How to use a Transistor as a Diode – Working and Equation:
The transistor can be modified to operate as a diode by short circuiting the two transistor terminals, as shown in Fig. 10.69 [collector and base terminals are shorted making VCB = 0] and for a practical P-N junction if voltage across it is less than cutin voltage, it can be assumed to be in a reverse bias state.
The configuration depicted in Fig. 10.69 makes use of emitter-base junction as a diode. The transistor is operating in forward active region because E-B junction is forward biased and C-B junction is reverse biased.
Applying Kirchhoff’s second or voltage law around the base-emitter loop, we have
With VBE = 0.7 V in active mode, the magnitude of current IC + IB will depend upon collector supply voltage VCC and collector resistance RC. If VCC and RC are assumed to be constant, IC + IB will be constant and if β ≫ 1, then