Common methods for improving the linear characteristics of amplifiers
At present, the commonly used methods to improve the linearity of power amplifiers mainly include the following: 1. The power rollback method, when the input power of the power amplifier is reduced by 1dB, the third-order alternating modulation coefficient will improve by 2dB, this method of improving the linearity of the power amplifier by reducing the output power is called the power rollback method. It is a common and effective method to improve the linearity of power amplifier. The advantage is that it is simple and easy to implement. The disadvantage is that the power of the power amplifier is reduced, the capacity of the power amplifier cannot be fully utilized, and the cost of the amplifier is increased. 2. Feedback method, which is a direct feedback method. It uses the nonlinear distortion signal output by the amplifier to feed back to the input terminal through the inverted network, thus offsetting a part of the nonlinear of the amplifier itself. The biggest difficulty faced by this method is how to ensure that the feedback network output signal is reversed to the input signal of the power amplifier in a wide enough frequency band. 3. Feed-forward method, due to the rapid development of broadband communication, feed-forward method has received more and more attention . The basic principle block diagram contains two loops: the main loop around the main amplifier is used to offset the signal, and the auxiliary loop around the auxiliary amplifier is used to offset the distortion, this ensures the high linearity of the final output signal. In this method, the input signal is input into the power amplifier and the nonlinear generator at the same time, and the two nonlinear distortion signals are just offset at the output, but at the same time the fundamental wave signal will also lose part. Similar to the negative feedback method, the difficulty of the feed-forward method lies in how to offset the nonlinear distortion signal generated by the power amplifier and the nonlinear generator in a wider band. 4. Pre-Distortion Method pre-distortion method is widely used because the circuit is relatively simple and can be attached as an independent component to existing amplifiers. The pre-distortion stage generates a nonlinear distortion signal and inputs a power amplifier to offset the nonlinear distortion generated by the power amplifier. Pre-distortion can be realized in microwave band and intermediate frequency band, as well as in baseband. The intermediate frequency pre-distortion circuit has the advantages of convenient adjustment and the nonlinear distortion signal of the upper frequency converter and the power amplifier can be offset together, so it is widely used.