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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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16th Oct 2012, 9:46 am | #1 |
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Transistor used as bias diode
During my recent radiogram repair, I encountered a transistor in the dc coupled amp with the base emitter junction used as a diode.
This I assumed to be for biasing. As the transistor tested okay I did not replace it. The transistor is a 2SC1129 wich is listed for vhf use. As the amp now uses BC558s ect. I wondered if I need to change the bias 'diode' so that it will have the same temperature characteristics as the rest of the transistors used or will it have little difference? Thanks, Rob |
16th Oct 2012, 10:44 am | #2 |
Dekatron
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Re: Transistor used as bias diode
What you have here is one of those "it shouldn't work, but it probably will if you don't tell anybody" situations.
Try warming the amplifier board with a hairdrier and cooling it with freezer spray, and listening out for any obvious distortion.
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16th Oct 2012, 10:58 am | #3 |
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Re: Transistor used as bias diode
Why shouldn't it work? All BJTs follow the Ebers-Moll model fairly accurately.
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16th Oct 2012, 5:04 pm | #4 |
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Re: Transistor used as bias diode
Transistors were often used in place of diodes for stabilisation. I suspect that the 2SC device would not be the original one from a Thorn radiogram, but should be as good as any other silicon junction. It's only the thermal characteristics that are being harnessed.
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16th Oct 2012, 5:15 pm | #5 |
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Re: Transistor used as bias diode
As long as you're replacing small-signal silicon with small-signal silicon, I'd expect it to work, as G8HQP Dave and AC/HL have observed.
If you were replacing with a 100A power transistor, then you may not get quite enough bias voltage generated. But that's not the case. As it's just used as a two-terminal device, you could even swap between NPN and PNP, with polarity reversal. |
16th Oct 2012, 6:25 pm | #6 |
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Re: Transistor used as bias diode
May well be used because it's easier to 'clamp' a transsitor case ot the heatsink rater than a diode.
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16th Oct 2012, 7:23 pm | #7 | |
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Re: Transistor used as bias diode
Quote:
I have memories of a Redifon-designed HF linear amplifier which had a couple of dirty great stud-mount power rectifiers fitted on the heatsink right between the output-transistor quad. Along with two OC35 transistors passing a couple of amps, they worked quite well to stabilise the bias up to heatsink-temperatures around 80 Centigrade. |
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16th Oct 2012, 9:03 pm | #8 |
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Re: Transistor used as bias diode
In this case, the transistor is not mounted on a heatsink, the ouput pairs have individual aluminium fins. The transiator does not even sit close to another semi-conductor so heat transfer is not an issue.
Rob |