So I'm looking to mess around with using germanium transistors as diodes (I have some less than stellar germaniums, and I'd like to put them to use somewhere.) So the internet tells me to tie Collector to Base when using the Base-Emitter junction as a diode. My Atlas Peak tells me that I see a serious drop in Vf when I do this vs. using the Base-Emitter junction and leaving the Collector unconnected. For my purposes, I kind of want the higher Vf. Is there any reason not to leave the Collector floating? Is it going to cause some sort of problem I'm not aware of?
Transistor as diode
- Thread starter hodad
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user 37518
Well-known member
When it comes to using transistor junctions as a diode, you must not only consider whether one is p and the other is n, but the actual dimensions as the diagram shows:
But you can try, it wouldn't be advisable if you have a relatively large current flowing through it, it is not designed for this. Also consider that the reverse breakdown voltage of the BE junction is relatively low, even something like 6V (or less in some cases) can make the junction go into reverse breakdown/zener/avalanche mode.
Transistors as diodes are really not very good diodes.
Edit: used a better diagram.
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I'm not totally sure that matters in my application--distortion pedal, with the transistor in series with a conventional diode. I actually have built the circuit, and I like the sound of it, but figured I'd inquire as to the wisdom of this setup before I actually build the circuit into a more permanent housing. Still not quite clear on the value of tying the Collector to Base, aside from getting a lower Vf (if that's what you're after.)
user 37518
Well-known member
Tying the collector to the base will make some of the carriers of each region to recombine with the other region. Which is probably why you are noticing a change in barrier or forward voltage. Some one more well versed in semiconductor physics might have a better answer.
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I wouldn't expect much difference but I never messed with germanium devices much. A lower voltage with the collector tied to the base suggests that the transistor is turning on. A higher voltage with collector floating suggests the base spreading resistance is giving you an IxR drop in series with the base diode junction voltage. How much current is your tester using? Does the curve look logarithmic, or do you see the linear resistance term in series.
Have you listened to it both ways yet? Which way sounds better?
JR
Have you listened to it both ways yet? Which way sounds better?
JR
user 37518
Well-known member
Yes, with the base connected to the collector the CB junction is still reverse biased (reverse biased does not necessarily imply a reverse voltage, in this case it just means that the CB junction is not forward biased), so the transistor is in its active region and turning on as you suggest, but always keeping the colector to emitter voltage of a diode drop.
By joining the CB junction you have a transistor with a voltage drop of that of a diode, but still working in transistor mode. Leaving the colector open and just using the BE junction, you are indeed using it as a true PN junction or diode, albeit not a very good one. That is the main difference.
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