Electronics > Beginners

BJT/MOSFET as switch

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Moriambar:
Hi there.

I have some doubts regarding the use of BJT vs MOSFET as switches in mainly digital circuits. Let me explain.

The applications I'm interested in are "as easy as they get" meaning switching on/off an LED, or something like that.

I usually find BJTs used/explained almost in every book and tutorial as "the easiest way" to implement an electronically controlled switch in a simple circuit.
I find way more easy, though, to understand MOSFETs, since they do not have "funny parameters" such as beta, and especially because they're voltage controlled and not current controlled. Also, if I understand the matter correctly, they're a bit more power efficient.

I also am thinking about real newbie applications, so nothing about "quick switching" or something like that. Perhaps power efficiency (in battery powered circuits) could be important.

So I really find it difficult to understand why there are so many things about BJT as "best switches", or "easiest", and I really find it hard to find an use of them as switches. Also for small hobby project, the price has almost no impact although I know that BJT cost half as much as MOSFETs (or less!).

My thought is, then, that I perhaps do not understand the real power of BJT and came here to ask how to better understand them and start using them when they're best w.r.t. MOSFETs.

I hope this post makes sense.

Cheers!

kosine:
I'm sure others will add more points, but to kick things off...

BJTs are generally better characterised than FETs. A FETs parameters are usually +/- a bit more than BJTs, so with a BJT you know more reliably how it will perform.

FETs normally require a relatively high gate voltage to switch fully, whereas BJTs require <1V. BJTs require more current off course, so the choice depends on your application. In a lot of basic circuits your input (control) voltage is usually the limiting factor, so BJTs are the go-to choice.

In both cases, there is always some limiting resistance through the channel. With a BJT this can be around 1Ohm, with a FET a few milliOhms. If you were pulling, say, 1A then the BJT might have about a 1V between collector & emitter (Ohm's law), whereas a FET would only have a few millivolts between drain & source. This is the "saturation voltage" and makes a difference when switching high current loads because the BJT will dissipate a lot more heat. (1 watt vs a few milliWatts in this example.)

FETs also tend to be more forgiving if they do overheat. With a BJT you can get thermal runaway - as it gets hotter it conducts more current, so gets hotter still. FETs don't suffer from this.

On the other hand, BJTs are generally more robust than FETs and less likely to suffer damage from handling (FETs can be killed by static electricity). BJTs also tend to be easier for beginners to get working.

Best way I've found to explain BJTs to people is to show them a Gummel plot. This shows two VI curves, one for the base and one for the collector. The two are linked, the collector current being base current x gain. Pick any point on the plot and a horizontal line will show you the corresponding currents and base-emitter voltage. Varying any of these parameters just moves you along the curve to a different operating point.

I.e., For a given base voltage, the transistor (= transfer resistor) will adopt whatever resistance is necessary to provide the corresponding collector current. Whereas for some given collector current, the transistor will instead adopt the corresponding base voltage and current values. A common mistake is trying to fix more than one of these parameters, transistors don't like that - just set one and give the transistor room to set the others.

When you use a BJT as a switch, you move up the curve to a point where the collector current can't keep up. The transistor resistance drops as low as it can (say 1Ohm) and the load resistance (usually between supply and collector for an NPN) then limits the maximum collector current. The transistor then saturates. It wants to deliver more collector current but can't. (Looked at from the other perspective, you're providing more base voltage and current than is needed, but that ensures saturation and full switching.)

Note that the Gummel plot also nicely shows that the "0.6V base-emitter voltage" is just a convenient rule of thumb for the kind of operating point normally used in practice. BJTs will happliy work down to 0.25V or so, you just get less amplifaction and lower currents.

Moriambar:
Thank you for the detail explanation... I still fail to see the simplicity of the bjts. I mean, they're more robust and whatnot but with a logic level mosfet (e.g. 2n7000 or FQP30n06L) I just provide 5V or 0V and the switch is open or closed, while  with the BJT I have to pick up the correct base resistance, depending on the collector current. I mean: am I really the only beginner who has always thought FETs are easier to understand (as switches)? Or perhaps I'm missing something about them?

Anyway your explanation is really detailed and interesting and makes me wondering: can I use BJTs as "voltage controlled resistors between collector and emitter"? I know it's out of this question scope but...


Cheers

kosine:
If you're only switching with logic level stuff, then yes a logic-level FET is a good choice. (Which is why most digital ICs are built with CMOS FETs.) BJTs can be more useful for analogue stuff, though in the vast majority of applications a transistor is a transistor. Use whatever you're comfortable with, but don't give up on figuring out how to use a BJT. You'll feel like you've earned your stripes once you can design working circuits with them! (LTspice is a great learning tool for this.)

(If you want to see a truly awesome BJT circuit, look up Bob Widlar's classic "bandgap voltage reference". He actually did two versions, both are works of genius in exploiting the finer points of BJT operation. Many common voltage regulator chips make use of it.)

Sadly BJTs aren't quite a voltage-controlled-resistor because you can't actually set their resistance. You can set the currents, however, and Ohms law then lets you work out the effective resistance the transistor will adopt. But it's not quite the same thing.

Alternatively, low down in their operating range, FETs can be used directly as a voltage-controlled-resistor. The range is somewhat limited, but it's a not-uncommon technique.

Moriambar:

--- Quote from: kosine on January 21, 2019, 01:57:21 pm ---If you're only switching with logic level stuff, then yes a logic-level FET is a good choice. (Which is why most digital ICs are built with CMOS FETs.) BJTs can be more useful for analogue stuff, though in the vast majority of applications a transistor is a transistor. Use whatever you're comfortable with, but don't give up on figuring out how to use a BJT. You'll feel like you've earned your stripes once you can design working circuits with them! (LTspice is a great learning tool for this.)

(If you want to see a truly awesome BJT circuit, look up Bob Widlar's classic "bandgap voltage reference". He actually did two versions, both are works of genius in exploiting the finer points of BJT operation. Many common voltage regulator chips make use of it.)

Sadly BJTs aren't quite a voltage-controlled-resistor because you can't actually set their resistance. You can set the currents, however, and Ohms law then lets you work out the effective resistance the transistor will adopt. But it's not quite the same thing.

Alternatively, low down in their operating range, FETs can be used directly as a voltage-controlled-resistor. The range is somewhat limited, be it's a not-uncommon technique.

--- End quote ---

Thanks, I do feel I'm missing out something by relying on mosfets only (especially for switching purposes), and never consider BJTs for the same application. I will try and see this more in depth, using also your suggestions.

Cheers

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