In this post I have explained 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
Questions & Answers
How would you manage to replace the bjt transistor with another type of Device?
What type of device are you referring to?
any replacement for D1 292 transistor. that might be similer to it
You can Google 100V 1 amp NPN transistor, you may find many good options.
Looooongest time sir.
I have been searching for your contacts or your site. I am happy I got you today. You helped me to understand a lot in the past
Thank you Ik, I am always happy to help….. All the best to you.
Hi SwagatamI sent you a msg in facebook asking for help with replacing mrf486’s and mrf422’s i sent it there because i attached a schematic if you could please help thank you
Thank you John,
According to me you cannot replace MRF486 with a MOSFET. You can try searching for 30 MHz 20 Amp Rf NPN power transistor, and find a suitable equivalent for the replacement.
maybe what i’m looking for is LDMOS not MOSFET’S ?
I think the basic replacement principle will be the same, just as any other mosfet, since LDMOS is also a type of MOSFET.
thanks for the reply , yes i been searching and found nothing even close to mrf486’s
i’m thinking they were only made for this one radio only
the next version of this radio that came out had mosfets in it
i compared both schematics to these radios and the pa unit circuit look pretty close to the same design
the thing is i don’t know that much as to how to make the proper modifications to make it work , i never tried it before
Yes, I agree, I too tried searching for “30 MHz power transistor” but could not find anything useful. Unfortunately in RF circuits a BJT cannot be easily replaced with a mosfet.
What ordinary transistor do I replace for Darlington 2SD1298 or D1298?
You can use BU406D and MJE13003 in Darlington pair.
Or you can search for any 400V 10 amp transistor and 400V 1 amp transistor and combine them to make a Darlington pair.
Hi sir how can i connect a universal power module to my smps 30amp cause i cannot get 13009 transistors on my local market
AS far as I know universal power module has a power mosfet or transistors with 3 wires corresponding to the 3 leads of the power device. You can carefully check these wire according to the given user manual and then connect these wires across the respective tracks of the SMPS PCB.
However I would strongly recommend replacing with a discrete transistor instead of a power module.
Hi,
Can you please review the MOSFET symbols given and the type of MOS labeled?
Irfp448n
Good day, i would like to know if its possible to change the 13009 transistors in a smps (cheap 12v 30amp) to N-Channel mosfets, the transistors are driven by a gate transformer. If its possible can you give me a example schematic of how to change it if possible, here is a schematic of the smps.
I would greatly appreciate any help you can give me.
Regards
Darius
Good day eant to add, primary has 33turns then a center tap with another 33turns after.
Then the secondary has two separate 9turns without a enter tap and then a feed back with 2 turns
The primary and secondary windings has a copper thickness of 0.4mm and the feedback is 0.65mm.
Regards
Darius
Hi, How is this transformer wired, which IC is used? I will need more information regarding your question to figure out the issue?
Hello, the IC is a TL494 and its a half-bridge topology, i see on my first post that i forgot to add the link to the schematic, i’ll add the link in the end.
(imajeenyus.com/electronics/20151028_smps_variable_voltage/index.shtml) or type in on google s-400-12 supply schematic and check on the images.
I hope this help, it has the full schematic and how the gate drive transformer is winded and the main transformer aswell.
Regards and thanks again
Darius
Thanks for the schematic, This is an SMPS circuit where the transformers are the most critical elements of the circuit. You can rewind it but the winding details must be exactly similar to the original data….you cannot change the data with your own specifications.
Hi, according to me a an 13009 BJT cannot be replaced with a MOSFET in an SMPS circuit.
Thank you for the fast reply, is there no way to rewind the gate drive transformer if I’m correct the transformer is wind in a 4:1 ratio and has a feedback winding aswell, the primary winding is 10v peak to peak and secondary is 2.2v peak to peak, so if you rewind it 1:1 will it work?
Thanks
Regards
Darius
Without seeing the full schematic it can be difficult for me to provide a useful suggestion.
Hello with respect sir
I have long range cordless phone senao-258 star handy 264MHz 350mW rf power output, transistor is blt50 but i want more power near 2 or 4 watts…
Is it possible to replace ldmos aft504???or please guide me
And another question is i know uhf mosfet kgf-1305, but not found in my area please tell me what are power mos-fet work very similar like kgf-1305
Hello Babak, I could not find any data regarding the mentioned mosfets, so cannot provide any suggestions. You can try other variants such as irf540 etc.
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
Hello swag, please what is the main point to take, considering replacing a MOSFET?
Hello Kenkenny, The power rating of the new device must be equal or higher than the original device. The pinouts must be correctly configured. The voltage and current spec of the new device must be equal or higher than the original device.
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!