help! transistor as diode

[vishu412k]

i need to know how to use 'transistor as a diode'......and its applications. i know of transistor as a switch, but as diode.....dos'nt make sense, unless its the same thing! the problem is, someone else has been asked to speak on 'transistor as switch'! seminar's in 10 hours ! HELP PLEASE!! :'(

 

[audioguru2]

Hi Vishu,
Simply connect the base of the transistor to its collector. Do not exceed the reverse-biased voltage rating of its base-emitter junction which is only 5V to 7V for silicon transistors.
ICs use transistors connected as a diode this way for low-voltage voltage references. They are better than an ordinary diode because their forward voltage is lower and doesn't rise as much with increased current.

For your seminar maybe you can make a demo showing the difference of forward-biased voltage between an ordinary diode and a transistor with its base connected to its collector. Show them at different current levels but the same current as each other.
You can call the latter "A Super-diode".

 

[vishu412k]

Hi Vishu,
Simply connect the base of the transistor to its collector. Do not exceed the reverse-biased voltage rating of its base-emitter junction which is only 5V to 7V for silicon transistors.

isnt it possible to use the base-collector diode as well. how does that work? also, in case of the base emitter diode, how do the negative charge carriers of collector take part in current flow. ???

 

[audioguru2]

Hi Vishu,
When you use a transistor as a "super diode" with its base connected to its collector, when forward-biased it is still a transistor with its rated current gain (check its Vce spec) and therefore produces a low forward voltage drop. Since a transistor's base-emitter junction has such a low reverse breakdown voltage, I recommend that it be used as a forward-biased voltage reference (like in ICs) or a very low voltage rectifier.

If you use only its base-collector diode then it is just an ordinary diode. If you connect a transistor "backwards", with its base connected to its emitter, its current gain will be very low and its forward-biased voltage drop will be similar to an ordinary diode.

 

[Kevin Weddle]

I have seen the base emitter diode used in practice. I have also seen it used in a circuit diagram. This is a bizzare thing because the transistor was not meant to operate like two diodes. It is a current amplifier. The two diodes exist because that is how a transistor is made. The reverse collector base junction conducts somewhat like a zener would do in it's place. So I would say although you get a diode potential, the device is more similar to a zener which can also give you .7V when forward biased.

 

[audioguru2]

Hi Kevin,
A forward-biased zener diode performs like an ordinary diode where its voltage will increase when its current increases.
A transistor "Super Diode" (its base connected to its collector) will have the base-emitter junction monitoring its output voltage and varies the collector conduction to keep its forward voltage low and fairly constant with changing current.

 

[surajbarkale]

http://www.allaboutcircuits.com/vol_3/chpt_4/12.htm try googling with term "current mirror" only place where i have seen such a construct. Then i am not that experienced ;D

 

[audioguru2]

Thanks Suraj,
You have found a good article that clearly explains that our "Super Diode" will have the same voltage across it as the base-emitter voltage of a transistor.
Since our "Super Diode" has current gain, the collector will handle most of its current and the base very little, reducing the conducting voltage drop to that of a base-emitter junction that is passing a low current. Therefore our "Super Diode" will have a lower voltage drop than an ordinary diode, and maintain (regulate) that voltage with changing current. The voltage regulation won't be perfect since the current gain of a transistor reduces at high collector current.