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a ridiculous 'overshoot' (Read 4720 times)

chaojixin

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Shanghai

a ridiculous 'overshoot'
Jul 26^th, 2011, 6:43pm

I encounter a ridiculous 'overshoot' in a PGA design as shown in (B) which is �is quite different from the normal overshoot as shown in (C). the phase margin is enough (80 degree), and the opamp is a cascode + gain-boost structure. but when I reduce the W/L of input MOS pair and keep bias current �unchanged, the response of Vo becomes normal as shown in (D). �
what does this ridiculous 'overshoot' come from? is it something to do with the parameter setting of input MOS pair?

thanks!

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chaojixin

chaojixin

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Lex Senior Member Offline Posts: 201 Eindhoven, Holland	Re: a ridiculous 'overshoot' Reply #1 - Jul 27^th, 2011, 2:37am First of all, let me ask u whether your transient repsonses are small signal, or large signal. For large signals, you might get some slewing effect in the way. If you simulated a small signal, I think this kind of overshoot should be predictable by AC analysis.
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RobG Community Fellow Offline Posts: 570 Bozeman, MT	Re: a ridiculous 'overshoot' Reply #2 - Jul 27^th, 2011, 4:29pm That looks like a zero in the transfer function not being cancelled exactly by a pole (i.e. a pole/zero doublet).
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chaojixin

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Shanghai

Re: a ridiculous 'overshoot'
Reply #3 - Jul 27^th, 2011, 10:51pm

I forgot something: it is large signal response. the ridiculous 'overshoot' is only several mV and its location is very closed to steady state value. but it did slow down the setting up

chaojixin wrote on Jul 26^th, 2011, 6:43pm:

I encounter a ridiculous 'overshoot' in a PGA design as shown in (B) which is �is quite different from the normal overshoot as shown in (C). the phase margin is enough (80 degree), and the opamp is a cascode + gain-boost structure. but when I reduce the W/L of input MOS pair and keep bias current �unchanged, the response of Vo becomes normal as shown in (D). �
what does this ridiculous 'overshoot' come from? is it something to do with the parameter setting of input MOS pair?

thanks!

chaojixin

chaojixin

278916917

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wave Senior Member Offline Posts: 117 Silicon Valley	Re: a ridiculous 'overshoot' Reply #4 - Jul 28^th, 2011, 1:18pm What "large signal" amplitude are you putting in? It could be you are slewing most of the way and small signal is not relevant.
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loose-electron Senior Fellow Offline Best Design Tool = Capable Designers Posts: 1638 San Diego California	Re: a ridiculous 'overshoot' Reply #5 - Aug 4^th, 2011, 7:55am It can also be a transient from input to output capacitively coupling thru the transistors. If you are trying a "step response" you will see the step couple through system.
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