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Old 12th July 2005, 11:57 PM   #1
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Default MOSFET or NPN for class A output stage

Looking to build a SE Class A amp...

either a MOSFET or NPN output..

what are adv vs disadv?

Can I use a choke to load either of these?

Thanks!
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Old 13th July 2005, 12:34 AM   #2
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NPN driving NMOSFET
cheers
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Old 13th July 2005, 12:38 AM   #3
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Default Re: MOSFET or NPN for class A output stage

Quote:
Originally posted by AudioGeek
Looking to build a SE Class A amp...

either a MOSFET or NPN output..

what are adv vs disadv?

Can I use a choke to load either of these?

Thanks!
Go to the Pass Labs forum. You will be able to find everything you want and more. Also check out http://www.passdiy.com if you haven't already.
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Old 13th July 2005, 07:10 AM   #4
PMA is offline PMA  Europe
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Quote:
Originally posted by darkfenriz
NPN driving NMOSFET
cheers
Or NMOS driving PNP power BJT.
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Old 13th July 2005, 08:23 AM   #5
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There are a lot of pro's and con's for each device. It's impossible to say which is better, because no device is ideal.

MOSFET advantages:

Very high input resistance
No second breakdown

MOSFET disadvantages:

low gm
high and nonlinear input capacitance (HEXFET & co)
sensitive to parasitic oscillations


BJT advantages:

high gm

BJT disadvantages:

low input impedance
second breakdown effects
base currents
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Old 13th July 2005, 08:35 AM   #6
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Not to mention that the Pd losses in a MOSFET output stage are considerably greater than a BJT stage - at least for emitter followers! Even greater waste of energy.
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Old 14th July 2005, 12:39 AM   #7
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Default so for a BJT output stage...

Ok... well back to basics:

how do I design an emitter follower class A SE output stage with BJT using a choke as a load?
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Old 14th July 2005, 10:43 AM   #8
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Quote:
Originally posted by amplifierguru
Not to mention that the Pd losses in a MOSFET output stage are considerably greater than a BJT stage - at least for emitter followers! Even greater waste of energy.
If the output swing is Rail to Rail and the output mosfets are only N-channel Vertical ones than NO BJT ON THIS PLANET DARES TO TOUCH THE EFFICIENCY CRITERIA OF N_CHANNEL VERTICAL MOSFETS...IN CLASS-AB AMPS

Greg,
I donot ..not at all agree with you......at this time.....

K a n w a r
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Old 14th July 2005, 11:04 AM   #9
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Kanwar
Greg considered emitter (source) follower. Do you know any NCHMOS P-P amp with follower output (low open loop impedance)?
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Old 14th July 2005, 11:14 AM   #10
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Quote:
Originally posted by darkfenriz
Kanwar
Greg considered emitter (source) follower. Do you know any NCHMOS P-P amp with follower output (low open loop impedance)?

In N-Mos PP upper rail mos act as source follower amplifier and lower rail mos act as common source amplifier but it can be made to achieve Av=1 using "current gain cell" which qsc uses....
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