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Components and Circuits For discussions about component types, alternatives and availability, circuit configurations and modifications etc. Discussions here should be of a general nature and not about specific sets.

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Old 17th Jan 2020, 12:04 am   #1
Martin G7MRV
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Default Convert an old positive earth PNP transistor circuit to negative earth NPN?

Say I have a simple transistor circuit using a germanium PNP device and a negative supply (positive earth), how would I go about changing that design to convert it to 'modern' NPN devices with a positive supply?

What rules/pitfalls are there? Is it just a case of changing the PNP for an NPN, and reversing any polarity sensitive components?
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Old 17th Jan 2020, 12:15 am   #2
paulsherwin
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Default Re: Convert an old positive earth PNP transistor circuit to negative earth NPN?

Yes, it's that simple if the devices are otherwise identical. Obviously, if you are using modern Si devices in a circuit designed for Ge then there may be other things to change.
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Old 17th Jan 2020, 12:31 am   #3
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Default Re: Convert an old positive earth PNP transistor circuit to negative earth NPN?

Change the PNP germanium device to a PNP silicon device?
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Old 17th Jan 2020, 12:39 am   #4
Martin G7MRV
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Default Re: Convert an old positive earth PNP transistor circuit to negative earth NPN?

Its a pretty simple three transistor pulse amp/integrator, I dont see anything in it that would be particularly different between Ge and Si devices,

Wouldnt changing just PNP Ge for PNP Si still require a negative supply?
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Old 17th Jan 2020, 12:50 am   #5
paulsherwin
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Default Re: Convert an old positive earth PNP transistor circuit to negative earth NPN?

PNP = +ve to emitter
NPN = -ve to emitter

It doesn't matter if the technology is Ge or Si, but the bias will be different.
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Old 17th Jan 2020, 5:57 am   #6
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Default Re: Convert an old positive earth PNP transistor circuit to negative earth NPN?

PNP versus NPN is what sets the polarity of supply needed.

The difference between Silicon PNP and Germanium PNP lies in the amount of bias voltage they need. Because both are PNP, the polarity stays the same.

The difference between Silicon NPN and Germanium NPN lies in the amount of bias voltage they need. Because both are NPN, the polarity stays the same.

So, if you want to chance from Ge to Si, then you may need to twiddle the resistor values in the bias circuitry. If you want to change from PNP to NPN (or vice-versa) then you need to reverse tho power supply polarity and reverse any diodes and electrolytics.

If you want to do both changes at once, say Ge PNP to Si NPN, then you need to both flip the supplies etc, and twiddle the bias resistor values.

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Old 17th Jan 2020, 12:48 pm   #7
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Default Re: Convert an old positive earth PNP transistor circuit to negative earth NPN?

It is perfectly possible to use PNP devices in what one of the Electronics mags. years ago called 'upside down' circuitry. Many Car radios using PNP devices were made dual polarity, and could be set to negative earth without changing any components. True that sometimes left decoupling resistors in the now grounded negative line, but they still worked OK that way.
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Old 17th Jan 2020, 4:52 pm   #8
Martin G7MRV
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Default Re: Convert an old positive earth PNP transistor circuit to negative earth NPN?

Yes, the biasing of course, all makes sense,

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Old 19th Jan 2020, 3:33 pm   #9
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Default Re: Convert an old positive earth PNP transistor circuit to negative earth NPN?

To me the most complex bit would be checking around for electrolytic capacitors and changing / swopping connection for polarity. Germanium transistors are still available (NOS and generally cheap equivalents as well) so I would personally not get too engrossed with radical change.
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