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Old 21st November 2008, 12:04 AM   #111
GK is offline GK  Australia
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Quote:
Originally posted by vynuhl.addict



Glen,

This is right, remember Aksa noted that after removing the Current Mirror the gain in the LTP will decrease now that it is R loaded, with lower fb factor the lag does not need to remain as large as the OLG as a whole has now decreased. If it were the opposite, replacing passive with a CM, then you would increase lag comp. The ever lasting balance between FB factor and the need to lower the dominant pole as olg is increased, you know this very well..




Colin

No it isn't right. With miller (lag) compensation increasing the open loop gain by decreasing the loading on the long tail pair does not change the unity loop gain frequency or stability it decreases the open loop bandwidth.
Attached below is a simulation of two differential VAS op-amps (with the same 68pF miller comp cap in each).
Op-amp A (green trace) has the current mirror LTP. Op-amp B (red trace) has a pair or resistors instead.

So no, by getting rid of the current mirror you will not be able to get away with a smaller miller compensation capacitor.
If you wan't to get away with a smaller miller comp cap then you will have to reduce the LTP's gm by increasing its emitter degeneration. This is fundamental.
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File Type: jpg comp.jpg (98.3 KB, 638 views)
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Old 21st November 2008, 12:28 AM   #112
vynuhl.addict is offline vynuhl.addict  Canada
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Ostrpper,


It looks good, love to see people out there building ideas, no better way to learn than hands on. C4 will of course take some heat off of the miller compensation cap and thats where the compensation gets fun, experimenting with the values/ration of each. The only change I would make is to tap C4 15pf off of the VAS output to the inverting base of the LTP, aside from that time to build!.



Colin
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Old 21st November 2008, 01:03 AM   #113
lineup is offline lineup  Sweden
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I see you use150R emitter for input pair.
At 2.2 mA you have drop >300 mV
If you read D. Self blameless input stage, you see he have 'only' 68 mV drop at one case.
This would be like you using 33R.

It is good to have a bit higher gain in input stage.
Because this is high quality gain = Fast transistors. low noise and good linearity.

Lower your emitter resistors from 150R to 33R / 47R
will increase Gain in first stage with a factor 3-4.
----------------

I am sure you studied my attach circuit very closely
but here it is again

I have 4 input transistors. Each of them is operating at a gain a bit less than x50. (5000 Ohm /100 Ohm)
As I run them at 1 mA, the drop across emitter resiistors is: 1 mA x 100 Ohm = 100 mV


I do not know if my input stage contributes 4x50 = Gvolt 200
Or if the contributed gain is only x50.
Glen, you can tell me, please. thanks

Lineup
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Old 21st November 2008, 01:13 AM   #114
ostripper is offline ostripper  United States
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Thanks , vynuhl.addict, I'm building the sim now. Just to see if I'm in the ballpark.
As far as C4 , you mean FULL NFB right from the VAS?? I've seen
that in some amps, but not many. maybe for the best for this
topology has a lot of OLG.

I'm pulling my hair out for a strange reason, these NJW
OP devices have some new plastic that acts as an insulator
interfering with my Vbias generator. I can't feel heat on the body
of the devices but the heatsinks are plenty warm.
I played with the Vbias ratio but it is right for a
BD139 (symasym amp uses my exact values, I noticed).
So, I'll try to mount on HS, see if that corrects issue.
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Old 21st November 2008, 01:30 AM   #115
ostripper is offline ostripper  United States
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Lineup, you mean MY emitter degens??? Put in 47R to give more gain?? I'll try it.
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Old 21st November 2008, 01:52 AM   #116
AKSA is offline AKSA  Australia
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Pete,

Remember that as you increase gm of the input stage by reducing emitter degeneration, OLG for the whole amp will increase, and miller comp will need to be bumped up slightly.

Self says that a CM will give 6dB more OLG, and so the same argument applies here. I've found that LTP degeneration has profound influence on lag comp, so be careful, don't connect up a speaker until you examine the output on a CRO, or at the least feel for heat on a Zobel resistor (put in 10R/100nF, it's excellent practice and a good detection tool for HF oscillation).

Good going, nearly there......

Hugh
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Old 21st November 2008, 02:15 AM   #117
GK is offline GK  Australia
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Quote:
Originally posted by ostripper
Lineup, you mean MY emitter degens??? Put in 47R to give more gain?? I'll try it.

Ugh. Did you read my post 111?
If you change the emitter degeneration from 150 ohms to 47 ohms you will need to increase the miller cap pretty much proportionally (not "slightly") to maintain the same unity loop gain frequency (unless you increase the closed loop gain ~proportionally instead, which you likely would not like to do).

Cheers,
Glen
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Old 21st November 2008, 02:30 AM   #118
ostripper is offline ostripper  United States
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Glen ,I had no idea which amp lineup was referring to ,1 or 2.
And being very tired, browsed right through yours.

I see you are a more advanced user of Pspice software , which do you use.?

Have patience,maybe the LTspice I have sucks, are there better.
I'll get it pronto.
Also, very hard to see your sims...
sorry..
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Old 21st November 2008, 02:37 AM   #119
GK is offline GK  Australia
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Quote:
Originally posted by ostripper
Glen ,I had no idea which amp lineup was referring to ,1 or 2.
And being very tired, browsed right through yours.

I see you are a more advanced user of Pspice software , which do you use.?

Have patience,maybe the LTspice I have sucks, are there better.
I'll get it pronto.

sorry..

OK. I'm using a year old + version of LT spice (to lazy to update because the downloadable updates take tooooo long ). Have a look in the example folders. The are some good files there (called "loopgain" or such) showing how to sim the (closed) loop gain and phase of an operational amplifier.

Cheers,
Glen
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Old 21st November 2008, 02:47 AM   #120
ostripper is offline ostripper  United States
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Quote:
by AkSA - least feel for heat on a Zobel resistor
I did that. In fact, I knew of the OLG/compensation ratio
previously, by looking at a thousand amps.

I know I shouldn't learn like this , but I often go through the
"back door" first to get it done, then learn the theory.

I know there an equation for this relationship.
Thanks, dean...

And Glen, I'm really not a dummy, just 3 hrs. sleep in 48..
OS
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