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Old 01-08-2011, 02:46 AM
Nano001 Nano001 is offline
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Default 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.
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Old 01-08-2011, 03:52 AM
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marshallf3 marshallf3 is offline
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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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Old 01-08-2011, 05:31 PM
gizmoman0 gizmoman0 is offline
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This page explains a few advantages of a cascode setup

http://en.wikipedia.org/wiki/Cascode
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Old 01-08-2011, 06:35 PM
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Originally Posted by marshallf3 View Post
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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Old 01-08-2011, 06:42 PM
gizmoman0 gizmoman0 is offline
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i think hes talking about one of these typical circuits

http://en.wikipedia.org/wiki/File:Long-tailed-pair.gif
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Old 01-08-2011, 07:54 PM
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Quote:
Originally Posted by Nano001 View Post
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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Old 01-08-2011, 08:17 PM
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marshallf3 marshallf3 is offline
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Quote:
Originally Posted by retched View Post
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.
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Old 01-09-2011, 10:14 PM
Nano001 Nano001 is offline
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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
File Type: png diff_amp.png (8.7 KB, 35 views)
File Type: png current mirror.png (24.7 KB, 39 views)
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Old 01-10-2011, 01:03 AM
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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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Old 01-17-2011, 06:11 PM
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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.
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