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Old 4th March 2009, 08:04 PM   #1
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Default Cascode VAS miller compensation

Hello All

I am using cascode the VAS for extended response in frequency in open loop. But I am not getting good results, the traditional topology says to put between the base input to emitter, but makes no sense use cascode because the response in frequency will be equal to common-emitter (unless you use values smaller than 1pF) complicated to make the PCB.

My question is, what is the best position to Miller capacitor in cascode topology , that increases the bandwidth in open loop?

Thanks

Traditional compesation cascode:
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Old 4th March 2009, 10:10 PM   #2
AndrewT is offline AndrewT  Scotland
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add an emitter follower after the cascode to lower the output impedance to feed the next stage.
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Old 4th March 2009, 11:00 PM   #3
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Quote:
Originally posted by AndrewT
add an emitter follower after the cascode to lower the output impedance to feed the next stage.
I'm using triple output, is the VAS loading problem.
The problem would be the rough cut of Cc, starting on 1KHz (>1pF)
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Old 4th March 2009, 11:10 PM   #4
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Rafael.luc,
your concern is legitimate. I would try to position the cap between the VAS output and inverting input.
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Old 4th March 2009, 11:29 PM   #5
pooge is offline pooge  United States
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Take a look at the Leach Superamp:

Leach Superamp
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Old 5th March 2009, 01:02 AM   #6
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Quote:
Originally posted by Lumba Ogir
Rafael.luc,
your concern is legitimate. I would try to position the cap between the VAS output and inverting input.
I tried this position, cut rough continues


Quote:
Originally posted by pooge
Take a look at the Leach Superamp:

Leach Superamp
this is my question the position of Cc, with this value(15pF) used in the Leach Superamp has no advantage used cascode in VAS stage, could achieve the same bandwidth using common-emitter
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Old 5th March 2009, 01:18 AM   #7
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Quote:
I tried this position, cut rough continues
It does, necessarily.
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Old 5th March 2009, 01:35 AM   #8
Bonsai is offline Bonsai  Taiwan
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Please post a full schematic and we can then point you in the right direction.
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Old 5th March 2009, 01:58 AM   #9
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Quote:
Originally posted by Lumba Ogir

It does, necessarily.
Not understand, what want to say?


Quote:
Originally posted by Bonsai
Please post a full schematic and we can then point you in the right direction.
Is the same design "blamelless" from Douglas Self, more with VAS cascode and triple output.
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Old 5th March 2009, 02:06 AM   #10
pooge is offline pooge  United States
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Quote:
Originally posted by Rafael.luc

this is my question the position of Cc, with this value(15pF) used in the Leach Superamp has no advantage used cascode in VAS stage, could achieve the same bandwidth using common-emitter
Leach, like others, prefers to maintain a defined GBP for the amp.
Leach used the cascodes to reduce the voltage across the transistors, not to increase bandwidth.

I wouldn't say the cascode still has no advantage, but increasing the GBP is not one that was of primary importance.

From Leach's VAS decription:

"I have been asked why I did not use cascode stages (common emitter followed by common base) for the second stage. The cascode stage minimizes the Miller effect and exhibits a lower input capacitance than the common emitter stage. It is used in applications where bandwidth is to be optimized. A feedback amplifier, however, must have a dominant pole to set its gain bandwidth product. It can be shown that the optimum method to set this pole is to place a capacitor from output to input of the highest gain inverting stage. If I had used cascode amplifiers in the second stage, a capacitor would have to be placed from the output to the input of each to set the dominant pole. Then the cascode stage would have the same bandwidth as the common emitter stage. A cascode stage cannot be used to improve the bandwidth of a feedback amplifier if the gain bandwidth product is to be held constant. If the gain bandwidth product is not intentionally limited to some reasonable value, the amplifier would not be stable. That is, it would oscillate. My Double Barrelled Amplifier page shows a cascode second stage with the Miller compensation capacitors that limit its bandwidth."
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