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Basic Electronics » Transistors Versus Diodes

October 02, 2010
by Scythium
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I'm wondering can a transistor act like a diode. If so, is it possible to have a transistor output a specific amount of volts, lets say 4 volts.

October 03, 2010
by mongo
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A transistor can be used as a voltage regulator. It can be used as a diode in a pinch but I wouldn't count on the performance. Actually, it is functionally like two diodes back to back. (or front to front, depending on how you see NPN and PNP)but the base is really not intended to act as a current source.

To use as a regulator, say a positive source, you need an NPN transistor. Connected in a voltage follower configuration, which connects the collector to the positive voltage source. The emitter is the output. At the base, connect to the tap in a voltage divider that is across the input voltage. The voltage sensed at the base is very close to what the output voltage would be.

That circuit is very basic and using a zener diode for the reference source is a lot more stable. There is a little noise and no feedback circuit for fine regulation but adding a sufficient capacitor between the emitter and ground (negative) the noise can be controlled.

The 2N3055 is a very useful transistor for these purposes.

October 03, 2010
by mrobbins
(NerdKits Staff)

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Hi Scythium,

I would also recommend that you take a look at the Power Supply Circuit section of our recent Digital Calipers DRO video which talks about using a NPN transistor to create a simple DC power supply. The text specifically talks about Thevenin equivalent output resistance and how using a transistor means that: "the big idea is that the effective output resistance has been basically reduced by a factor of 100 with essentially no extra power consumption".

This circuit is basically the voltage divider plus voltage follower approach that mongo described.

Mike

October 04, 2010
by Scythium
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Thank you Mike and Mongo. I learned a lot by reading your posts.

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