Help me analyze the distortion in feedback opamp

[betty1007]

hi, everyone
i want to analysis the distortion in feedback opamp, but i find it is a hard work.
only HD2 and HD3 is considerated.
first, in open loop:
vout=a1*vin+a2*vin^2+a3*vin^3
then, in unit agin close loop :
vout=a1*(vin-vout)+a2*(vin-vout)^2+a3*(vin-vout)^3
it is very difficult to solve this equation
Does anyone can tell me how to do this?

Thanks in advence

 

[hannerfherder]

Re: about distortion

Hi betty1007,

Please check Laker and Sansen or Grey and Meyer for a low frequency op amp analysis. Also, Razavi in RF Microelectronics cranks through the math, but it is difficult and he makes (valid) approximations to get a closed form solution.

However, the Laker and Sansen approach uses the feedback principle to show that the closed loop harmonics equal the open loop harmonics divided by the loop gain.

For example, say you have 100 dB of amplifier open loop gain at the HD2 and HD3 frequency. When you close the loop with a unity gain negative feedback configuration, the HD2 and HD3 harmonic go down by 100 dB. This is a fairly idealized feedback oriented result, but it is very good for low frequency applications (like audio).

I hope this helps.