Author Topic: Logic Level Mosfet Vs Transistor (BJT)  (Read 2258 times)

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Offline Harsh Chandola

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Logic Level Mosfet Vs Transistor (BJT)
« on: March 17, 2017, 09:57:50 am »
Hello,
I have a very basic question, I am designing a circuit where the mosfets are being turned ON via Atmega328p chip, now since the o/p voltages on its digital pins are 5Volts I am using logic level mosfets for my purpose as other MOSFETs would not turn ON fully with 5 volts at Gate. What I gave a thought to was that can I use normal NPN BJT instead of logic level MOSFETs? I still want to maintain minimum current consumption by ATMEGA328P. Also, if not, are there any devices like logic level BJTs?
Thank You,
Harsh Chandola
 

Online Kleinstein

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Re: Logic Level Mosfet Vs Transistor (BJT)
« Reply #1 on: March 17, 2017, 10:59:23 am »
BJTs need a base current to turn on. Usually gain is on the order of 100, smaller for high current types. So the ┬ÁC has to provide something like 1% of the maximum output current to drive. There are darlingtons with higher gain / less base current, but they also have a higher minimum voltage drop for the load. So something like 0.7 V are lost for the load.
The base emitter voltage to turn an BJT on is around 0.6 V and usually there is a resistor to set the base current to the desired level. There are a few so called logic BJTs with integrated base resistor.
 

Offline Codebird

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Re: Logic Level Mosfet Vs Transistor (BJT)
« Reply #2 on: March 17, 2017, 11:42:53 am »
In short: You can, yes. It's almost exactly the same circuit. The only difference is that you need to connect a resistor between your logic pin and the transistor base. The value of this resistor depends upon your choice of transistor and the load current you are switching. Though current consumption is slightly higher for a BJT, the difference is negligible in almost all applications.

That said, there isn't really any advantage either. As a general rule, BJTs are used for analog purposes. Amplifiers, oscillators, that sort of thing. For simple digital switching, the FET rules.

It's very common to use a small, logic-level transistor (BJT or FET) driven off the microcontroller in conjunction with a pull-up resistor in order to control the gate voltage on a much larger power MOSFET for switching higher currents. It's cheap and reliable, though not usable at high frequencies.
 

Offline MagicSmoker

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Re: Logic Level Mosfet Vs Transistor (BJT)
« Reply #3 on: March 17, 2017, 11:44:52 am »
What Kleinstein referred to as "logic BJTs" are found at Digikey, for example, under "Pre-biased" (both single and arrays). For direct connection to an MCU you'll want a BJT that includes both the series base resistor (which limits base current) as well as the shunt base-emitter resistor (which ensures the BJT is off if the MCU pin is floating, as happens during reset or power up/down).

 

Offline danadak

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Re: Logic Level Mosfet Vs Transistor (BJT)
« Reply #4 on: March 17, 2017, 11:51:16 am »
In general if you want to sat the BJT you have to force the beta, to a
rule of thumb value of 10. So if you need 100 mA in the collector,
plan on driving 10 mA into the base. You can see this effect in charac-
teristics curves for the device. This minimizes Vcesat, hence Pdiss of the
BJT.

If you are driving many bjts your processor probably has a limit on total
I/O buss current, consult datasheet.

Some BJTs will exhibit even lower Vcesat if you interchange Emitter and Base
leads.


Regards, Dana.
Love Cypress PSOC, ATTiny, Bit Slice, OpAmps, Oscilloscopes, and Analog Gurus like Pease, Miller, Widlar, Dobkin, obsessed with being an engineer
 

Online Zero999

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Re: Logic Level Mosfet Vs Transistor (BJT)
« Reply #5 on: March 17, 2017, 12:37:24 pm »
Hello,
I have a very basic question, I am designing a circuit where the mosfets are being turned ON via Atmega328p chip, now since the o/p voltages on its digital pins are 5Volts I am using logic level mosfets for my purpose as other MOSFETs would not turn ON fully with 5 volts at Gate. What I gave a thought to was that can I use normal NPN BJT instead of logic level MOSFETs? I still want to maintain minimum current consumption by ATMEGA328P. Also, if not, are there any devices like logic level BJTs?
Thank You,
Harsh Chandola
What voltages are you switching?

How much current?

What's the highest frequency?
 


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