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Old 8th April 2006, 01:24 PM   #1
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Default Calculating VAS current for mosfet output

1. How to calculate vas current for the mosfet output stage?
2. What are the differences in calculations when driving mosfets from the vas stage,directly, and when driving with bjt buffer stage?
3. Anyone tried to experiment the sound quality of normal biased vas stage versus overbiase vas stage?

Thanks!
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Old 8th April 2006, 02:01 PM   #2
jcx is online now jcx  United States
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mosfet outputs require surprizing amounts of current drive at higher frequencies due to gate capacitances

source followers don't completely bootstrap the Cgs and you always see all of the Cgd

a point that i think can be seen in http://www.ne.jp/asahi/evo/amp/J200K1529/report.htm going from fig 7 to fig 13 (lots of mods but i think the performance couldn't get near these levels without the buffering of the output fet gate impedance)
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Old 8th April 2006, 02:34 PM   #3
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I discovered that higher VAS stage bias(only tried in simulations), gives lower distortion on high freq. I tried it in several circuit designs and vas stage bias 50mA at 10KHz, gives 0.2%THD and 10mA at 10KHz, VAS iddle curent gives 1.5%THD. THis is extermly BAD and i dont know is it about the simulation software? I triad several standard designs and I had same pour THD levels...
I use SIMETRIX software for simulation.
The same circuits had very good THD for 1KHz (up to 0.0025%THD).

HELP!

I dont have osciloscope or THD meter to try it in practice...
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Old 8th April 2006, 03:05 PM   #4
AndrewT is offline AndrewT  Scotland
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Hi Jcx,
I have seen links to that site on previous occasions.

Can you help me read it?

What is it telling me?

Do the complex circuits sound better?
__________________
regards Andrew T.
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Old 8th April 2006, 03:21 PM   #5
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Quote:
Originally posted by jcx
http://www.ne.jp/asahi/evo/amp/J200K1529/report.htm

I had to chuckle a bit on this one. If you take figure 1 and change the input FETs to bipolars, add an emitter follower to drive the output MOSFETs, and add a bootstrap on the input transistors to raise the PSRR, you have Greg's "simple killer amp"...
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Old 8th April 2006, 04:28 PM   #6
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anyone to answer my question?
It`s not japanese amps thread!!!
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Old 8th April 2006, 04:41 PM   #7
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Good analysis of this question make years ago Erno Borbelly in The Audio Amateur at paper about his 60 W mosfet amp...I don't remember, which number it was ( maybe half of 80's )....
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Old 8th April 2006, 05:14 PM   #8
ilimzn is offline ilimzn  Croatia
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Quote:
Originally posted by bogdan_borko
I discovered that higher VAS stage bias(only tried in simulations), gives lower distortion on high freq.
Yes, this is normal because the MOSFETs are a capacitive load, so load increases with frequency.

Quote:
I tried it in several circuit designs and vas stage bias 50mA at 10KHz, gives 0.2%THD and 10mA at 10KHz, VAS iddle curent gives 1.5%THD. THis is extermly BAD and i dont know is it about the simulation software? I triad several standard designs and I had same poor THD levels...
I use SIMETRIX software for simulation.
The same circuits had very good THD for 1KHz (up to 0.0025%THD).
There is something odd going on, as for small signals (well below the power rails) the MOSFET Cgd is fairly linear. This would mean that the open loop gain decreases 10x from 1k to 10k, distortion should increase by the same amount.
Make sure your output stage is biased properly!!! If not, there is a large discontinuity in tge MOSFET transconductance, where it drops at crossover, resulting in lowering the 'bootstraping' action for Cgs, which is often up to an order of magnitude higher than Cgd, so current demand on the VAS suddenly increases. At higher frequencies this crossover phenomenon takes a larger 'angle' out of the sinusoidal, therefore increasing distortion far beyond whet a mere capacitance at the VAS output would.

In order to check this, you may want to sheck the waveform of the current flowing into the MOSFET gates in your simulation.
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Old 8th April 2006, 09:18 PM   #9
mikeks is offline mikeks  United Kingdom
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Default Re: Calculating VAS current for mosfet output

Quote:
Originally posted by bogdan_borko
1. How to calculate vas current for the mosfet output stage?
Just buffer the sucker...and you're home free!!
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Old 8th April 2006, 11:26 PM   #10
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Quote:
Originally posted by bogdan_borko
anyone to answer my question?
It`s not japanese amps thread!!!
JCX gave most of the answer in post #2. One thing to consider is that vertical MOSFET output devices have a much higher Cgd (and much more non-linear as well) than do lateral devices. So you need to tell us which output devices you are planning to use and also the configuration (eg, source-follower).
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