Advantages of pos ground?

[raulgrell]

I have seen alot of circuits (even silicon based) that use PNP trannies and positive ground. Why not just make it silicon NPN negative ground? Besides the obvious power supply issue, what keeps pos ground from being used?

IIRC the obsidian uses pos ground to eliminate background hiss. Does this work for every circuit, or just mosfet circuits?

[petemoore]

I have seen alot of circuits (even silicon based) that use PNP trannies and positive ground. Why not just make it silicon NPN negative ground? Besides the obvious power supply issue, what keeps pos ground from being used?
  Fuzzface/Tonebender and Rangemaster [ge fuzzes]...started out as PNP Pos Gnd., and work fine. If you have PNP transistors and want one of these circuits, Pos Gnd. is the way to go, some say NPN FF's sound different.
  IIRC the obsidian uses pos ground to eliminate background hiss. Does this work for every circuit, or just mosfet circuits?
  Obsidian is a case of *multiple high gain/clipping' stages. As such noises amplified can be proportionately large compared to signal [BMP is similar in this way] and the P channel Mosfet was found to be lower noise in this application.
  Maybe there's an applicable 'disclaimer clause', but it'd seem that the P channel mosfets Joe chose for the Obsidian might produce less hiss in other mosfet builds, especially if similar *conditions were in place.

[raulgrell]

I'm aware of the FF, RM being pos gnd... My questin was more regarding the reason why this is so. For these specific circuits, IIRC it was simply because that was all that was available. But even circuits developed after the Si revolution when pretty much everything jumped to NPN neg gnd sometimes use PNP/Pos ground... In  essence, my question is this:

Say I'm on a quest to design a distortion stompbox, transistor based...
Would there be any general advantage (sonic/tonal) to having PNP BJT's and pos ground, rather than NPN and negative ground? eg less noise, less RF prone, etc?

Ditto for P/N channel (MOS/J)FETs?

[PerroGrande]

One reason for the existence of positive ground circuits dates back to the days of Germanium transistors.

Due to the nature of the beast (actually, the semiconductor physics of the beast) the performance (notably gain and leakage characteristics) of Germanium were generally better with a PNP configuration than with an NPN configuration.

Your choice today depends on the design goals and available components.  One is not inherently "better" than another -- if all else was equal.   Negative ground is "better" only in so much as it seems to be the defacto "standard." 

[brett]

Hi
Here's how I understand it: back in the sixties, the lack of quality NPN transistors, and ready availabability of reasonably good PNP devices dictated that pos ground power supplies were used.

While there are "hybrid" systems where the input and output are neg ground, but the transistors are isolated by capacitors and powered as in pos ground systems, nobody was doing this in the 1960s.  Many people still consider these systems undesirable and unreliable (due to "motorboating", etc).  The problems relate to separation of DC ground from AC ground.

Once silicon transistors and NPN devices were cheap, neg ground dominated.  (NPN and pos ground can be done, but it raises the same problems as PNP and neg ground).

In conclusion, if you need to use PNP BJTs or P-channel FETs or any similarly doped materials, pos ground is simplest, but neg ground is possible.  And vice versa.  

[ambulancevoice]

the reason joe used those p channel fets is cause like those specific fets are low noise transistors (which is a very good thing in high gain circuits) unlike other n channel fets
i dont think they provide any sort of dramatic tonal difference

[niggez]

Possibly lame question:
If I want to use a power supply instead of battery to power a negative ground MK2 Tonebender, could i just use a regular 9v power supply and switch the polarity of the device??

[ambulancevoice]

depends on the transistors, if your gonna use silicon (or germanium) npn then you can use a negative ground supply.
but if your using pnp, positive ground is needed
there are ways to power a pnp device with a 9v+ supply (neg ground) by running the supply into the emitted instead of the collector, but this usually causes noise and unwanted stuff

[drewl]

Yes, but you'll need to isolate the case.

[niggez]

aw dang...so ill have to stick to the battery for now...grrrrr