Transistors -- Elementary Question

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MARTIN MARRIS:
I have an outstandingly elementary question regarding transistors. I've pulled some nice output transformers and a speaker from a beat-up 1950s transistor radio. I have the schematic for the radio's push-pull output stage which I would like to use as the template for the additional audio output stage that I'm adding to my 3-transistor receiver, which currently functions only with high-impedance headphones. This would enable me to use low-impedance headphones and get an adequate 250mW output from the 3-ohm speaker.

But the original schematic has PNP transistors and my receiver has a negative ground. The schematic uses the 1950s equivalent of today's NTE100 germanium transistors. Can I just use the NTE NPN equivalent (the NTE102) and the same circuit, merely reversing the polarity of the supply?

The link the to picture of the schematic is now fixed and you can see it here:

http://tinyurl.com/luk6hbn

(Sorry, the little knowledge that I have about electronics is almost all tube-based but I unexpectedly ended up in a transistor-based building spree!)

73 de Martin, KB1WSY

Clark McDonald:
There are plenty examples of newer designs that would use the Silicon NPN output scheme. 

Suggest you look for one of those instead. 

You might also consider simply using an AF power amp IC as well...


73

MARTIN MARRIS:
Quote from: KE3WD on June 23, 2013, 05:29:24 PM

There are plenty examples of newer designs that would use the Silicon NPN output scheme.  


Indeed. For that matter, this AF output stage is going into is a 45-year-old receiver design and I could have done better with a more recent design, or just dropped in an IC, or just bought a modern receiver off the shelf. What I'm doing is a deliberate excercise in trying to stay within the technology of that period -- not everyone's cup of tea but that's the idea....

Quote from: Clark McDonald on June 23, 2013, 05:29:24 PM

There are plenty examples of newer designs that would use the Silicon NPN output scheme.  


Yes but if you humor me and look at it within the technological limitations of the time, can I just use NPN germaniums in place of the PNPs? I've read a bit of theory about transistors and this is never really explained. For instance this p/n comes in both NPN and PNP versions:

http://www.nteinc.com/specs/100to199/pdf/nte102.pdf

I do realize that I am probably asking some dumb questions ... sorry.

73 de Martin, KB1WSY

Wesley Moore:
You can use the same basic circuit but you will need to
adjust the resistor values to account for biasing
differences.  The circuit you are trying to use  utilized
germanium devices and I suspect you will be using
more modern silicon products. Also the newer devices
will have much higher dc gains then their earlier siblings
which you need to address.

Sorry to be such a wet blanket.

Chuck WD4HXG

Nelson Derks:
CLICK HERE
...then look through the list of headset and low wattage amplifiers. Not much to building what you need using discrete parts.

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