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 V_CB = 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 V_BE = 0.7 V in active mode, the magnitude of current I_C + I_B will depend upon collector supply voltage V_CC and collector resistance R_C. If V_CC and R_C are assumed to be constant, I_C + I_B will be constant and if β ≫ 1, then