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Old 21st March 2012, 02:42 AM   #111
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Jim, you got it right!
Ihquam, thanks for the answer also!
I lost the whole day checking it but couldn't find the problem...
That's why NP Citation 12 wasn't working also.
Besides IRFP240, 2SJ74 and MJE15031 were also inverted.

Now I can bias it, but I need 30V at input to produce 500mV at output. Perhaps the JFET models are problematic? Jim, what models are you using?
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Old 21st March 2012, 02:59 AM   #112
lhquam is offline lhquam  United States
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Quote:
Originally Posted by Daniel(Del) View Post
Jim, you got it right!
Ihquam, thanks for the answer also!
I lost the whole day checking it but couldn't find the problem...
That's why NP Citation 12 wasn't working also.
Besides IRFP240, 2SJ74 and MJE15031 were also inverted.

Now I can bias it, but I need 30V at input to produce 500mV at output. Perhaps the JFET models are problematic? Jim, what models are you using?
Your wiring to the base of cascode transistor Q3 is wrong. R18 is connected to ground, not the voltage from the resistor divider.
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Old 21st March 2012, 03:47 AM   #113
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Thanks for finding it out, Ihquam!

Besides that I also had to change 2SK170 by Multisim original model and 2SJ74 by 2SJ108 original model too, because the models I was using didn't work at all. I had to have very large input levels.

What about R19? I was using 10k because I thought it should be symmetrical with upper side, but I've just noticed it is really 47,5k on schematics, while R1 is 10k. What's the right value?
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Old 26th March 2012, 04:06 AM   #114
delecoy is offline delecoy  United States
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Hello all!

It may already be buried in a thread somewhere around here, but i dont seem able to find it. What idss shoudl one aim for on the 2SK170's for the F5T V3? 10ma? Trying to gather parts.......slowly.

Thank you much!

Cody
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Old 26th March 2012, 10:32 AM   #115
AndrewT is offline AndrewT  Scotland
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Old 4th April 2012, 05:38 AM   #116
tinitus is offline tinitus  Europe
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just now i stumbled over this, from F5 turbo article

First off, you get the 44 amps. In addition, with Fets you get an output stage
that will deliver more Class A power at a given bias figure due to the square
law character of the Fets. Unfortunately you also tend to get a thermally
unstable circuit that is prone to bias hogging. This is true of Vertical Mosfets
and even more so with Bipolar transistors. There are a couple of examples of
high end amplifiers on the market which boast no ballast resistance, but the
praise for their sound is accompanied by rumors about reliability.

As an alternative, you can consider carefully matched Lateral Mosfets which
have a declining temperature coefficient. Unfortunately they tend to have
lower current ratings, and so are not suited to this particular goal.

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Old 4th April 2012, 06:37 AM   #117
tinitus is offline tinitus  Europe
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Quote:
Originally Posted by Daniel(Del) View Post
May someone help me finding out what is going wrong with the following simulation?
can I ask why you show schematic using cascoded jfets, with only 25V rails
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Old 4th April 2012, 05:43 PM   #118
lhquam is offline lhquam  United States
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Default FQA36P15 FQA28N15 SPICE models?

Does anyone have SPICE models for FQA28N15 and FQA36P15 ?
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Old 4th April 2012, 05:54 PM   #119
Tea-Bag is offline Tea-Bag  United States
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Quote:
Originally Posted by lhquam View Post
Does anyone have SPICE models for FQA28N15 and FQA36P15 ?
Isn't the input capacitance too high on these?

FQA36P15
2550typ
3320max
pF

FQA28N15
1250
1600
pf

FQA19N20c
830
1080
pf

FQA12P20
920
1200

pf
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Old 4th April 2012, 06:58 PM   #120
lhquam is offline lhquam  United States
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Quote:
Originally Posted by Tea-Bag View Post
Isn't the input capacitance too high on these?

FQA36P15
2550typ
3320max
pF

FQA28N15
1250
1600
pf

FQA19N20c
830
1080
pf

FQA12P20
920
1200

pf
I bought 25 each of FQA36P15 and FQA28N15 because of the apparent unavailability of FQA19N20c and FQA12P20.

Where can one obtain FQA19N20c and FQA12P20?
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