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Old 11th January 2002, 05:21 PM   #1
Won is offline Won
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Default Folded cascode question

Not being an electrical engineer by training, there are pretty big gaps in my circuit knowledge. I'm sure many people here are the same way, having learned basic electronics by staring at enough amplifier circuits.

One particular circuit that always eluded my reasoning is the folded cascode circuit. Would someone care to explain this basic circuit building block? Thanks...Won
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Old 11th January 2002, 05:56 PM   #2
sonnya is offline sonnya  Denmark
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If we start of with a common emitter stage designed around a npn transistor (2n3904) you have three parasitic caps.

between CE
between BE
between BC

BE is ~ 8pF
BC is ~ 4pF

But in a common emitterstage BC is multiplyed by the voltagegain.

If your gain is around 100 times your source will see 8pF(BE) + 400pF(BC*voltage gain).

With a cascode or folded cascode the collector voltage only changes few millivolt on lower transistor. So your source will only see 8pF + 4pF.

You will also get a faster stage because of the smaller capacitive load of stage before this one.

That is the short story!

Try Borbeley Audio's homepage. He has some papers on this subject with JFET instead. But it have the same effect on them.

Sonny
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Old 11th January 2002, 06:25 PM   #3
Won is offline Won
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Sonny, I knew what the cascode/folded-cascode did and why it was useful. Sorry, I should've been more specific.

My question was how to implement one, or use one in an amplifier circuit. The cascode is easy, because all you need is to stack some transistors, but there's a trick in the "folding" that I can't quite figure out from what I've seen.

Thanks, Won
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Old 11th January 2002, 07:10 PM   #4
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Another reference is the A75 article at either passdiy
or passlabs.com, where it explains in some detail the
operation of a folded cascode circuit and gives the
example to apply to that amplifier.
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