how to use BJT as a DIODE

transistor as a diode

it will be useful if we are (i am) aware of how to use BJT as a diode. could some one throw some light on this? since it is a three region element , so to convert it into two region diode certainly any two region must be shorted.here we are having various ways to achieve it ie base with emitter or base with collector AND SO ON........ .but my question is which combination will be more benefical to us

The emitter-base junction acts as a simple semiconductor diode.
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.
Short circuiting the collector-base ensures that the collector-base junction is always reverse biased.
Regards,
IanP

For a NPN, where P is base, N+ is emitter, connect P and N+ accordingly to be used as a simple PN-junction diode. N as collector can be (1) left disconnected or connected to (2) base or (3) emitter. 1 and 2 configure NPN into a single diode, 3 configures NPN into 2 parallel diodes. All methods are valid, afterall Vbe is still 0.7V at room temperature. The net output current is the same as the input current to base in any configuration. 2 creates a special flywheel diode to protect it from inductive or/and capacitive energy discharge at the input end to base.

For PNP, P+ as emitter, N as base. Connected emitter and base accordingly as a simple PN-junction diode. If you connect collector to emitter, it's the same as having 2 parallel diodes with net current to base equates to input current. If you connect collector to base, this creates a special flywheel diode to protect the output end to the base. Or you can let collector disconnected.

Connecting collector to base or collector to emitter are commonly used, especially collector to base to have additional protection.

You can use a base-collector juction as a diode too. The emitter can be left open, or connected to base. Using so a high voltage transistors, you can have a diodes
with much smaller reverse current ratings, than a conventional diodes, with the
similar voltage and current.
Varuzhan

Hi

I am interested in building a rectifier using transistors instead of diodes.
The transistors have to be controlled using a DSP. ( somewhat like the PWM switching of transistors in an inverter)

Can the same concept of using a transistor as a diode apply?

Honeybee,

Yes, you can built a simple full-wave rectifier using a PNP (emitter, base, collector),
where emitter-base PN-junction as one diode and collector-base PN-junction as another diode. Both the emitter and collector are connected to a sinusoidal signal source. The base is the output of the rectified signal.

WARNING! This is only meant for signal-level or low-power rectification. Do not use this for power rectification.

I want to know why are only base and collector shorted to make a diode as most of you have siad earlier. Is thre any special reason> We can also short base and emitter. What are the advantages in first case and disadvantages in the second case?