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Old 20th June 2007, 06:00 PM   #1
pooge is offline pooge  United States
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Default Diff amp input cap for improved phase margin

In his description of his cascode differential input stage at:

http://users.ece.gatech.edu/~mleach/lowtim/instage.html


Leach states:

"In addition to being part of the input low-pass filter, C1 improves the bandwidth of the feedback signal path through the diff amps for improved phase margin in the loop-gain transfer function."

I can't find any textbook or other such explanation of how this capacitor operates to extend the bandwidth of the feedback signal path through the diff amps, and how it improves the phase margin.

Can anyone give me an explanation of this, or point me to some helpful explanation? Thanks.
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Old 20th June 2007, 10:14 PM   #2
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Long tail pair stage likes to be driven from relatively low source impedance at higher frequencies, because base-collector capacitance plays lesser role then, but this is not as much an issue when LTP is cascoded. If it really plays such a role here, I guess the effect is subtle.
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Old 20th June 2007, 11:26 PM   #3
pooge is offline pooge  United States
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Hmmm. Thanks. I was expecting something else. Do you have any info on how to predict the behavior due to various source impedance?
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Old 21st June 2007, 06:53 AM   #4
Bonsai is offline Bonsai  Taiwan
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Pooge,
this question was asked a few months ago in another thread and the more experienced people were asked to comment (Bob Cordell, JC, some of the IC design folks et al).

If you run Spice simulations, you indeed do see a significant improvement in phase margin with an input filter.

I have my input filter pole set at 150Khz on my amp.

Without the filter, if you stick a square wave in with unrealistic rise/fall times (<<1uS) you can see ringing etc. with the filter of course, this is removed.

So, the reason I use it is to limit the input bandwidth so I dont get the ringing/overshoot. There will be those that will argue (correctly) that a normal music signal does not have very fast rise times, so why use a filter. I use it anyway (also good to kill RF and other out of audio band garbage).

If anyone can offer an explanation as to why the phase margin of the amp is actually improved, please let us know.

thank you
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Old 21st June 2007, 06:54 AM   #5
Bonsai is offline Bonsai  Taiwan
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. . . . with the filter of course, this is removed . . .

Just to clarify, what I mean here is the output waveform no longer exhibits the ringing
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Old 21st June 2007, 07:57 AM   #6
AndrewT is offline AndrewT  Scotland
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search Graham Maynard.
He gave a good (understandable) explanation a week or so back.
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Old 21st June 2007, 11:10 AM   #7
pooge is offline pooge  United States
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Are you talking about this post?:

http://www.diyaudio.com/forums/showt...54#post1226754
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Old 21st June 2007, 11:15 AM   #8
pooge is offline pooge  United States
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Quote:
Originally posted by Bonsai
Pooge,
this question was asked a few months ago in another thread and the more experienced people were asked to comment (Bob Cordell, JC, some of the IC design folks et al).

If you run Spice simulations, you indeed do see a significant improvement in phase margin with an input filter.

I have my input filter pole set at 150Khz on my amp.

Without the filter, if you stick a square wave in with unrealistic rise/fall times (<<1uS) you can see ringing etc. with the filter of course, this is removed.

So, the reason I use it is to limit the input bandwidth so I dont get the ringing/overshoot. There will be those that will argue (correctly) that a normal music signal does not have very fast rise times, so why use a filter. I use it anyway (also good to kill RF and other out of audio band garbage).

If anyone can offer an explanation as to why the phase margin of the amp is actually improved, please let us know.

thank you

Bonsai, not asking about the function of the cap as a part of an input filter for reducing bandwidth. I'm asking about its function of "improving" the bandwidth of the input differential.
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Old 21st June 2007, 11:33 AM   #9
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Hi pooge

Often in power amps, the effects of low impedances are overlooked. People tend to think of a diff. input pair as being optimum but then feed them with moderate impedances (for example, on the feedback side perhaps a 10k feedback resistor is used with about 330 ohm feedback ratio resistor). You can see the effects of a 330 ohm versus 33 ohm resistor on the frequency response using a simulator. (Of course you can run this simulation without a complete amplifier; and if you use 33 ohms you'll need to change the feedback reistor as well)

Therefore keeping the input stage operating from a low input impedance (which is given by the shunt input capacitor) actually has the effect of increasing the bandwidth of the input stage because the impedance is low. Of course there is the loading effect on any series resistance, which acts to filter the upper frequency response if there is a resistor present. You can achieve similar results by using low input impedances (e.g. 50 ohms) but generally preamps aren't designed to drive such low impedances.



cheers
John
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Old 21st June 2007, 11:40 AM   #10
AndrewT is offline AndrewT  Scotland
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Quote:
Originally posted by AndrewT
search Graham Maynard.
He gave a good (understandable) explanation a week or so back.

Quote:
Originally posted by pooge
Are you talking about this post?:

http://www.diyaudio.com/forums/showt...54#post1226754
Sorry, not that one.
It was about the two halves of the LTP operating in common emitter and common base mode. and both halves doing the same thing depending on whether the signal was entering from the inverting side or the non-inverting side.
When the signal comes in from the NFB loop to the inverting side, it forces the input transistor to operate in common base mode. To do this well the base should be tied to a good stable voltage reference (ground), the lower the source impedance the better that common base works. But he says it a lot better than I can manage.
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