Using a PNP instead of an NPN in a circuit

lednpnpnp

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The figure shows my circuit.

The transistor I am currently using is a MJH6287 (PNP) as opposed to whats shown in the figure (MJH6284 – NPN)

The circuit with the PNP transistor worked just once in turning the LED on and off, Would it make a difference if I replace the NPN transistor with a PNP, if the circuit is designed for an NPN?

Best Answer

PNP and NPN transistor are diametrically different: NPN transistor conducts current / switched on when saturated (specified current applied to its base) while PNP transistor inhibits current / switched off when saturated current applied to its base (cut-off condition).

The NPN transistors on your circuit diagram is arranged in Darlington pair, a simple voltage polarity swap will NOT work on this configuration.

Anyway, you can use PNP transistors by changing the position of the resistors (see: PNP Darlington Pair) and put an inverter (you can built it with a transistor and two resistors, or simply use inverter IC such 7404) right after the opAmp output. But why would you do it (adding complexity and more components to your circuit), while the NPN transistor is as available as it's PNP counterpart?