All About Circuits Forum 2 MOSFETs in series
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#1
01-08-2011, 02:46 AM
 Nano001 Member Join Date: Jan 2010 Location: FL, USA Posts: 82
2 MOSFETs in series

Hi I have a simple question. Why is it that 2 MOSFETs in series are used as a current source instead of 1? For example in a differential amplifier, or in a cascode current mirror. What is the advantage of using 2 over just a single one? Thank you.
#2
01-08-2011, 03:52 AM
 marshallf3 Senior Member Join Date: Jul 2010 Location: Oklahoma City, OK. Large enough to be modern but plenty of country left around it. Posts: 2,358

Please provide an example circuit, contrary to popular belief we can't read minds nor guestimate unless we know what we're looking at.
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#3
01-08-2011, 05:31 PM
 gizmoman0 Junior Member Join Date: Jan 2010 Posts: 26

http://en.wikipedia.org/wiki/Cascode
#4
01-08-2011, 06:35 PM
 retched Senior Member Join Date: Dec 2009 Location: Baltimore, MD Posts: 5,198 Blog Entries: 14

Quote:
 Originally Posted by marshallf3 Please provide an example circuit, contrary to popular belief we can't read minds nor guestimate unless we know what we're looking at.
I dunno.... we are better at mind reading then most sites...

It is just EXHAUSTING!
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#5
01-08-2011, 06:42 PM
 gizmoman0 Junior Member Join Date: Jan 2010 Posts: 26

i think hes talking about one of these typical circuits

http://en.wikipedia.org/wiki/File:Long-tailed-pair.gif
#6
01-08-2011, 07:54 PM
 thatoneguy Senior Member Join Date: Feb 2009 Location: Midwest USA Posts: 6,356 Blog Entries: 4

Quote:
 Originally Posted by Nano001 Hi I have a simple question. Why is it that 2 MOSFETs in series are used as a current source instead of 1? For example in a differential amplifier, or in a cascode current mirror. What is the advantage of using 2 over just a single one? Thank you.
One transistor is set up for properties such as high input impedance, but little gain, while the second is set up for high gain and lower output impedance.

In a current mirror, one transistor is biased to always carry a certain current, which biases the second transistor properly to process outside signals.

Check out This Section of the e-book that explains the properties of common emitter, common base, and common collector circuits. The MOSFET equivalents have similar properties and reasons for use.
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#7
01-08-2011, 08:17 PM
 marshallf3 Senior Member Join Date: Jul 2010 Location: Oklahoma City, OK. Large enough to be modern but plenty of country left around it. Posts: 2,358

Quote:
 Originally Posted by retched I dunno.... we are better at mind reading then most sites... It is just EXHAUSTING!
I'll agree, there are a lot of people here that go far out of their way to help on a valid question. Some of them stump even the best, others are so simple I don't even bother responding as I'm hoping some of the younger, less experienced people will take it to their heart to try and research the answers as they often end up learning something that way too.

Of course I've got a lot of holes in my education so, as with anyone, I may not have a clue. I occasionally give the wrong answer as well but everyone's rather quick at correcting my mistakes so I learn from that.
__________________
-
The very first course in engineering school should be how to use Google and Wikipedia
-
I very often misspell or miss things while making posts so come back and double check a few times.
I also have a full time job and often 10 projects going on at the same time so I'm not always online.
#8
01-09-2011, 10:14 PM
 Nano001 Member Join Date: Jan 2010 Location: FL, USA Posts: 82

I ttached 2 pictures of what I am talking about. The first is a differential amplifier. I am curious why 2 MOSFETs are needed instead of 1 (the 20/2 FETs) and what the effect of 2 separate biasing voltages for them are.

Also, I attached a picture of a cascode current mirror. I understand what a current mirror does, however I don't understand the advantage of the cascode, and how the current flows through the right two MOSFETs in series. Iref flows through the left 2 MOSFETs, and if the transistors sizes match, Io=Iref. What is the advatange of cascode, I saw the link gizmomano posted, however it is still a bit unclear. I just need a simple explanation to clarify. Thanks very much.
Attached Images
 diff_amp.png (8.7 KB, 35 views) current mirror.png (24.7 KB, 39 views)
#9
01-10-2011, 01:03 AM
 marshallf3 Senior Member Join Date: Jul 2010 Location: Oklahoma City, OK. Large enough to be modern but plenty of country left around it. Posts: 2,358

Back up and go through the theory of how Op Amps were originally designed, it may help to shed a bit of light.
__________________
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The very first course in engineering school should be how to use Google and Wikipedia
-
I very often misspell or miss things while making posts so come back and double check a few times.
I also have a full time job and often 10 projects going on at the same time so I'm not always online.
#10
01-17-2011, 06:11 PM
 Nano001 Member Join Date: Jan 2010 Location: FL, USA Posts: 82

Ok, I was reading and I found out that the cascode current mirror is used to increase the output resistance so the current becomes relatively independent of the output load. However, I still don't understand the application of the diff amplifier that I posted, and what the biasing on the separate MOSFETs does.

 Tags mosfets, series

 Related Site Pages Section Title Textbook The cascode amplifier : Bipolar Junction Transistors

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