PNP better than NPN for non-complimentary diff buffer inputs

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mack

New member
Joined
Sep 25, 2005
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Location
New Hampshire
Greetings,

Have been looking at some "modern" discrete/hybrid mic pre designs and noticed that some designs seem to prefer PNPs rather than the ol' tried and true NPNs so loved in engineering school (with the explanation of PNPs just have everything upside down).

I am referring to complete differential buffers made entirely of PNPs including active loads, etc. on the front ends of opamps.

I wonder if there is a something that I am missing or is not obvious in various search engine queries regarding the use of PNPs in this capacity? Mobility funnies perhaps?

Cheers,
-chris
 
i thought i remember reading that one input to an op amp used the power supply harder than the other. so i always attributed that to whether it was pnp or npn. sorry for such a vague response, but i hope some one else could elaborate on it.
 
[quote author="mack"]Greetings,

Have been looking at some "modern" discrete/hybrid mic pre designs and noticed that some designs seem to prefer PNPs rather than the ol' tried and true NPNs so loved in engineering school (with the explanation of PNPs just have everything upside down).

I am referring to complete differential buffers made entirely of PNPs including active loads, etc. on the front ends of opamps.

I wonder if there is a something that I am missing or is not obvious in various search engine queries regarding the use of PNPs in this capacity? Mobility funnies perhaps?

Cheers,
-chris[/quote]

Mobility funnies is correct. The N base of the PNP is better than the P base of the NPN. The difference is not as dramatic as that between N and P FETs, but noticeable.

It's a little harder to make high beta PNP's than NPN's though, so the PNP superiority may not hold at moderately high source impedances. But for trafoless mic pre's with all else equal the PNP's have the lower voltage noise due to the lower r sub bb'.

PS: Welcome to the forum.
 
[quote author="bcarso"]
Mobility funnies is correct.....
It's a little harder to make high beta PNP's than NPN's though...
[/quote]

Ah ha... Thank you folks: this thread has filled the gap in my head from all the analog VLSI courses at school years ago (but no real application except RF HBT NPNs a year ago since then). There is not mention of this in my old texts.

[quote author="bcarso"]
But for trafoless mic pre's with all else equal the PNP's have the lower voltage noise due to the lower r sub bb'.
[/quote]

Excellent.. This is good to know... My latest fun, a year ago, was playing with Heterojunction Bipolar Transistors (HBTs) for RF power amplification near 1GHz of which only "NPNs" were available in the GaAs process that we were using.

Definitely not a mic pre in any regard.

[quote author="bcarso"]
PS: Welcome to the forum.
[/quote]

Thanks for the warm welcome. I have been remiss even thinking about transistors since I have been mostly swamped with writing automation and test software for other high technology jobs unrelated to analog design...

harrumph! Time to get back into the analog design that I miss...

Cheers,
-chris
 
Some of that HBT stuff could have application to audio but no one is likely to work on lowering the excess low freq noise. Barrie Gilbert told me they had some experimental devices at Analog D*vices that had betas of >100,000....
 
> prefer PNPs rather than the ol' tried and true NPNs

If you remember 1958: all we could get (or afford) was a few PNP types. We made it work.

Then when Sillycon was developed, for a long time NPN was all you could get, or at least NPN were a dime to a dollar cheaper.

There are people who like the old BBC and RCA Ge PNP mike preamps; early Neve Silicon that was all NPN, and people who play with the Flickinger and other early Silicon where they used as few PNP as possible. It was mid-late-1970s before we could get either polarity with good performance at good price.

In many fields, you only need one polarity and it hardly matters which. AM radio. Digital Logic. RTL and DTL were totally NPN jobs; I think TTL is total NPN. With NPN, it is easier to think with negative ground, hence the fad for +5.3V supplies for digital (though there was ECL).

Such fashion can dominate new topologies. Before CMOS there was MOS and a lot of that was PMOS. It was RTL implemented with P-type MOSFETs and wired upside-down on a +5V supply.

Negative ground power supplies have dominated since tube days. NPN often makes the most sense in that world. But anything was possible. When we had radio tubes that would do RF and IF and low-audio with a +12V supply, we made cars with 12V positive-hot batteries, and wired the one Delco PNP Germanium POWER transistor upside-down, collector toward ground. That was easy because the ~5K driver tube needed a transformer to couple to the ~5 ohm transistor input, and just as easy to wire the secondary to +12V where the base and emitter pins were. In fact you could ground the collector to the heatsink and take output from the emitter, though still working the transistor as common-emitter.

99.8% of the time, choice of PNP or NPN is suggested by available power polarity and simple thinking, existing designs and boards, or penny-pinching costs.

It is sometimes a great blessing to have both PNP and NPN available, roughly complementary, at similar prices. (But it is interesting how far you can go with just one polarity, or one good polarity and a low-spec "complement" implemented as a parasitic in the main polarity process.)

When you are reduced to sorting through data-sheets and testing samples to find THE BEST, you are forced to take the polarity of the device you pick. While theory may say PNP should have lower noise, that has not always been true of available devices, and when it is "true" the difference may be lost in production spread. NPN may make Beta easier, but in audio systems you often don't need mega-beta. Given that Ft is rarely over 300mHz anyway, higher beta just means beta starts dropping at a lower frequency. If you "depend" on high beta, you run out of bandwidth sooner.
 
they had some experimental devices at Analog D*vices that had betas of >100,000....

I thought of this many times whilst laying out a GaAs ("GAS") chip on the back-end CAD system (yes, I have back-end GaAs experience :twisted: )
Also, the inferred Early voltage was quite interesting for these HBTs.. Our betas weren't that high though (at RF) unless we arrayed them (and ballast resistored the base) Although, we mostly relied on the simulator models as even the fab process was kinda new and "mildly" variable...

If you remember 1958:
just a little before my time... 14 years before my time... None the less I do enjoy reading (and cranking through the math) of the tube/transistor books that I have from the 1940s and 1950s.

The practical advice is as always most useful.. Thanks everyone.

Cheers,
-chris
 

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