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Old 8th May 2013, 03:46 AM   #1
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Default Darlington VAS

In Cordell's book in the chapter about evolving a simple amp, he introduces the use of a darlington VAS.

In his example, and pretty much every example of this circuit I've looked at, the collector of the first transistor is grounded, instead of being connected to the collector of the common emitter VAS transistor. He states early in the section that there is "a very good reason for this", and that it will be expanded upon. Unfortunately, he never mentions why, or at least I can't find it. Why is this done?

When I look at it, the first thing that strikes me is that the grounded version can source a lot more current, where as if it were connected to the collector of the other transistor, it would be limited by the current source. I suppose the common emitter transistor would also command less current since it would be sharing with the other transistor. Is this the reason?

The thing is, in that same section he uses another transistor to limit current in the event of negative clipping, so it seems like too much current here is considered a bad thing, and that the current source would limit this from happening, and thus making my earlier thought incorrect.

What is going on in this circuit that I am missing?
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Old 8th May 2013, 04:06 AM   #2
jcx is offline jcx  United States
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"beta enhanced" is another term for this VAS/TIS improvement - calling it Darlington is unfortunate

we want the extra current gain of the added Q - but not the parasitic Miller multiplied Zcb feeding back to the input

by connecting the input Q collector to gnd the parasitic AC feedback is avoided, the low Z EF input Q drives the "Voltage Gain" Q Miller multiplied Zcb without trouble
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Old 8th May 2013, 04:23 AM   #3
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ok, so is the short version that we have the enhanced beta of a darlington, but since the e.f.'s collector is grounded, and thus has no voltage change, it's input capacitance does not change with signal?
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Old 8th May 2013, 04:29 AM   #4
jcx is offline jcx  United States
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yes, or rather - close enough

Last edited by jcx; 8th May 2013 at 04:31 AM.
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Old 8th May 2013, 05:28 AM   #5
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care to get me closer?
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Old 8th May 2013, 05:37 AM   #6
jcx is offline jcx  United States
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just hedging since others will find details to point out and claim a simple "yes" was "wrong"

http://www.essex.ac.uk/csee/research...%20cascode.pdf may be helpful
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