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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 30th Jan 2019, 7:25 pm   #1
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Default Germanium transistors, new?

To make a simple current limiter with a low voltage burden a germanium transistor would be ideal. As it's for a commercial product are there new germanium transistors available?
 
Old 30th Jan 2019, 7:35 pm   #2
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Default Re: Germanium transistors, new?

For new production I would use a P-gate MOSFET.
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Old 30th Jan 2019, 7:41 pm   #3
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Default Re: Germanium transistors, new?

Not that I'm aware of. There are still lots of NOS parts available if you're prepared to pay the prices, particularly from the former USSR, but it may be a better approach to design around mainstream modern devices.
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Old 30th Jan 2019, 8:50 pm   #4
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Default Re: Germanium transistors, new?

Quote:
but it may be a better approach to design around mainstream modern devices.
I have done that, it works but with a 0.6V burden, I just thought I could reduce that to 0.2..0.3V with a germanium device, doesn't seem like much of a difference but every little helps. I only have a few microamps to play with so a voltage ref. and comparator are out of the question.
 
Old 30th Jan 2019, 10:31 pm   #5
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Default Re: Germanium transistors, new?

At the uA level won't leakage in Ge devices screw things up?
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Old 30th Jan 2019, 10:43 pm   #6
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Default Re: Germanium transistors, new?

I'm sure RS still list the AC127 and AC128, and maybe a few others. I don't think they would list obsolete devices.
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Old 30th Jan 2019, 10:56 pm   #7
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Default Re: Germanium transistors, new?

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At the uA level won't leakage in Ge devices screw things up?
Not where it is only sensing the current, I can't have much leakage to 0V.
 
Old 31st Jan 2019, 12:57 am   #8
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Default Re: Germanium transistors, new?

I have seen P-gate MOSFETs used in E-cigs where they switch a couple of amps from a 4V lithium battery. They are SMD without a heat sink. They just have a pull down with a resistor and a button and timing capacitor.
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Old 31st Jan 2019, 7:39 am   #9
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Default Re: Germanium transistors, new?

I have several OC28's Merlin, your welcome to a few to play with.

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Old 31st Jan 2019, 3:26 pm   #10
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Default Re: Germanium transistors, new?

I have gone back to using Germanium transistors for very low voltages. Russian ones are readily available, and some seem particularly geared toward medium current low frequency use.
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Old 31st Jan 2019, 4:05 pm   #11
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Default Re: Germanium transistors, new?

The switching is the easy bit, NOS germaniums are available, I need a currently manufactured one for traceability. Seems they aren't made anymore, ah well I will cope with 0.6V.
 
Old 31st Jan 2019, 5:12 pm   #12
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Default Re: Germanium transistors, new?

Low power op-amp? Any burden voltage you like, and if board area is a concern there are plenty available in SOT23.
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Old 31st Jan 2019, 8:19 pm   #13
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Default Re: Germanium transistors, new?

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Low power op-amp?
The previous version had one, I haven't the current available in "standby" to power one, let alone a voltage reference for comparison.
 
Old 1st Feb 2019, 12:20 am   #14
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Default Re: Germanium transistors, new?

There are many ways to make a transistor current source. I guess from what you're saying that you're thinking of a 2-transistor "ring of two", with the germanium transistor controlling a silicon pass transistor?

You'll need to supply current to the base of the pass transistor and collector of the Ge transistor. And even if the pass transistor is a MOS-FET, don't forget that reasonable collector current is needed by the sensing transistor if it is to have enough gain to play its part in making a half-decent current source.

Micropower op-amps and references would draw less current than that. I've seen op-amps and voltage references that need less than 10uA. I'd be amazed if you could make a couple of transistors work down at that level sensibly. But then, we don't know the application or the required performance...
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Old 1st Feb 2019, 3:11 pm   #15
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Default Re: Germanium transistors, new?

What suggests itself to me is an LM10, a fairly venerable IC which I have used successfully (combines an op-amp and a precision 200mV reference). One of Bob Widlar's triumphs! It will operate with 1.2V supply - I have powered one with the voltage drop across a LED.

The op-amp is a bit slow and gutless, but has reasonable output drive (sinks current well, sources slightly less well).
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Old 1st Feb 2019, 4:30 pm   #16
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Default Re: Germanium transistors, new?

The LM10 is a lovely little IC. Time Electronics built their 1030 Microcal around it: https://www.markhennessy.co.uk/time_...1030_microcal/

It's nice to have a decent reference that is only 200mV rather than the usual 1.25V or some multiple thereof. And the op-amp inputs work down at ground, so making a current sink with a low dropout only requires the LM10 plus a MOS-FET and the sense resistor. Very easy indeed.

Only downside is the ~300uA consumption. That's very respectable indeed, frankly, but from the few breadcrumbs we have been given so far, that might be too much?

This is a nice active load realised with an LM10: https://www.tekbox.com/product/tboh0...d-active-load/
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Old 1st Feb 2019, 5:10 pm   #17
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Default Re: Germanium transistors, new?

300uA is about 100 times too much.
 
Old 1st Feb 2019, 5:32 pm   #18
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Default Re: Germanium transistors, new?

So what have you done so far?
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Old 1st Feb 2019, 6:00 pm   #19
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Default Re: Germanium transistors, new?

Done it with silicon, I just wanted it to be a bit better (the engineers curse).
 
Old 1st Feb 2019, 6:05 pm   #20
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Default Re: Germanium transistors, new?

I know, but how about a schematic or description of the circuit?

You have the whole thing in front of you, but we're trying to piece it all together from what little you've let on. It's very hard to make suggestions when we're in the dark.

And apart from anything else, I'd like to see how you've made it work on such a small current. I'm sure I'm not alone.
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