Op Amp bandwidth

Discussion in 'Homework Help' started by sharkDawg, May 21, 2008.

May 21, 2008
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Hey guys,

Whats the best way to determine the bandwidth of an op amp apart from looking it up in the datasheet?

2. Papabravo Expert

Feb 24, 2006
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Use a signal generator to create a sinewave and measure the output. Find the corner frequency by changing the frequency until the output is 3 dB down

May 21, 2008
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Thanks Papabravo

Dec 25, 2006
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5. Audioguru New Member

Dec 20, 2007
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Many opamps have "slew rate limiting" where the max output level drops above a certain frequency because the output waveform becomes a triangle-wave. A lousy old 741 opamp has a frequency response to 1MHz when its gain is 1 and its output level is low but its max output level drops above 9kHz due to slew rate limiting.

• uA741.PNG
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May 21, 2008
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Unfortunately not.

So do I just take it that the bandwidth of the op amp is the frequency at which the gain drops 3dB below the output peak? Also did I read somewhere that at the bandwidth frequency the output should be roughly half the output peak, or did i just imagine that?

7. Papabravo Expert

Feb 24, 2006
10,340
1,850
3 dB down is the same as 1/SQRT(2)

As Audioguru's graphic illustrates there is the "bandwidth" of the output being 3dB down and there is the "unity gain bandwidth". Two entirely different animals.

May 21, 2008
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Thanks for the help with the bandwidth guys, got it sorted. One other question, I want to measure the step response of the op amp. To do this I am inputting a 1Vp-p square wave at a low frequency, but I can't see any overshoot on the ouput?

9. Distort10n Active Member

Dec 25, 2006
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This certainly is not a bad thing. Minimal overshoot with no capacitive load would suggest that the amplifier has plenty of phase margin >45°.

What amplifier are you looking at? You can determine phase margin from the open-loop gain vs. phase curve in the datasheet.

May 21, 2008
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Its the AD8651 , the graph your talking about is on page 9 of the data sheet. Im working with a gain of 100, so by my calculation the phase margin is approx. 67°?

11. Distort10n Active Member

Dec 25, 2006
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A closed loop gain of 100 is 40dB. If you drew in a closed loop curve across 40dB, it would intersect the open loop curve ~600kHz and the phase there is -90°. So you have 90 degress of phase margin.

Beautiful, textbook curve IMO.