Views: 4 Author: Site Editor Publish Time: 2018-01-24 Origin: Site
The output impedance of the operational amplifier and the capacitance of the capacitive load may form a resistive and capacitive oscillation. The output impedance and capacitive load form a R-C oscillation at the output stage, thus causing an additional phase lag in the feedback signal. The CMOS amplifier has a high output impedance which will cause the electrode to approach or lower the unity gain frequency of the operational amplifier. The additional phase lag of the electrode will weaken the phase margin of the operational amplifier. The total phase lag of the amplifier will increase the phase angle of the unity gain frequency by more than 180 degrees. In the oscillator, this result results in a total feedback phase shift of more than 180 degrees per unit gain. The output impedance of the CMOS amplifier is between 100 and 500, leading to a relatively low pole frequency. Similarly, the output impedance of the high speed bipolar operational amplifier ranges from 1 to 100, leading to a much higher pole frequency than the CMOS operational amplifier. Thus, the poles are kept away from the unity gain frequency of the device. The capacitive load drive of CMOS amplifiers can be improved by placing output resistors and external "positive feedback" capacitors at the output end. There is a section of the data manual for CMOS operational amplifiers on how to compensate for capacitive loads.