If the magnitude plot of your closed-loop gain zooms off to infinity, you're unstable. The equation for closed loop gain is A/(1+ A*beta). You can do the analysis with the whole equation, but the A*beta part is the key part for stability. By just focusing on that piece rather than the whole equation, most folks find the analysis easier. I'd recommend the previous item in the series, "Loop Gain in Operational Amplifiers" if you haven't already seen it.
A bit late answer... The *closed* loop gain is going to be 10. The attenuator block gain (beta) is going to be 1/10, which is the one discussed in the video. Closed loop gain is not discussed here. Some definitions (as a refresher): Open loop gain: A. The internal gain of the op-amp. Usually really large. Attenuator block gain: B (beta). The gain of the beta network. In this case the resistor divider, that feeds back a fraction of the output to the input. Since the resistor divider feeds back 1/10 of the signal back to the input, we say that B = 1/10. Loop gain: A*B. Closed loop gain: A / (1 + A*B). This is the gain of the entire circuit. Recall that the open loop gain is usually really large, so we can say that the closed loop gain will be 1/B. If you put in B=1/10, you'll get that the closed loop gain is 10.
Hi David, Thanks for your feedback, we are glad that you liked the video!
Nice explanation of stability theory in op-amps. This is a much better way to view op-amp analysis than using the two port models I learned in school.
great video series! thanks Analog Devices
You are welcome!
Great explanation Matt!
Thanks Mohamad!
Can you explain why the phase shift is 90° at cut-off frequency?
Why is it not possible for you to show if an op-amp is stable or not from the magnitude and phase plot of the closed-loop gain??
If the magnitude plot of your closed-loop gain zooms off to infinity, you're unstable. The equation for closed loop gain is A/(1+ A*beta). You can do the analysis with the whole equation, but the A*beta part is the key part for stability. By just focusing on that piece rather than the whole equation, most folks find the analysis easier. I'd recommend the previous item in the series, "Loop Gain in Operational Amplifiers" if you haven't already seen it.
Fantastic...
why is the gain 1/10
, shouldnt it be 10 and the bode plot start at 20db?
A bit late answer...
The *closed* loop gain is going to be 10. The attenuator block gain (beta) is going to be 1/10, which is the one discussed in the video. Closed loop gain is not discussed here.
Some definitions (as a refresher):
Open loop gain: A. The internal gain of the op-amp. Usually really large.
Attenuator block gain: B (beta). The gain of the beta network. In this case the resistor divider, that feeds back a fraction of the output to the input. Since the resistor divider feeds back 1/10 of the signal back to the input, we say that B = 1/10.
Loop gain: A*B.
Closed loop gain: A / (1 + A*B). This is the gain of the entire circuit. Recall that the open loop gain is usually really large, so we can say that the closed loop gain will be 1/B. If you put in B=1/10, you'll get that the closed loop gain is 10.
Thank you gurenig
The scratchy sounds tho....wwwwwwhhooooowowwaaaaaahhhh thanks