Any reason why opamp based current sources arent uses? - diyAudio
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Old 18th September 2012, 07:34 AM   #1
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Default Any reason why opamp based current sources arent uses?

When looking through schematics I almost always see some kind of vbe/r setup for current sources. Any reason why opamp based current sources arent used? Are there some stability or transient issue with opamps or something? Something like what I have attached for a 1A current source for biasing a class A output.
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Old 18th September 2012, 08:09 AM   #2
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I think one reason is that current sources tend to be floating. If you base it on an opamp, you must make arrangements for the opamp power supply to float as well which adds complexity.

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Old 18th September 2012, 08:11 AM   #3
jitter is offline jitter  Netherlands
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Why make something simple that works more complicated?

In a commercial design I'd say that cost is an important consideration. A cheap opamp is probably at least 10 times more expensive than a transistor.
In cheap commercial audio devices, you tend to find zener + transistor rather than an integrated voltage regulator. I think for the same reason...

Last edited by jitter; 18th September 2012 at 08:15 AM.
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Old 18th September 2012, 08:12 AM   #4
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Originally Posted by janneman View Post
I think one reason is that current sources tend to be floating. If you base it on an opamp, you must make arrangements for the opamp power supply to float as well which adds complexity.

jan didden
Why would the current source be floating? In, for example, an common collector stage it will by the output of the amp stage.
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Old 18th September 2012, 08:28 AM   #5
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Quote:
Any reason why opamp based current sources arent used?
Because in 99% of all cases, a simple standard implementation is good enough. There's no point in improving a circuit element which is not a significant performance limitator.

Samuel
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Old 18th September 2012, 05:21 PM   #6
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Originally Posted by Samuel Groner View Post
Because in 99% of all cases, a simple standard implementation is good enough. There's no point in improving a circuit element which is not a significant performance limitator.

Samuel
Well if you use a simple 2 transistor current mirror you're risking very inaccurate current since youre relying on matching and vbe. Plus burning more idle power. If you use vbe/r by biasing with diodes again youre relying on vbe/r, so inaccurate.

If you use a 50 cent opamp, instead, you get extremely accurate current that does not depend on a random value such as vbe plus much higher rout. How is that not a good trade off given how cheap general purposes opamps are?
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Old 18th September 2012, 05:47 PM   #7
jaycee is offline jaycee  United Kingdom
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Most opamps are +/-15V. This doesn't lend itself well to power amps where the rails are often much higher. Also, it really isn't that critical and even a Wilson mirror is often overkill.
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Old 18th September 2012, 06:08 PM   #8
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Originally Posted by jaycee View Post
Most opamps are +/-15V. This doesn't lend itself well to power amps where the rails are often much higher. Also, it really isn't that critical and even a Wilson mirror is often overkill.
So if you are using a amp with lower rails like +/- 15 there isn't really a downside to using this current source besides "cost"?
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Old 18th September 2012, 06:28 PM   #9
FdW is offline FdW  Netherlands
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Originally Posted by Fusion916 View Post
So if you are using a amp with lower rails like +/- 15 there isn't really a downside to using this current source besides "cost"?
This is an idea I had earlier the FFT is while simulating 20Vpp in 4Ohm (it is a class A amp at 4Amps bias). The amp is based on 2 voltage controlled current sources; the current sources are opamp based (as you can see ) The rail is at +/- 35V's.
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Old 18th September 2012, 06:44 PM   #10
qusp is offline qusp  Australia
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cost, and..well, you know their evil right?

only joking, I think their handy little suckers for some purposes, but i'd hazard a guess thats why you dont see them so much.

I actually recently got some LT3092 to try out for ccs when roughing up prototypes

Last edited by qusp; 18th September 2012 at 06:46 PM.
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