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Old 2nd July 2011, 10:51 AM   #61
AndrewT is offline AndrewT  Scotland
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would bsp122 be a suitable alternative?
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Old 2nd July 2011, 11:32 AM   #62
dadod is offline dadod  Croatia
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Quote:
Originally Posted by homemodder View Post
Oh it is, think of the high input impedance of a mosfet and how it will affect loading on the LTP, on this forum there is only one other member I know of that has recently discovered it too, thinking he was the first, it was patented in late 1980s. Ive been using it for 15 years.

Soon D Self will have it in his book, I bet.
I use emitter follower before VAS(beta enhancer) and in this case I've got high impedance.
Even small MOSFET are with high imput capacitance(50-200pF) and this is not so good, loads LTP as well.
JLH used small MOSFET in his 80W mosfet amp(1984).
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Old 2nd July 2011, 11:52 AM   #63
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Dadod, the mosfet has much higher impedance than a BJT. I hope you understand me correct, Im saying the beta enhancer should be a mosfet. Just put one in your sim and be surprised, no need for any other circuit modifications except when you tune for sound as a higher current through mosfet sounds a bit better. The much higher input impedance far outway the input capacitance.
I can give more info but it will have to wait till Monday.

Andrew it should work fine.
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Old 2nd July 2011, 12:22 PM   #64
jcx is offline jcx  United States
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it doesn't matter since input Z of vas isn't limiting gain

the mirror output conductance is a limit, can be bootstrapped away
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Old 2nd July 2011, 12:32 PM   #65
dadod is offline dadod  Croatia
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Quote:
Originally Posted by jcx View Post
it doesn't matter since input Z of vas isn't limiting gain

the mirror output conductance is a limit, can be bootstrapped away
Could you elaborate it, preferably with circuit diagram?
dado
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Old 4th July 2011, 03:13 PM   #66
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Originally Posted by jcx View Post
it doesn't matter since input Z of vas isn't limiting gain

the mirror output conductance is a limit, can be bootstrapped away
Im not worried about the gain, the idea is to lower the vas output impedance, one can nearly halve the distortion by using a mosfet as beta enhancer if you choose the mosfet carefully.

JCX have you looked into lowering THD due to vas loading effects ?

Dadod try it out on your sim, then come tell if it drops THD or not.
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Old 4th July 2011, 05:51 PM   #67
Bigun is offline Bigun  Canada
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the beta enhancer adds gain within the VAS/cdom loop to make it more linear but you are still relying on cdom to close this local loop, for good or bad. I think you can enjoy the same distortion reduction by cascoding the VAS device.
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Old 4th July 2011, 09:25 PM   #68
jcx is offline jcx  United States
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Default the vas open loop gain depends on both source and load circuit Z too

there are some complicating issues, degen of the mirror raises the output Z

combining mosfet vas "beta enhancer" with degen in the mirror does show a region of increased vas open loop gain - basically single pole over audio

but bjt in the same position can give quite good (sim, ~140 dB) gain at audio and has a higher high frequency gain

at such high levels of low frequency gain I expect more parasitic limitations to show up


in sum - mosfet vs bjt "beta enhancer" - it depends on surrounding circuit properties but mosfet can show increased low frequency gain, bjt can be faster due to the better gm,C tradeoff

if you use Bob's 20 KHz THD as a metric I expect the bjt to give better numbers, mosfet could be better @ 1 KHz THD - again depending on other circuit properties - such as output stage current gain - just 2 Q "Darlington" output with bjt's low frequency input Z will completly swamp the potential added vas open loop low frequency gain of the mosfet "beta enhancer"

Last edited by jcx; 4th July 2011 at 09:44 PM.
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Old 5th July 2011, 07:34 AM   #69
dadod is offline dadod  Croatia
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Quote:
Originally Posted by jcx View Post
there are some complicating issues, degen of the mirror raises the output Z

combining mosfet vas "beta enhancer" with degen in the mirror does show a region of increased vas open loop gain - basically single pole over audio

but bjt in the same position can give quite good (sim, ~140 dB) gain at audio and has a higher high frequency gain

at such high levels of low frequency gain I expect more parasitic limitations to show up


in sum - mosfet vs bjt "beta enhancer" - it depends on surrounding circuit properties but mosfet can show increased low frequency gain, bjt can be faster due to the better gm,C tradeoff

if you use Bob's 20 KHz THD as a metric I expect the bjt to give better numbers, mosfet could be better @ 1 KHz THD - again depending on other circuit properties - such as output stage current gain - just 2 Q "Darlington" output with bjt's low frequency input Z will completly swamp the potential added vas open loop low frequency gain of the mosfet "beta enhancer"
I did quick test with mosfet "beta enhancer" and, really, mosfet was better at 1kHz(cca half distortion) but bjt was a bit better at 20kHz.
I wanted to eavaluate it more but I have problem with LTspice now. I bought a new PC with Winows 7 home edition 64bits and LTspice blocks PC after a few steps.
Does someone had similar problem and what is a solution??
dado
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Old 5th July 2011, 08:53 AM   #70
dadod is offline dadod  Croatia
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Quote:
Originally Posted by AndrewT View Post
would bsp122 be a suitable alternative?
I am not sure, quite high imput capacitance. Could you send spice model, I can't find it?
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