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Old 29th March 2012, 08:48 PM   #21
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Quote:
Originally Posted by godfrey View Post
@ Nigel:
It sounds like you're talking about Vbe multipliers for output stage bias, rather than voltage gain stages.
lol sorry its a while since I designed any class AB amps and got the VAS and Vbe mulitpliers mixed up.
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Old 29th March 2012, 08:49 PM   #22
godfrey is offline godfrey  South Africa
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Quote:
Originally Posted by Bonsai View Post
I especially like the collector resistor R1. This doubles up as a current limiter as well.
Yes, the whole point of it is to provide bidirectional current limiting in conjunction with D1 and D3. In practice, there's a couple of gotchas. The trick is to make sure that neither Q1 nor Q3 can turn off. IIRC, the main things to be aware of are Q2's collector current being added in at Q3's emitter, and that the full input stage current flows through D1/D3 during clipping.

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Originally Posted by AndrewT View Post
Does R1 act similarly to Puefue's "magic" resistor? i.e. trying to maintain constant power dissipation.
I guess it does, but that wasn't a design consideration. I expect that allowing voltage swing on Q1's collector actually makes distortion a bit worse, especially as Vce is so low to start with. I considered that a small price to pay in exchange for civilized clipping behavior though.
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Originally Posted by AndrewT View Post
Does the compound current route R9+R2 as the input collector load, change the way the VAS works?
Not really, it just lowers distortion a bit and gives higher gain. A more normal approach might be to connect the bottom of R9 to the -30V rail, but if you do that, then the voltage across R9 = Vbe of Q1 + Vbe of Q2. The problem is that the signal voltage across Q1's base-emitter junction is much larger and more distorted than that of Q2. With a current mirror, it's not an issue as there's no input resistor.

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Originally Posted by AndrewT View Post
Would a pair of 25V Zeners from the +-30Vdc supply rails work as the voltage limiting +-25V references?
Or.
Would it be better to run the next stages @+-25V and then the VAS limit would match the next stages capability?
I'd want to clip the output a few volts before the output stage reaches it's limits. My approach would probably be to run the front end from the same rail voltage as the output stage but with extra filtering, and then set the clipping level 5 volts or so below the front-end supply voltage.

IMHO it's a mistake to use higher voltage rails for the VAS than the output stage to maximize output power. Sure you get a couple of extra watts, but clipping behavior is horrible.
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Old 29th March 2012, 09:21 PM   #23
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If you have output transistor current limiting then you also need current limiting in the VAS.
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Old 29th March 2012, 10:13 PM   #24
godfrey is offline godfrey  South Africa
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Agreed, that's the purpose of R1, D1 and D3. The voltage across R1 is proportional to the VAS current. When the VAS current gets high (or low) enough, D3 (or D1) will start conducting to prevent any further input to Q2's base.

Last edited by godfrey; 29th March 2012 at 10:16 PM.
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Old 29th March 2012, 10:44 PM   #25
tvrgeek is offline tvrgeek  United States
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godfrey,
Makes sense. I bet where the higher rails are used, additional protection circuitry to make it more beguine is employed. That's one thing I noticed between top amps of the 80's and now. New ones have a LOT of parts for management.

Why such high VAS current? The other amps I have looked at are more like 6 or 8 mA.
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Old 29th March 2012, 10:50 PM   #26
Bonsai is offline Bonsai  Taiwan
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jcx, I am using a beta enhanced VAS that feeds a cascode. And the whole amp has TMC and it appears to be quite stable and I have done a lot of testing on the physical amp. I prefer to see TMC as 'transitioning' the output stage (triple in my case) out of the global feedback loop at HF rather than positive feedback, or are you perhaps talking about a different mechanism?

I think in a classic Darlington connection to the VAS you talk about Homeodder then Cob indeed is an issue, but the VAS feeding the cascode is a little different. Sims show a very big improvement at HF (both with and without TMC). The whole idea of fixing the collector voltages is so that changes in Vce cannot take place and changes in Vcb are minimal anyway (a few mV) However, I'd be interested to see you findings - I might be missing something here.
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Old 29th March 2012, 11:13 PM   #27
godfrey is offline godfrey  South Africa
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Quote:
Originally Posted by tvrgeek View Post
Why such high VAS current?
It is a bit over the top. I was using a double EF output stage and wanted a VAS with very low output impedance and high output current capability, to drive it with a "fist of iron". (The low output impedance happens with local feedback connected, but requires high current gain, hence the Darlington)
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Old 30th March 2012, 12:38 AM   #28
tvrgeek is offline tvrgeek  United States
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Thanks. Long learning process.

I have seen two pole Miller compensation, called TMC correctly or not, implemented several ways. The leg returned to ground, to a rail ( which makes no sense to me), or to the output. Sometimes they seem to have huge effects, sometimes nothing in basic sim. Where do I get a more complete read on the subject?
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Old 30th March 2012, 03:36 AM   #29
djoffe is offline djoffe  United States
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See Douglas Self's Audio Power Amplifier Design Handbook. In the 5th edition, he covers it some on pgs. 218-222, and 362 (look under compensation in the index).

Douglas puts a bit more information on TMC (transitional miller compensation)
at this link, the "Baxandall Papers".

The Baxandall Papers: Transitional Miller compensation

Jan Didden has available more of that here:

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Old 30th March 2012, 07:55 AM   #30
godfrey is offline godfrey  South Africa
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Quote:
Originally Posted by tvrgeek View Post
...two pole Miller compensation, called TMC correctly or not...
There's been a lot of discussion of TPC (two pole compensation) and TMC (transitional Miller compensation) in Bob Cordell's threads, especially this one: Bob Cordell Interview: Negative Feedback, but also these two: Bob Cordell's Power amplifier book and Bob Cordell Interview: Error Correction.

Long threads, but worth reading if you ever have the time.
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