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Old 26th November 2008, 10:32 PM   #1
jgedde is offline jgedde  United States
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Default Complementary input stages with current mirrors

OK, I know this has been covered again and again. I have read threads regarding this subject many times over. Nonetheless, I need to make sure I have a firm grasp of this subject before I continue experimenting.

I see two fundamental problems with this topology. I need the gurus to tell me if I've got it.

1) There will likely be a "balance" issue between complementary halves of the input stage. A circuit needs to be included to keep them balanced. If not included, I would anticipate one half riding the rail and the other doing all the work. I'm not sure I understand why this isn't a problem with a no current mirrors topology. I'm assuming the current mirrors raise the OL gain causing this to occur.

2) Using a Slone style straight push-pull VAS without a current source won't work because the VAS idle current is completely undefined.

Yes?

Thanks,
John Gedde
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Old 26th November 2008, 10:54 PM   #2
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I'm certainly not a guru, but I do remember seeing something somewhere that a complimentary input shouldn't use current mirrors.
I might be (probably am) wrong though.
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Old 27th November 2008, 01:55 AM   #3
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Default Re: Complementary input stages with current mirrors

Quote:
Originally posted by jgedde
OK, I know this has been covered again and again. I have read threads regarding this subject many times over. Nonetheless, I need to make sure I have a firm grasp of this subject before I continue experimenting.

I see two fundamental problems with this topology. I need the gurus to tell me if I've got it.

1) There will likely be a "balance" issue between complementary halves of the input stage. A circuit needs to be included to keep them balanced. If not included, I would anticipate one half riding the rail and the other doing all the work. I'm not sure I understand why this isn't a problem with a no current mirrors topology. I'm assuming the current mirrors raise the OL gain causing this to occur.

2) Using a Slone style straight push-pull VAS without a current source won't work because the VAS idle current is completely undefined.

Yes?

Thanks,
John Gedde
You are correct. A complementary input stage with current mirrors, like Slone, has its idle current undefined if transistor betas go large. There are ways around this to largely have your cake and eat it too. One approach is to return a resistor from the output of the current mirror to a fixed voltage that is the voltage needed to be applied to the VAS input to establish the desired VAS standing current. The fixed voltage source can be made to have the right temperature coefficient to match the input characteristic of the VAS. For example, a Darlinton VAS with a 22 ohm emitter degeneration resistor and a standing current of 10 mA will want a fixed voltage source of about 2 Vbe plus 220 mV below the rail. This is a bit oversimplified, because the Vbe's may not all be the appropriate value due to different current densities, but you get the idea. The added resistor can often be of a fairly large value compared to the size of the load resistor that would be used in a non-current-mirror-loaded input pair.

Another issue of concern with complementary input pairs has to do with transconductance matching. This is a bigger concern with JFETs, where the P-channel and N-channel pairs may not have the same transconductance at the same operating current. Mismatched transconductances can cause increased distortion.

Cheers,
Bob
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Old 28th November 2008, 01:10 AM   #4
jgedde is offline jgedde  United States
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Thanks Bob. You certainly qualify as a guru!

I'm not sure what you mean about the "output" of the mirror. You mean the "load terminal"? I would expect your suggestion to mean the tied bases of the mirror transistors.

In any case, how about this?

Thanks!
John
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Old 28th November 2008, 02:01 AM   #5
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Hi,
Sorry, the non-guru again with a question: What is gained? Other than cost and complexity?
That is an audio amp isn't it?
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Old 28th November 2008, 02:25 AM   #6
GK is offline GK  Australia
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Hi

I thought we covered this already?

My new amp design - the Overkill 180

http://www.diyaudio.com/forums/showt...51#post1413451

BTW, how did the OK180 work out?

Cheers,
Glen
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Old 28th November 2008, 02:36 AM   #7
jgedde is offline jgedde  United States
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Good question. A question to which I don't have a convincing answer. It is an audio amp.

Because it's there?. The challenge to get the hypercomplex to work? An obsession with measurements? Good old fashioned psychosis? Perhaps all of the above....

I design "out there" electronics for a living in an industry where better is not always the enemy of good enough. $1200 MOSFETs, $200 2N2222As, $400 LM139's and $6000 FPGAs are routine. The design always must outperform specification even when operating under "outside nominal" conditions. On the other hand, cutting edge components are shunned as not having "heritage." The trick is to use old technology to achieve bigger, better, and faster goals.

Gentlemen, we have the technology...

One can never have too much money, a lady too pretty, too much horsepower, or low enough distortion...

John
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Old 28th November 2008, 02:45 AM   #8
jgedde is offline jgedde  United States
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Quote:
Originally posted by G.Kleinschmidt
Hi

I thought we covered this already?

My new amp design - the Overkill 180

http://www.diyaudio.com/forums/showt...51#post1413451

BTW, how did the OK180 work out?

Cheers,
Glen
Hey Glen! Thanks for chiming in. We did indeed cover this already. I needed confirmation that I truly understood the challenges at hand.

As you can clearly see, your Kleinschmidt 12 was an inspiration. The OK180 is still an elusive goal unfortunately. You're looking at "son of OK180."

I'd love to hear more about the K12. How did it sound? Did it perform as well as you expected or hoped? How's that Land Rover coming along?

I see you've changed your avatar... Where's Rachel?

Cheers,
John
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Old 28th November 2008, 04:12 AM   #9
jcx is online now jcx  United States
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maybe my simmed "solution" will help you see if you've understood the issue:

"optoisolator VAS bias for Comp Diff?"
optoisolator VAS bias for Comp Diff?

also its hard to tell which thread by the default link #, have you seen:

"Unstable VAS current in amp from Slone book"
optoisolator VAS bias for Comp Diff?
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Old 28th November 2008, 12:05 PM   #10
jgedde is offline jgedde  United States
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Now that's clever! I hadn't seen that before...

The second link was the same as the first. It seems when I search for something I only get results that aren't in the archive...

Thanks,
john
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