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OpAmp Frequency Response Theory 
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#1
Feb2212, 04:08 PM

P: 123

1. The problem statement, all variables and given/known data
We did a lab analyzing this inverting, negative feedback circuit for a 741 opamp: We measured the closedloop gain and phase shift of the signal for several values of the input frequency with [itex]R_2/R1=1000[/itex],[itex]R_2/R1=100[/itex] and [itex]R_2/R_1=10[/itex]. The gain curves all looked like horizontal straight lines for low frequencies, and then some sort logarithmic decrease for larger frequencies. The phase shift curves looked sort of similar; they all started with π radian shift for low frequencies with some sort of logarithmic decrease before leveling off slightly near the end. The problem is, I have no idea what the theory is behind the shape of these curves. We know that the ideal case is [itex] G = \frac{R_2}{R1}[/itex], but the max values of the measured gain weren't even close to that. If someone could point me in the direction of a source that deals with the theory (equations) for this circuit's frequency response, that would be great. 2. Relevant equations Exactly what I need to know. 3. The attempt at a solution I did find one pretty good paper here: http://coe.uncc.edu/~dlsharer/ETEE32...ectionH/H7.pdf On page 4 it gives: [tex]G = \frac{G_o}{1 + s/{\omega_o} + {G_o}{\gamma}}[/tex] But I don't think it's for exactly the same circuit as I have, and it doesn't have any equations about phase shifts. 


#2
Feb2212, 07:58 PM

Mentor
P: 41,274

You can also do some reading about "Dominant Pole Compensation" that is used inside opamps like the LM741: http://www.analog.com/library/analog...2/appleng.html . 


#3
Feb2312, 12:06 PM

P: 123




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