two signals viz. -90 degree shifted (top) and -180 degree phase shifted (bottom). The two signal outputs of amplifiers are 90 degree out of phase. This will create more issues. Hence quarter wave impedance transformer is used in top path as shown at the output. This will bring both the signals to same phase. Both of these signals are added at the output. This combined amplified RF output signal is passed through quarter wave impedance transformer to bring it back to normal phase. The use of both class AB amplifier and Class C amplifier in doherty architecture restores signal fidelity at the output.
Benefits or advantages of Doherty Amplifier
Following are the benefits or advantages of Doherty Amplifier:
➨It maximizes power efficiency while maintaining linearity of amplifier for high PAPR signals usually observed in OFDM modulated signals.
➨It reduces signal regrowth in high PAPR signals in comparison to class AB amplifier operating near compression region.
➨It is applicable for both low power and high power amplifiers.
➨There are many ways one can optimize doherty architecture for different applications such as biasing, phasing etc.
➨The balanced input circuit at doherty architecture reduces variation in return loss and magnitude over power range.
Drawbacks or disadvantages of Doherty Amplifier
Following are the drawbacks or disadvantages of Doherty Amplifier:
➨The doherty circuit topology is more complex compare to classical class AC amplifier.
➨Doherty architecture is not suitable for applications requiring constant carrier modulation types such as FM, FSK, PSK etc.
➨It is very difficult to tune all the parameters in order to find best operating point in doherty architecture.
➨Input signal levels affect the operating characteristics of the circuit.
➨Doherty amplifier gain is lower (~3 dB) than class AB amplifier due to power splitter at the input.
➨Device parasitics complicate the design of real world amplifier.