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Old 11th October 2012, 10:00 PM   #2461
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Quote:
Originally Posted by Bob Cordell View Post
Hi Doug,

I'm sorry I'm late to get onto this. It looks like you have already gotten some pretty good answers. The circuit in Randy Slone's book is where I first saw this problem circuit. Most of us like current mirror loads for a lot of good reasons you and I have explained. Many of us also believe strongly in a push-pull VAS. Many also like the symmetry of the double complementary input stage (it looks so nice and symmetrical when you draw it, at least). Put those ideas all together and you get this problem circuit.

The key to understanding the problem is that it is at minimum unwise to design a circuit that depends on the non-ideality of the devices. For this reason, we must be able to calculate the VAS standing current with infinite beta and no Early effect in the NPN and PNP VAS Darlinton pairs. We must also assume no Early effect in the other transistors as well. But now we must also factor in other real-world mismatches, like input transistor diff pair Vbe offset and differences in emitter degeneration resistor values due to tolerance. It is then easy to see that one can arrive at virtually any VAS standing current, given small mismatches and extremely high beta transistors.

I'm surprized that you were able to calculate a standing current value, and I'm gussing you had to make some assumptions about finite beta, etc.

I suspect a SPICE run will readily show the problem if you use high-beta transistors and make all of the degeneration resistors 470 ohms except one in my Figure 7.9. Take that one and vary it from 440 to 500 ohms, and you should see the VAS standing current go all over the place.

Note in Figure 7.10 I show how to greatly mitigate this problem in this circuit while still retaining most of the advantages of the current mirrors. I just add a helper transistor to the current mirror and add some "load" resistance.

Cheers,
Bob
Hi Bob

It's interesting you're touching on this circuit as the circuit crossed my thoughts when I was mulling over what traits I wanted my MF500 design to fulfill. The fun thing here is that this can also be described in the manner of "what wants to happen" in a circuit. The situation assumes a proper working GNFB:

An mirror loaded LTP output always wants to 'throw' its voltage to make sure the mirror balances out. In a symmetric LTP scenario, there are now two outputs that each try to settle their outputs so the result would be a zero output signal when there's no input signal.

That's where the fun starts. Assume both LTPs settle 'right' for a given VAS current to run. There are an infinite number of voltage points for both legs at which the output is effectively zero. The only difference? A wildly varying VAS quiesent current! So what happens? The circuit is highly eratic, neither LTP can't settle against anything and will most likely result in blown drivers the second power is applied. Current likes to find the lowest resistance path and WILL abuse the inability for the LTPs to define bias current.

That's why I designed my VAS in such way that I no longer need a symmetrical input stage; I can use a single-ended differential pair to drive my VAS with a current mirror loaded IPS. The VAS itself is a fully symmetric push-pull setup so I'm thinking I managed to combine the 'best' of different worlds
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Old 11th October 2012, 10:29 PM   #2462
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Originally Posted by DouglasSelf View Post
Thank you all for your input. I thought this subject would benefit from a bit of ventilation. I must admit to being intrigued by the very low THD produced by a circuit that in theory cannot work at all. I will try to find how it does with such a tiny standing current when I get some spare time. (hollow laughter)
Hello Douglas,

That you ended up with a 'working' prototype is a fluke. The reason why it can not work is because the input stage generates two control signals of which neither is referenced. The 'only' reference is the reference generated by the GNFB of the circuit, derieved from the input signal. You will have two control signals operating independantly trying to 'fight' for a proper output value. The more ideal and similar those control paths are, the more undefined the circuit becomes.

It's because of the actual differences between each control signal path's impedances, device parameters etc that causes one control signal path loop (in context to VAS current biassing) to be slower or faster than the other, making one of them dominant. Your circuit differences in the two paths force the VAS standing current to a nearly-off state, the few uA standing current. You're lucky; any transient in the circuit could nudge either loop into opening the flood gates... err VAS gates, or immediately start up in that state.

In process control terms, there is no absolute reference within and so it will chase its own tail.

You wondered about incresed instability at 20KHz + right? The nudging is getting strong enough right there to make the other control path dominant, ending up with two control paths fghting for domination over the same process output.
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Old 11th October 2012, 10:45 PM   #2463
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Hi MagiBox,
Quote:
It's because of the actual differences between each control signal path's impedances, device parameters etc that causes one control signal path loop (in context to VAS current biassing) to be slower or faster than the other, making one of them dominant.
This is reflects my way of thinking about this topology since years.

