Transistors and FETs

[kamilon]

I use transistors quite a lot when breadboarding. I've only used MOSFETs a couple of times and mostly because I kept seeing them used in circuits. For complex circuits I typically search for lots of examples of how other people do it and then kinda cobble them all together learning along the way. That said, I have created some quite complex circuits completely of my own design, but I typically take my time with calculations and lots of time spent on the bench with my multimeter and oscilloscope making sure things play together as planned. This information is provided so it is clear where I am at in my stages of learning

I'm trying to learn when to pick a MOSFET over a standard BJT transistor. I have always seemed to be able to get a circuit to do what I want it to do with a BJT. However I've noticed more and more recently that when people ask questions about BJTs (like how to properly calculate the base current) people say things like "or you could just use a FET).

I guess the question is, when do you pick BJT vs MOSFET? Is this one of the few parts that you can just pick to have one or the other on hand (when all you have is a hammer everything is a nail )?

[Dave]

Just to make things clear, a MOSFET is a transistor. Saying "transistors and FETs" is as silly as saying "food and pies".

Picking the type of transistor depends on the application. There are pros and cons to every type of transistor.
Here is a nice comparison of the two. You have the advantages of each listed at the bottom.

[kamilon]

By transistor I meant BJT, but point taken  lol

So I know that BJTs are current controlled and FETs are voltage controlled, but for many scenarios for me it seems like both are just as controllable/available (digital logic mostly).

I know I'm just missing that fundamental aha! But I still don't have it. I know when to pull out some different 7400 logic for example. I just feel like I'm missing a fundamental building block by not using FETs in my designs

[Ice-Tea]

Fun thing about FETs is that they don't load the controller. Doesn't really matter if you'd draw 1mA otherwise but it makes life a lot easier if you'd have to pump in 10mA+ with a BJT

[Zero999]

So I know that BJTs are current controlled and FETs are voltage controlled, but for many scenarios for me it seems like both are just as controllable/available (digital logic mostly).

That's a bit of an oversimplification. I'm sure you're not aware of it but that statement is controversial round here.
https://www.eevblog.com/forum/beginners/transitor-the-base-pin/

For small signals BJTs are treated as voltage controlled devices. The voltage between the base and emitter controls the current through the collector. Within reason the collector current doesn't vary much with the load resistance, so the transistor is treated as a voltage to current converter. A an amplifier who's input is a voltage and output is a current, is known as a transconductance amplifier.

http://en.wikipedia.org/wiki/Transconductance

When switching large signals it's easier to treat a BJT as a current controlled device. The base is supplied with sufficient current to saturate (turn on the transistor fully), so there's very little voltage drop across the collector and emitter.

BJTs have a higher voltage gain (transconductance) than MOSFETs, so they're more commonly used for amplifying small signals.

MOSFETs have a very high current gain (the gate current is near zero) but lower transconductance than MOSFETs, so they're more commonly used for switching large currents.

Quite often an amplifier will use both field effect transistors and BJTs. FETs are used for the input stage because they have a very high input impedance and BJTs are used for the rest of the amplifier because of their high transconductance. Take a look at the old CA3140 and TL072 (the schematics/datasheets can be found using a search engine) for examples of op-amps with FET input stages and high gain BJT stages.

[kamilon]

FETs are used for the input stage because they have a very high input impedance and BJTs are used for the rest of the amplifier

Now that makes sense to me... The whole thing that started this question for me was the video Dave did on what he claimed was the simplest on-off latching circuit. It used a FET on the input and then 2 BJTs. I was really confused as to why you wouldn't just use all FETs or all BJTs. This drives it home for me. I'm going to have to find some FETs to play around with on the breadboard now.

Any good circuits any of you guys can think of that will really show strengths and weaknesses of FETs and transistors? I have an oscilloscope so 'seeing is believing'

[Smokey]

I know I'm starting to be a broken record on this but...
Build some audio power amplifiers. 

This is a great book with a lot of example circuits and does a great job of explaining things step by step.
http://www.amazon.com/Designing-Audio-Power-Amplifiers-Cordell/dp/007164024X

It covers both bjt and fets in all sections of the amplifier design from front end to voltage amp to output section. And it uses LTSpice for simulation if that's your thing.

[atferrari]

Just to make things clear, a MOSFET is a transistor. Saying "transistors and FETs" is as silly as saying "food and pies".

Picking the type of transistor depends on the application. There are pros and cons to every type of transistor.
Here is a nice comparison of the two. You have the advantages of each listed at the bottom.

Hola Dave

Any other way to get that info? The links seems to have difficulties to download anything.

EDIT

Working now.

/EDIT

[dannyf]

when do you pick BJT vs MOSFET?

It is going to be an oversimplification and lots of confusions:

Typically, BJTs work well with amplification and switching, most mosfets you see today are designed for switching (there are exceptions to that).

Generally, BJTs are good for high voltage, low and very high current switching and mosfets for medium voltage and high current switching (typically tops out at 100s of volts and 100 amps or so).

BJTs suffer from 2ndary breakdown easily. MOSFETs also suffer from 2ndry breakdown but not nearly as easily.

So if you are switching, start with a mosfet and if it doesn't work, to go a BJT; if you are amplifying, start with a BJT and if it doesn't work, go to a MOSFET.

Having said that, MOSFETs are very difficult to drive for high frequency high amperage applications - the gate capacitance is a nightmare; Being a current-controlled device, a BJT is far easier to deal with at high frequencies.

Now, there are MOSFETs designed for analog uses, like the lateral mosfets or most RF mosfets.

There are also hybrid defices, like IGBT (=MOSFET buffered BJT), or ESBT (a bjt switched on the emitter by a mosfet)....

Again, over-simplification and excessive generation but you get the picture.

[Zero999]

Being a current-controlled device, a BJT is far easier to deal with at high frequencies.

BJTs can be just as much of a pain, apart from having to supply enormous base currents, the turn off time can be very slow. To get a BJT to switch off quickly, you need to apply a negative voltage to the base, to discharge the base-emitter capacitance, which is a nuisance.

[Sigmoid]

I keep reading that BJTs are more robust than FETs, which are pretty easy to destroy. How much truth is in this as of 2015?

[dannyf]

I keep reading that BJTs are more robust than FETs, which are pretty easy to destroy.

I use mostly low-frequency devices (power fets for example). since the 1980s, I have used no less than a thousand mosfets of different kinds and I have yet to destroy one due to static. For those devices, I consider the saying to be an urban legend.

However, RF mosfets are very fragile. Touching the gates on a RF mosfet / LNA sometimes is enough to kill them.

[Zero999]

I think that stems from the fact that it's more easy to destroy a MOSFET by ESD than a BJT. A BJT will just go into avalanche breakdown and providing the current isn't too high, no damage will be done. A MOSFET's gate oxide layer can easily be zapped by voltages as low as 50V.