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Old 12th December 2003, 06:21 AM   #1
gfle is offline gfle  Greece
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Question Amplifier "polarity" ?

I've seen amp designs using NPN LTP and PNP VAS and the other way around, like Slone and Self designs.

What are the design criteria that make an engineer chose one way and not the other?

Is it cost, performance or availability?
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Old 12th December 2003, 04:03 PM   #2
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My
NPNs: more variety, especially in matched pairs; faster; probably cheaper if high-volume part.
VAS stage is slugged so a fast device is not a benefit. LTP matching is very important as is low capacitance to reduce collector-base coupling.
PNP VAS means NPN current cources, again lower capacitance and a slight cost advantage.
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Old 12th December 2003, 04:32 PM   #3
sreten is offline sreten  United Kingdom
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PNP's have the lowest noise, thats why Self uses
them in the LTP input as it determines amplifier noise.

The VAS has to be the opposite of the LTP for current
drive into the virtual earth of the VAS TR's base.

If understand correctly the N bit of the PNP has higher
mobility and lower resistance than the P bit of a NPN.

sreten.
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Old 12th December 2003, 05:10 PM   #4
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IMO input noise is not important in this decision. The input stage noise is insignificant compared to other noise sources. I believe Self uses this as a criteria in the absence of superior criteria.
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Old 12th December 2003, 07:23 PM   #5
sreten is offline sreten  United Kingdom
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Quote:
Originally posted by traderbam
IMO input noise is not important in this decision. The input stage noise is insignificant compared to other noise sources. I believe Self uses this as a criteria in the absence of superior criteria.
For what its worth I agree with you - his bootstrapping of the input to get even lower noise is even more pointless -
- but 'justified' by lower output offset - his own books indicate the pre-amplifier completely dominates effective noise.

sreten
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