How to Replace a Transistor (BJT) with a MOSFET

In this post we discuss the method of correctly replacing a BJT with a Mosfet, without affecting the final outcome of the circuit.

Introduction

Until mosfets arrived in the field of electronics, transistors or BJTs to be precise ruled the power switching circuits and applications.

Though even Bipolar Junction Transistors (BJTs) can not be ignored due to there immense flexibility and low cost, mosfets also have certainly become  hugely popular as far as switching heavy loads is concerned and due to the high efficiency associated with these components.

However even though these two counterparts may look similar with their functions and style, these two components are completely different with their characteristics and configurations.

Difference Between BJT and Mosfet

The main difference between a BJT and a mosfet is that, a BJT operation depends on current and needs to be proportionately increased with the load, whereas a mosfet depends on voltage.

But here the mosfet gets an edge over a BJT, because voltage can be easily manipulated and achieved to the required degrees without much  trouble, in contrast increasing current means greater power that's to be delivered, which results in bad efficiency, bulkier configurations etc.

Another big advantage of a mosfet against the BJT is it's high input resistance, which makes it possible to be integrated with any logic IC directly, no matter how big the load may be that's being switched by the device. This advantage also allows us to connect many mosfets in parallel even with very low current inputs (in mA).

Mosfets are basically of two types, viz. enhancement mode type and depletion mode type. In common words these are referred as N-type and P-type mosfets.

The N-type mosfets can be turned ON or activated by applying a specified positive voltage at their gates while P-type mosfets will require just the opposite that is a negative voltage to get turn ON.

How to Replace a Transistor (BJT) with a Mosfet

As explained above a BJT depends heavily on current, and its base current needs to be increased proportionately with increase in its collector load current. But the voltage does not matter as they require just 0.6 to1 volts to get switched ON.

With mosfets its just the opposite, you can switch ON them with any voltage between 3 and the maximum rated voltage with current as low as 1 to 5 mA.

Since the voltage is never an issue with any source like from an IC or any other relevant source and the current being so low, mosfets can be easily interfaced with any source irrespective of the load current.

So it means, in general we can easily and blindly replace a BJT with a mosfet, provided we take care of the relevant polarities.

Compatible Pinout Replacement Diagram for NPN BJT with N-Channel Mosfet

For an NPN BJT, we may replace the BJT with a correctly specified mosfet in the following manner:

• Remove the base resistor from the circuit because we don't typically need it anymore with a mosfet.

• Connect the gate of the N-mosfet directly to the activation voltage source.

• Keep the positive supply connected to one of the load terminals, and connect the other terminal of the load to the drain of the mosfet.

• Lastly, connect the source of the mosfet to ground.......DONE, you have replaced the BJT with a mosfet within minutes.

The procedure will remain as above even for a PNP BJT to be replaced with a  P-channel mosfet, you will need to just reverse the relevant supply polarities.

Compatible Pinout Replacement Diagram for PNP BJT with P-Channel Mosfet