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.
BJT Base Resistor vs MOSFET Gate Resistor
As explained above, a the base switching of a BJT is current dependent. Meaning its base current needs to be increased proportionately with increase in its collector load current.
This implies that the base resistor in a BJT plays an important role and must be correctly calculated to ensure that the load is optimally switched ON.
However, the base voltage for a BJT does not matter much, as it can be as low as 0.6 to 1 volts for a satisfactory switching of the connected load.
With MOSFETs it's just the opposite, you can switch them ON with any voltage between 3 V and 15 V, with current as low as 1 to 5 mA.
Hence, a base resistor may be crucial for a BJT but a resistor for the gate of the MOSFET may be immaterial. That said, a low value gate resistor must be included, just to safeguard the device from sudden voltage spikes and transients.
Since voltages above 5 V or up to 12 V are easily available from most digital and analogue ICs, a MOSFET gate can be quickly interfaced with any such signal source, irrespective of the load current.
How to Replace a Transistor (BJT) with a MOSFET
In general we can easily replace a BJT with a MOSFET, provided we take care of the relevant polarities.
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, I was wanting to try and replace the Darlington Transistor a circuit with an FET or MOSFET. I am not sure if I should do this or what changes would need to be made. It charges the capacitors when plugged in, then turns off when the capacitors discharge through the coils attached to the output with the momentary on switch. The capacitors would discharge for 20ms to maybe 200ms to pulse the coils, but don’t go through the FET. It is the initial charge surge current I need to worry about. I can’t seem to attach an image, but the power source is 12-20V, the current goes through the Darlington right to the + end of a 4700uF capacitor. When the capacitors discharge through a diode, the current to the base of the transistor goes to 0, it switches off, and the diode protects the transistor as the current has to go to the coil. The button on the coil is released, that pat of the circuit is open again, voltage goes to the transistor, and current flows to the base, so it switches on again and charged the capacitor again. Thanks for any help.
Hi, if your load is on the collector pin of the BJT, then you can replace the BJT with a MOSFET and place the load on its drain, however, if the load is connected at the emitter of the BJT then MOSFET might require the load to be placed on its source pin, which might alter the results.
Alright my specs
Are jfet j201 , or any 2n5457 series
I searched for depletion mode that could replace them but couldn’t find one only found 2n7000 unfortunately it enhance mode MOSFET.
Can you please suggest some to me sir
You will have to also provide the required current and voltage specifications of your MoSFET or the JFET, to be able to search an equivalent
Interesting , I really love this it has enlighten me more , in dealing with related components,
Thanks so much for this ,help my novice understanding
Here is my question
JFET are very rare at my Local store i found it highly frustrating. It has serve as barrier to my Inspired DIYs
is there a possible replacement or alternative
I found they good for guitar sound effect , amplifiers am working towards phaser effect (using opamp LFO) but i still found i need those jfet
Please help my novice knowledge
Glad it helped you! You can definitely replace JFET with a depletion mode MOSFET. There are plenty of variants online, you can select one of them depending on the VDS and ID of your JFET specs.
Thanks for the reply, that is what I thought but I have been watching a Utube video of an attiny drive the gate of P channel mosfet to blink an led and it had no pull down resistor so I was confused
Pull down resistor is recommended for MOSFETs for ensuring a failproof working, but it is not mandatory if the switching input is from a high quality logical source.
Am I correct in thinking if I replace a pnp bjt (BD136) with a mosfet I will still need a pulldown resistor on the gate to enable the mosfet to turn off?
For MOSFETs it is recommended to have a permanent resistor across their gate/source terminals.
Hi Swagatam!
I have an amplifier with power transistor sc5200 and sa1943 with 45-0-45v supply, i would like to convert it to mosfet. Do you have suggestions what pair of mosfet will be good? thanks!
Hi Nitsya, in amplifiers it may not be so easy to replace BJTs with MOSFeTs, since a BJT may start conducting fully with base potential as low as 1 V while MOSFETs may require 6 to 12 V.
Oh! Thanks alot Swagatam.
Hello Swagatam, can I replace D718 & B688 in audio amplifier. Any recommended MOSFETs,also do I need to replace BD139 & BD140 to use 45-0-45 Volt 8 Amp transformer.
Hello Ajay, the first two transistors are 120 V 8 amp NPN/PNP pairs, you can replace them with a matching pair having the same V and I specs.
You can use BD139/BD140 with 45 V supply
Hello Swagatam,
Thank you very much for the response, any suggestions for MOSFETs as I want to increase the wattage of the amplifier.
Regards,
Thanks Ajay, I can’t suggest much about the MOSFEts, because it can be a bit complex to replace BJTs with MOSFETs in an audio amplifier design
Thank you Swagatam, let me try som experiments, if successful will update
Sure, no problem!
2SC4054 BJT Transistor Failed Need a Replacement
I have an SMPS using a 2sc4054 BJT transistor that failed, since that transistor is no longer available I would like to replace it with a Mosfet. What Mosfet should I use and what other modification should be made to the circuit so the Mosfet receives the right gate voltage ? here is the link to the power supply circuit diagram
Try MJE13005 and see if that works or not.
Hi,
I want to change my BJT BC547 with a MOSFET, but i dont know which MOSFET should i choose. Can you help me with it?
Hi, you can try BS170 mosfet
Thank you for your help
Can I replace a K10A60 N Channel mosfet with a D13007 npn transistor
Please check and compare the VDSon and ID of the two devices, if they are similar then you ca interchange them…
Thanks for sharing such a beautiful article. It’s very helpful to me. Please keep sharing more about mosfet transistor
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://www.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!