Hi,
I'm working on a homework problem and I'm stumped.
The question is:
A measurement of the open loop gain of an internally compensated opamp at very low frequencies shows it to be 98dB; at 100kHz this shows it is 40dB. Estimate values for Ao, fb, ft:
where: Ao is open loop gain; fb is the break frequency; ft is the unity gain frequency.
From what I have learned:
The open loop gain is 98dB.
Now I figured I would use the point slope formula to get the break frequency.
(y2y1) / (x2x1) = m
rearranging gives me:
x1 = x2  (y2y1)/m
where m = 20dB/dec, y2 = 98dB, y1 = 40dB, x2 = 100kHz
Now I know I have to be careful because the xaxis is logarithmic, so x2 would be 10^5 (100kHz)
The second part of the equation => (y2y1)/m gives units of dB/(dB/decade) => decades.
So the first part of the equation (x2) has to be in decades as well.
So then x2 = 100kHz = 5 decades
So then the equation should be:
x1 = 5  (9840)/(20)
x1 = 5  (2.9) = 7.9 decades which would be 10^(7.9) which is definitely WRONG as it should be somewhere down near 10Hz (10^1).
I can't figure out where I'm going wrong, I spoke to the instructor yesterday and he said using the point slope approach was correct. So I'm doing something wrong but I can't figure it out.
I'm working on a homework problem and I'm stumped.
The question is:
A measurement of the open loop gain of an internally compensated opamp at very low frequencies shows it to be 98dB; at 100kHz this shows it is 40dB. Estimate values for Ao, fb, ft:
where: Ao is open loop gain; fb is the break frequency; ft is the unity gain frequency.
From what I have learned:
The open loop gain is 98dB.
Now I figured I would use the point slope formula to get the break frequency.
(y2y1) / (x2x1) = m
rearranging gives me:
x1 = x2  (y2y1)/m
where m = 20dB/dec, y2 = 98dB, y1 = 40dB, x2 = 100kHz
Now I know I have to be careful because the xaxis is logarithmic, so x2 would be 10^5 (100kHz)
The second part of the equation => (y2y1)/m gives units of dB/(dB/decade) => decades.
So the first part of the equation (x2) has to be in decades as well.
So then x2 = 100kHz = 5 decades
So then the equation should be:
x1 = 5  (9840)/(20)
x1 = 5  (2.9) = 7.9 decades which would be 10^(7.9) which is definitely WRONG as it should be somewhere down near 10Hz (10^1).
I can't figure out where I'm going wrong, I spoke to the instructor yesterday and he said using the point slope approach was correct. So I'm doing something wrong but I can't figure it out.
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