Quote:
That's why I designed my VAS in such way that I no longer need a symmetrical input stage; I can use a single-ended differential pair to drive my VAS with a current mirror loaded IPS. The VAS itself is a fully symmetric push-pull setup so I'm thinking I managed to combine the 'best' of different world.
Is there any link where the schematics of your VAS can be seen ?
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Old 11th October 2012, 10:50 PM   #2464
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There is, but it is an unfinished concept. I was busy prototyping the concept, but due to time and budget constraints I had to shelve the project It's the first thing to pick up again as soon as I get some time for it. You might want to search for the term "MF500" in this forum, it should show you the thread detailing the amp concept.
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Old 12th October 2012, 01:54 PM   #2465
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Originally Posted by MagicBox View Post
Hi Bob

It's interesting you're touching on this circuit as the circuit crossed my thoughts when I was mulling over what traits I wanted my MF500 design to fulfill. The fun thing here is that this can also be described in the manner of "what wants to happen" in a circuit. The situation assumes a proper working GNFB:

An mirror loaded LTP output always wants to 'throw' its voltage to make sure the mirror balances out. In a symmetric LTP scenario, there are now two outputs that each try to settle their outputs so the result would be a zero output signal when there's no input signal.

That's where the fun starts. Assume both LTPs settle 'right' for a given VAS current to run. There are an infinite number of voltage points for both legs at which the output is effectively zero. The only difference? A wildly varying VAS quiesent current! So what happens? The circuit is highly eratic, neither LTP can't settle against anything and will most likely result in blown drivers the second power is applied. Current likes to find the lowest resistance path and WILL abuse the inability for the LTPs to define bias current.

That's why I designed my VAS in such way that I no longer need a symmetrical input stage; I can use a single-ended differential pair to drive my VAS with a current mirror loaded IPS. The VAS itself is a fully symmetric push-pull setup so I'm thinking I managed to combine the 'best' of different worlds
Hi MagicBox,

You've provided a very good description of the problem with that circuit. Although the problem can be overcome, I also prefer to have a single unipolar input stage, made with good N-channel duals that are readily available from companies like Linear Systems.

Cheers,
Bob
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Old 13th October 2012, 04:25 PM   #2466
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Hi Bob,

Yeah basically you'd be turning one of the control signal paths into reference signal paths (just like a CCS output is for example). I've come across what seems to be very interesting types of mosfets. I'm considering to negotiate samples to see how these would do for inputstages and mirrors. They have a tight matching and can dissipate up to 500mW. With the number of transistors in a package, one package could house a cascoded input stage. I really wonder how they'll do. They have a very low input capacitance as well and near infinite DC gain.

Check them out They'll probably be expensive, but specs wise I think these could be used to build very linear stages when cascoded to honour their voltage/power limits. What would you think of these?

Cheers,
Magic
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Old 14th October 2012, 04:59 PM   #2467
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Originally Posted by MagicBox View Post
Hi Bob,

Yeah basically you'd be turning one of the control signal paths into reference signal paths (just like a CCS output is for example). I've come across what seems to be very interesting types of mosfets. I'm considering to negotiate samples to see how these would do for inputstages and mirrors. They have a tight matching and can dissipate up to 500mW. With the number of transistors in a package, one package could house a cascoded input stage. I really wonder how they'll do. They have a very low input capacitance as well and near infinite DC gain.

Check them out They'll probably be expensive, but specs wise I think these could be used to build very linear stages when cascoded to honour their voltage/power limits. What would you think of these?

Cheers,
Magic
Hi Magic,

Sounds like an interesting approach to check out. Thanks for the link. I'll check it out when I get back from RMAF. It has been a great show, with some very good-sounding rooms, and the usual cadre of rooms with good equipment that are playing poor material that makes their stuff sound bad. Lots of exhibitors and many, many rooms to see. Quite a bit of good tube equipment and Vinyl equipment on display.

Cheers,
Bob
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Old 14th October 2012, 07:53 PM   #2468
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Originally Posted by Bob Cordell View Post
Hi Magic,

Sounds like an interesting approach to check out. Thanks for the link. I'll check it out when I get back from RMAF. It has been a great show, with some very good-sounding rooms, and the usual cadre of rooms with good equipment that are playing poor material that makes their stuff sound bad. Lots of exhibitors and many, many rooms to see. Quite a bit of good tube equipment and Vinyl equipment on display.

Cheers,
Bob
What good material should they have used then?
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Old 3rd December 2012, 10:53 PM   #2469
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Hello,
I was reading some replies ago about the ill defined vas current with the Slone circuit. Nothing new but but there is another way to avoid the fighting vas current. At least something that works since i built it and could see for myself.

however i didnt progress since one year, so this one is just to post for those interested. Design based on information from E Stuart.

Cheers

HEEEELLLPPP : M. Randy Slone Mirror Image Topology Construction - Troubles
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Old 3rd December 2012, 10:55 PM   #2470
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Here is the link to the thread where to find schematic etc.
cheers

HEEEELLLPPP : M. Randy Slone Mirror Image Topology Construction - Troubles
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