Quick Fundas: BJT# 1

What happens to VOUT when the source V1 is varied from +12V to +20V? Also, what will be the nature of variation of the current through R1 during this time? How could this circuit be useful? The Zener diode D2 has a breakdown voltage of 10V.

Figure 1: PNP Transistor Circuit

Hints: The circuit drives nearly constant current through R1 even though voltage V1 varies from +12V to +20V. Due to constant current through R1, voltage VOUT remains almost constant with respect to the positive rail (VOUT gets clamped with respect to positive rail of V1 irrespective of the variation of V1).

Using NPN transistors, the circuit below would provide nearly constant voltage ~9.6V at VOUT with respect to ground even if the input voltage varies between 12V and 30V.

Figure#2: NPN Transistor Clamp Circuit

One application of this circuit is to use this circuit to clamp the voltage of the gate drive of the MOSFET when the voltage used for the gate drive could vary over a wide range.