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. Enhancement type is more frequently used and is the prevalent one.
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 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
hi what could i replace a irfz34n with could i used a irf 44n
Please check the datasheet of the two and compare their VDs and ID rating, it will clarify your doubt.
What is the alternative transistor RFP50N06 for a 300W inverter?
IRFZ44N
Educational article sir
Hi mr Mr swagatam,if i had a circuit that need NBJT,
Q1: can i replaced it with any NFET if from datasheet that NFET have my own requirements such as voltage, current, temperature?even more great than NBJT.
Q2: Is there any transistor built in oscilators?
thanks for any advice.
Thanks Skhan, yes you can replace a BJT with an FET, just make sure their polarity are identical, use a low value resistor at the gate and make sure the gate has at least 10V for the switching
for transistor oscillator you can refer to this post:
https://www.homemade-circuits.com/easy-two-transistor-projects-for-school-students/
Hi Swagatam, I have to replace all my BJT from all my PCBs.
Which N-ch Mosfet do you advise me to change the MMBT2222L (NPN)?
and…
Which P-ch Mosfet do you advise me to change the MMBT2907AL (PNP) ?
Considering:
– They be economical
– All my BJT are SOT-23 package, thus Mosfet must be also.
– Both BJT type are working under TTL logic, so gates will be drive directly by PIC microcontroller 5v. And Load 12v (no need much Amp).
Hi Yany,
I would like to know the reason for changing those devices, because BJTs have their own significance depending on the application area, and should not be changed with a MOSFET when they are well suited for the need.
Moreover a MOSFET will not turn ON efficiently at 5 V. I think in your case the BJTs are perfectly fine!
Hi Swagatam,
The reason is just what you said, to reduce dependencies and look for best commutation efficiency, and also to reduce PCB sizes for my next designs.
And to improve it for my next design, which Mosfet would you advise me for I begin testing?
(considering all the previous, and turn ON efficiently at 5v)
Hi Yani,
you can try this for the 2222
https://www.onsemi.com/pub/Collateral/FDB8860_F085-D.PDF
or you could try this for a 2-in-1 package
http://ww1.microchip.com/downloads/en/DeviceDoc/tc6215.pdf
HI BROTHER THIS METH ODE NOT WORKING FOR ME BY REPLACING 2N 3055 WITH IRFP240
Hi, it will surely work, but remember MOSFeTs are very sensitive, and will blow without a hint, not like BJTs. Please specify what kind of driver are you using for the gate drive, I’ll try to help!
Good evening Swagatam Ji.
I need one more clarification.
Let me give 24 dc supplies to solar-e-rickshaw-circuit (or )
I have to reduce 12volt dc by the way of 12volt regulator.
If I have to give only 12volt dc supply then please give me simple
and trouble free solution to get 12volt from 24volt battery supply.
(I have crossed my age about 68years. I am just helping my friends by the help of fresh EEE students )
Thanks.
sakthivel.
Hello Sakthivel ji,
you can use 24V, and use a 7812 IC for converting it to 12V only for the IC. Meaning, the motor can be tied to +24V, but the IC 555 must be fed from a 7812 IC connected with the 24V source
Thank you very much Swagatamji for your suggestion!
sakthivel.
You are welcome sakthivel!
TIP142/ IRF540 Mosfet compatiable pinout diagram please
gate will go to IC 555 output, source will go to the ground line, drain to the motor:
https://homemade-circuits.com/wp-content/uploads/2013/01/irf540ndatasheet252Cpinouts.png
Just wanted to point out that “triggered” is not a suitable description for turning on a BJT or FET, since it has a very specific meaning regarding semiconductors, and is used instead to describe the activation of UJTs, SCRs, DIACs, TRIACs, etc. You “turn on” or “switch on” a BJT or FET, you don’t trigger it.
Thank you for your suggestion!