Design Of Low Phase Noise Amplifier

The basic circuit of the oscillator is composed of an amplifier and a feedback network.The phase noise of the amplifier contributes a lot to the phase noise of the oscillator output signal.With the development of microwave technology,the communication system requires the phase noise of the oscillator to be as low as possible,which requires us to design an amplifier with a low phase noise level.This article carefully analyzes the source of amplifier's phase noise,summarizes the design methods to reduce amplifier phase noise,and designs two low-phase noise amplifiers on the basis of this theory.The first amplifier adopts an active low-frequency negative feedback structure,a lowphase noise amplifier circuit with working frequency of 480-520 MHz and three active lowfrequency negative feedback networks with gain bandwidth products of 10 MHz,800 k Hz,and 100 k Hz are designed.Then build the absolute phase noise test circuit and the saturation method residual phase noise test circuit to test the phase noise of the amplifier under different input powers and different feedback networks.The experimental results show that the amplifier with a feedback network gain bandwidth product of 800 k Hz,working in a linear state,has the lowest phase noise,as low as-168 d Bc/Hz@1k Hz,which shows that the gain bandwidth product of the feedback network is different,and the amplitude of reducing the phase noise of the amplifier is also different.In the operating frequency,the amplifier's gain is greater than 15 d B,the input/output return loss is less than-15 d B,the noise figure is less than 3.5 d B,and the input 1d B compression point is 7.5 d Bm.The second amplifier is a broadband monolithic low phase noise distributed amplifier.It uses a Ga As HBT process with low 1/f noise and biases the amplifier in a Class A linear state to obtain the lowest phase noise.The gain unit is designed by adopting the Cascode structure,which improves the gain of the amplifier at high frequency and expands the bandwidth.A matching structure is introduced at the input of the amplifier to improve the input return loss.The layout simulation results show that in the 4-10 GHz operating frequency band,the amplifier gain is greater than 20 d B,the input/output return loss is less than-10 d B,the noise figure is less than 3.6 d B,the gain flatness is less than 1 d B,and the input 1 d B compression point is-4.8 d Bm.