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Old 18th January 2003, 04:00 PM   #1
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Question NPN vs PNP input stage

Can anyone explain the apparent preference for PNP transistors as the input devices in a differential amplifier? I'm designing an amp using 2N3904/2N3906 in a differential input circuit with current mirror collector loads and current source tail. I can't decide which polarity to use for which, though - SPICE simulations haven't helped me clear up the issue either.

I see PNP inputs in many published designs; is there a reason they are considered superior to NPN in audio applications?


Thanks
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Old 18th January 2003, 04:11 PM   #2
halojoy is offline halojoy  Sweden
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Default Re: NPN vs PNP input stage

Quote:
Originally posted by zapo
Can anyone explain the apparent preference for PNP transistors as the input devices in a differential amplifier? I'm designing an amp using 2N3904/2N3906 in a differential input circuit with current mirror collector loads and current source tail. I can't decide which polarity to use for which, though - SPICE simulations haven't helped me clear up the issue either.

I see PNP inputs in many published designs; is there a reason they are considered superior to NPN in audio applications?
Thanks
It does not matter which you use.
What matters is the quality of the input pair.
This is the important. Not if they are P or N.
--------------------------
But often you first decide what output transistors
and Output stage configuration you will use.
And the second stage, the voltage amplifier,
what transistor will you use there?

Have you found some good NPN for voltage amplifier.
Well then you better use PNP in the input pair to drive that
VAS-transistor.

So the later stages can sometimes decide what you use for input.
Sometimes the other way.
Most important is that the total works good together.

And that your design is adjusted to take advantage of
different available transistors.
------------------------
If you have PNP as voltage amplifier,
it is most suitable to have NPN in input.

Quality is more important, than the polarity of the transistors.

/halo - has built amplifiers with either kind of input pairs.
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Old 18th January 2003, 04:28 PM   #3
dhaen is offline dhaen  Europe
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Originally PNP's were lower noise.
I don't know if it is still true or not.

Cheers,
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Old 18th January 2003, 04:33 PM   #4
dimitri is offline dimitri  United States
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Yes, this is still true, the pnp bjts are usually have lower noise, because of the lower base bulk resistance (n-Si). pnp bjts are obligatory for example in moving magnet RIAA amplifier.
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Old 18th January 2003, 04:50 PM   #5
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Lightbulb Re: NPN vs PNP input stage

Quote:
Originally posted by zapo
Can anyone explain the apparent preference for PNP transistors as the input devices in a differential amplifier? I'm designing an amp using 2N3904/2N3906 in a differential input circuit with current mirror collector loads and current source tail. I can't decide which polarity to use for which, though - SPICE simulations haven't helped me clear up the issue either.

I see PNP inputs in many published designs; is there a reason they are considered superior to NPN in audio applications?


Thanks
If you design a full complementary amplifier you will need both transistor polarities i.e. NPN and PNP.
See f.a.:
http://home.kimo.com.tw/skychutw/amp...pzilla_sch.jpg
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Old 18th January 2003, 05:32 PM   #6
halojoy is offline halojoy  Sweden
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Default Re: Re: NPN vs PNP input stage

Quote:
Originally posted by Elso Kwak

If you design a full complementary amplifier you will need both transistor polarities i.e. NPN and PNP.
See f.a.:
http://home.kimo.com.tw/skychutw/amp...pzilla_sch.jpg
Fully complementary, symmetric amplifiers do not exist.
Not with todays transistors.

We can come close to left and right symmetry. (compare Pass designs)
But not Positive Negative symmetry.

The best amplifier designers, have realized this fact.

JFETs & MOSFETs have even worse Complementary Performances.


/halo - almost never designer of comlementary amplifiers,
at least not in the input stage.
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Old 18th January 2003, 05:56 PM   #7
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Cool Re: Re: Re: NPN vs PNP input stage

Quote:
Originally posted by halojoy

Fully complementary, symmetric amplifiers do not exist.
Not with todays transistors.

We can come close to left and right symmetry. (compare Pass designs)
But not Positive Negative symmetry.

The best amplifier designers, have realized this fact.

JFETs & MOSFETs have even worse Complementary Performances.


/halo - almost never designer of comlementary amplifiers,
at least not in the input stage.
Hi Halojoy,
The example I gave was clear enough I think. It is a excisting design!
If you prefer a single frontend not complementary like the JLH design be my guest.
As early as in the seventies I remember a French designer claiming that complementary transistors never could be complementary.
You understand all his schemes were NOT complementary. But James Bongiorno designer of the example in the link claims that all audio signals are bipolar and ineed to be amplified by a full complementary design. John Curl, who I know you admire, also uses complemenatary design in his poweramplifiers. John uses complementary JFET's at the inputs: 2SK389 & 2SJ109.
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Old 18th January 2003, 06:22 PM   #8
halojoy is offline halojoy  Sweden
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Wink Okay ELSO - We differ in likings.

John Curl is a very experienced and great designer.
So I have to think again.

I state my views with whole my heart
good then, that i am not the only voice around.
And that my friends can help to broden the view
as seen from their horizon.

thanks elso
---------------

/halo - takes a stand, when he has one
- and doesn't feel bad for doing so on the contrary!

-------------------------------------------------------------------
PS. you mention one pair of transistors, you claim to be
"fully complementary" in their qualities.
This is what is needed for a full complementary working amp.
I do not know any such N+P, judging from the curves and data.
Maybe there are more fully complementary pairs I do not know of?

I want to know ... I always do
- as far as I know. DS.
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Old 18th January 2003, 06:31 PM   #9
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quote

Fully complementary, symmetric amplifiers do not exist.
Not with todays transistors.


I totally disagree. A fully complementary amp can be built
as long as it is fully balanced input and fully balanced
(i.e. H bridge) output.

Every signal ends up going thru the exact same amount
of pnp and npn transistors. The resulting rise and fall
slew rates are absolutely identical.
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Old 18th January 2003, 06:38 PM   #10
dimitri is offline dimitri  United States
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Hey guys, may be it will be good idea to start new Forums named halojoy and Elso Kwak, where you can put your opinions on nothing and compete with the number of posts? Are all the pubs closed in your city? No girls to piffle with?
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