An open loop amplifier which has three poles was built ,its open loop voltage gain corresponding to - 180° phase difference was found from the frequency respond which was got from the AC Analysis in Multisim. The extent of feedback coefficient was calculated according to the stability working condition, the feedback resistance of voltage series negative feedback was calculated, then feedback amplifier was built. Changing feedback resistance, observing wave forms of input and output using oscilloscope and doing Fourier analysis using Fourier Analysis in Multisim. The result shows that the wave of oscillation is mainly composed of two components: one is the amplification of input, another comes from self oscillation, whose frequency is equal to the theory analysis, apart from that there are still some other small amplitude harmonics. The process of the start of oscillation,the decrease of the amplitude and in the end the stop of oscillation with the increase of feedback resistance can also be watched.%利用运放构建有3个极点的开环放大电路,由Multisim的AC Analysis获得该电路的频率响应,找到对应相位差为-180°的开环电压放大倍数.根据放大电路稳定工作条件,分析出反馈系数范围,据此计算出电压串联负反馈的反馈电阻,构成反馈放大电路.改变反馈电阻,用Muhisim中的示波器同时观察输入输出波形,并用Fourier Analysis对输出信号进行傅里叶分析.结果显示振荡波形主要由两部分频率成分迭加而成,一部分是对输入信号放大产生的输出信号；另一部分是由于自激振荡产生的输出信号,其频率值与理论分析一致,除此之外,还包含幅值较小的其他频率成分的谐波分量.通过仿真,同时还可以观察到自激振荡的起振过程,随反馈电阻增大,振荡幅度减小到振荡消失等过程.
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