The importance of MOSFET in everyday design. - Page 1
Forum section: electronics / beginners

author: zorthgo
author_rank: Regular Contributor
timestamp: November 13, 2012, 06:13:32 pm
content: Hi Guys, I have a question for y'all. We are learning about MOSFET in class right now. How important is MOSFET to everyday design. I know that it is very important if you are designing an IC, but how about when you are just designing a simple circuit. Are BJTs more used by hobbyist?Thanks!Zorthgo

author: Kremmen
author_rank: Super Contributor
timestamp: November 13, 2012, 06:30:15 pm
content: As a rule of thumb BJTs are more common in amplifier circuits whereas MOSFETs are nowadays more commonly used as switches.Practically all low and medium power switchmode power management circuits use MOSFETs as switching elements these days. For the highest power and voltage ranges you have IGBTs or insulated gate bipolar transistors. Those are kind of hybrid between the 2 - insulated gate like MOSFET but bipolar main conduction path like a BJT.So MOSFETs are right at the top and it pays to learn them well.

author: Paul Moir
author_rank: Frequent Contributor
timestamp: November 13, 2012, 09:56:31 pm
content: I think one thing that might not be apparent is that up until recently (last ~15 years) MOSFETs were a little on the pricey side and weren't as friendly to use as BJT. With the advent of logic level gates so you don't need a separate supply & driver, good RDS(on) & good VGS things have gotten a lot nicer. And then there's plug-and-play gate drivers for high efficiency switching.So from my perspective (a hobbyist since the '80s), MOSFETs have just continually gotten more important.

author: NEDM64
author_rank: Contributor
timestamp: November 13, 2012, 10:46:14 pm
content: The great thing about MOSFET is that they have almost zero input impedance, as you can see for the (simplified) small signal model:

author: Zero999
author_rank: Super Contributor
timestamp: November 13, 2012, 11:26:08 pm
content: Quote from: NEDM64 on November 13, 2012, 10:46:14 pmCorrected.

author: Mechatrommer
author_rank: Super Contributor
timestamp: November 14, 2012, 03:43:54 am
content: Quote from: NEDM64 on November 13, 2012, 10:46:14 pmCorrected.

author: Tube_Dude
author_rank: Regular Contributor
timestamp: November 14, 2012, 03:50:14 am
content: Zero input DC current, or infinite DC input impedance, are actually the same thing...

author: vk6zgo
author_rank: Super Contributor
timestamp: November 14, 2012, 04:19:13 am
content: Back in the 1970s & early 1980s MOSFETS were widely used in RF stages of HF & VHF receivers,especially the dual-gate variety.Increased concern about the need for good dynamic range in receivers caused a swing away from the use of such devices.MOSFETS nowadays are more commonly found in switching circuits.

author: Zero999
author_rank: Super Contributor
timestamp: November 15, 2012, 10:53:57 am
content: Quote from: Tube_Dude on November 14, 2012, 03:50:14 amLol.

author: zorthgo
author_rank: Regular Contributor
timestamp: November 15, 2012, 12:08:55 pm
content: Great explanation guys. Thanks!Sorry if this is a NEWB question but, how can you tell that NEDM64's circuit has 0 impedance? Do you just look at it and with your experience you know. Or is it because of the formulas that you have written there? Also, these formulas look very similar to those given by my teacher, are they always the same? Thanks!

author: Mechatrommer
author_rank: Super Contributor
timestamp: November 15, 2012, 12:21:30 pm
content: Quote from: zorthgo on November 15, 2012, 12:08:55 pmwhat is NEDM64? 0 impedance what? input or output?. if you are talking 0 impedance input, you didnt read the errata. maybe what you need is to read how the mosfet is constructed, i assume you have the book. i'm no expert someone else may elaborate it.

author: zorthgo
author_rank: Regular Contributor
timestamp: November 15, 2012, 12:25:25 pm
content: NEDM64's simplified small signal diagram that he posted. Sorry, I don't know much about MOSFET. We've only have one lecture on it so far.

author: Mechatrommer
author_rank: Super Contributor
timestamp: November 15, 2012, 02:03:05 pm
content: Quoteduh! i just typed that into alldatasheet. cant anybody find a better name?i'm no expert but let me... mosfet only sucks very little current from source through its base. compared to bjt which need higher current to operate, hence source loading is greater and is your concern if your source has high output impedance. the rest i think have been explained by Kremmen in the 1st reply.

author: Kremmen
author_rank: Super Contributor
timestamp: November 15, 2012, 03:09:02 pm
content: Quote from: zorthgo on November 15, 2012, 12:08:55 pmActually it is infinite impedance, as in no input (gate) current at any voltage.NEDM64's diagram is a conceptual 2 port device with idealized circuit elements. The input port is G-S, and output port is D-S. Now, as you see the input port is not connected to anything so there can be no current flow. There can still be an applied voltage across the port, as shown by Vgs. This is the gate-to-source voltage and it is a real thing that you can measure in a physical circuit.The other elements across D-S are a resistor denoting the intrinsic resistance of the main conduction path, and a voltage controlled current source. The voltage is the gate-to-source voltage Vgs and the other factor is the the transconductance of the device, gm, i.e. A/V. Transconductance times the gate voltage gives amps through the main channel.But note that this is a very simple idealized model of a MOSFET and ignores most of the real semiconductor physics. About the only thing that is accurate is that there really will not be any gate current in a stable DC operating point. There _will_ be gate currents, sometimes considerable, as charge currents to charge/discharge the gate capacitances that real devices exhibit. There are parasitic capacitances between all device nodes and these can absolutely not be ignored in real designs at any frequencies above trivial.Edit: typos.

author: Noize
author_rank: Regular Contributor
timestamp: November 15, 2012, 04:36:23 pm
content: Quote from: zorthgo on November 15, 2012, 12:25:25 pmSo you are just learning the large signal model of the mosfet at the moment? If so don't worry about that diagram, that is a small signal analysis diagram and you will be more than likely be learning that straight after the large signal model. It will just confuse you if you haven't learnt the large signal model.

author: zorthgo
author_rank: Regular Contributor
timestamp: November 15, 2012, 09:06:26 pm
content: Thanks Kremmen for the awesome explanation!Well we just started learning about mosfet, so I guess we are learning about large signal. But we did learn about small signal amplifier when we were learning about BJTs. Is it the same idea? The formulas seem similar.