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Old 12th January 2004, 01:25 PM   #1
DarkOne is offline DarkOne  Slovakia
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Default Input stage idea

Hello,
recently I've been thinking about best possible input stage for my amplifier and I've got an idea. I don't know if it's something new, or already used before, but I haven't seen anything similar yet.
Principle is in differential input stage fed from current source, but output is attached to 2 current mirrors and then to another current mirror. I've made this with my old mosfets (IRF540) and everything seems that it works, but I don't have anything to measure performance. So I'm trying to ask you (proffesionals (some of you)).
Schematic is attached (i hope)
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Old 12th January 2004, 03:28 PM   #2
azira is offline azira  United States
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That topology is very much like an OTA, operational transconductance amplifier. It's a voltage to current amplifier. If you observe, with zero differential input, you'll have zero current going through the load.

If you want to use it for your input stage, you'll have to size a load resistor to set your voltage gain.

One thing you should notice is that the output resistance this looks like the parallel combination of two current mirrors which is fairly high. Good for current source, bad for voltage source.
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Danny
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Old 12th January 2004, 10:00 PM   #3
Steven is offline Steven  Netherlands
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This topology is used quite often, both in opamps as well as in dicrete amplifiers. And both with BJTs and MOSFETs.

Steven
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Old 13th January 2004, 07:24 AM   #4
DarkOne is offline DarkOne  Slovakia
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Thanks for your replies.

If this is used in operational amplifiers, I think that overall performance is good.
High output impedance is not a problem, I'll be driving power mosfets directly from this stage and I've set circuit currents quite high (200mA for current source, and 40mA AC for driving output mosfets). Output stage is ciclotron (so I'll need this circuit twice, inverting and non inverting).
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Old 14th January 2004, 01:45 PM   #5
ratu is offline ratu  Spain
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Default IRF 540

Sorry, im not shure if i understand you, but you want to use the IRF 540 for an imput stage?

IRF540 is a power mosfet, try to find an smaller mosfet.

Ratu
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Old 15th January 2004, 06:05 AM   #6
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I am thinking of using the same ckind of circuit.
My main problem is how to match the upper current mirrors and the current source in the tail of the diff-stage !


grtz

Simon
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Old 15th January 2004, 08:27 AM   #7
DarkOne is offline DarkOne  Slovakia
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Of course I'm not going to use IRF540 for an input stage. I just had a dozen of them, so I've been testing. I'm going to use IRF610 & IRF9610, maybe in combination with BC550 or some similar small-signal bjt (don't know how you call that circuit when small bjt controls big BJT or FET)
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Old 15th January 2004, 08:35 AM   #8
DarkOne is offline DarkOne  Slovakia
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Quote:
Originally posted by blu_line
I am thinking of using the same ckind of circuit.
My main problem is how to match the upper current mirrors and the current source in the tail of the diff-stage !


grtz

Simon

Current mirrors must consist of matched pairs of transistors and they must be thermaly coupled, because one side has much more power dissipiation. If they were not matched and thermaly coupled distortion will occur and there will be more DC component in the output.

I use the same source resistors in current mirrors as in differential stage, so every part of circuit works with the same current and the same Vgd - distortion will cancel.
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Old 15th January 2004, 11:25 PM   #9
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Very noisy input stage.
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Old 16th January 2004, 08:05 AM   #10
DarkOne is offline DarkOne  Slovakia
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Quote:
Originally posted by john curl
Very noisy input stage.
Why do you think so??
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