NPN on both sides of the output

[sapito]

Hi!
I think someone interrupt a forum.
This question stands:

Using NPN transistors in both sides of the OPS in an amplifier changes the signal for better?

Please answer this.

Regards

Pedro Martins

[janneman]

Hi Pedro,

I think people will have trouble answering your question because it doesn't really say what your problem is. Do you have a specific schematic in mind that you want to change? Do you just have this idea or what? You do of course realise that just changing a PNP for an NPN ends in disaster.
Maybe you should look up the thread on the circlotron amp with power fets, that is using like-polarity devices in the output stage.

Jan Didden

[peranders]

Most designers tend to avoid N-channel or NPN output stages. I think the main reason for this is assymmetry (spelled right?). You get emitter out (low output impedance) for positive signals and collector out (high output impedance) for negative. You can reduce the negative aspects with "compound" transistors and/or feedback. I would say that there are no high performance amp with this kind of design. Am I wrong?

[sapito]

Well, never passed in my mind to change PNP with NPN. That will not cause a disaster. Only a BUMMM!!

I've forgot one thing, The output stage is all with NPN and the drivers are designed to work with that.

The question is: what are the differences in the signal? Is it better, or not?

Tanks 1 more time.

Pedro Martins

[UrSv]

I am not the person to answer this question but just to clarify. I think the question is:

Does the use of NPN transistors only in a push-pull OPS (quasi-complimentary?) result in a better performance than using the conventional complementary NPN and PNP OPS?

I suspect the question is based on the PSSAUDIO (they use is I think) statements in another thread?

A MOSFET version of this concept often discussed the N-Channel?:
http://www.aussieamplifiers.com/downloads/

/UrSv

[sapito]

I like the designs of PSS audio and I've made one of it few years ago and it still work.

Well,

Does the use of NPN transistors only in a push-pull OPS (quasi-complimentary?) result in a better performance than using the conventional complementary NPN and PNP OPS?

Yes, that's what I need to know.


Pedro Martins

[skaara]

I think peranders answered your question, read his post again. I think its better to use npn/pnp than all npn..

[SemperFi]

If the NPN devices are driven by identical, but exactly 180degrees outta phase signals, the output stage will in theory be more linear since the devices are more alike. NPN / NPN pairs have much closer input/output characteristics than NPN / PNP pairs.

As for different impedances as the signal swings positive and negative is wrong. The load sees transistors, transformer, wiring, etc, in series no matter which order they're connected. An amp with NPNs at the negative rail, PNP at the positive and load connected to collectors will have same output impedance as one where the transistors are emitter followers,(with all devices being equal).

Regards Nick

[sapito]

So, I conclued that I must follow the PNP/NPN complementary type of OPS.
I can achieve full simmetrical sinal swing from input to output. Is that right? I must never forget to insert a biasing transistor between the pre-drivers. Is an obligation that this transistor have 'thermal tracking' with tor output transistors?
I'm new in designing amplifiers.
I've always constructed from designs in books and webpages.

When I full design my own amplifier, I'll put it in these forums for discussion.

Many thanks to all of you who reply.

Best Regards,

Pedro Martins

[janneman]

Originally posted by sapito
I like the designs of PSS audio and I've made one of it few years ago and it still work.

Well,

Does the use of NPN transistors only in a push-pull OPS (quasi-complimentary?) result in a better performance than using the conventional complementary NPN and PNP OPS?

Yes, that's what I need to know.


Pedro Martins

Pedro,

Two different issues here:

- The quasi-complementary OPS is using an NPN output device and PNP driver to *simulate* a hi-power PNP. That is not very good, generally.

- The circlotron topology uses two NPN (or FETs) in a symmetrical topology that is much better (like two tubes in a PP arrangement). Again, check the treads, all you ask is there!

Jan Didden