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Old 13th March 2003, 01:32 PM   #1
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Default Help me with my JFET / BJT design

OK... I tried to attach an image.. but its in emf? format.

Simply put... the pre-amp was simulated with 2N3819's and BC547's.

It has a cascode 2N3819 with a 560 ohm source resistor and a 10K drain resistor.

Running from a +36v supply... that places the upper 2N3819 drain at around 18v.

This is taken from this point into 3 paralleled BC547 emitter followers with a common 270 ohm resistor.

Output is taken from this point via a 1uf cap and a 10K load resistor.

Simulating the circuit... I get 0.0001% THD with 10v peak output at 1Khz.... but at 10Khz get 3.53% THD...

Why the huge difference?

Should I be running more current in the cascode?

btw... I need a voltage gain of 10.

Thanks!!!!
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Old 13th March 2003, 09:58 PM   #2
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Default Simulations

Hi Richard

First of all it would help with a schmatic

The THD simulation is difficult, since it's based on samples, and a FFT (Fast fourier transform) The problem is problably the simulator and not your amp.

Also the THD needs a center freq to do the calculations. What program do you use?

My experience is that simulations are less than perfect regarding THD, but a nice tool to make sure all your DC levels are right.

\Jens
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Old 13th March 2003, 10:10 PM   #3
mlloyd1 is online now mlloyd1  United States
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yeah, we need a schematic.

and I think most of us have come to the conclusion that distortion numbers on a simulator are about as close to worthless as you can get. there was at least one thread on this a while back.

sorry

mlloyd1
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Old 14th March 2003, 01:00 PM   #4
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Default schematic?

I am using a SIMetrix demo software.

Ahh... I think I figured it out:
Attached Images
File Type: jpg schematic.jpg (32.0 KB, 340 views)
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Old 14th March 2003, 01:30 PM   #5
jam is offline jam  United States
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What happens if you replace R4 with a current source?

Regards,
Jam
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Old 14th March 2003, 01:39 PM   #6
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Default Advantage?

The output darlington's don't really add a lot to THD I think... so what would be the advantage?

Would it be better to put one on the Source of the cascode?

Thanks!
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Old 14th March 2003, 01:52 PM   #7
jam is offline jam  United States
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Possibly,

Why don't you try both and report your results.

Regards,
Jam
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Old 14th March 2003, 01:55 PM   #8
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Default split supply

oh... well since the source of the FETS are about 1v above ground.

And btw I changed the drain resistor to 12K and the source to 680. Better symmetrical clipping...

I would have to go with a split supply to build a CCS for the source of the input FETS.

I could try a CCS on the output NPN....
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Old 14th March 2003, 01:59 PM   #9
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Default CCS on output

Remember that I am simulating all of this... and don't have anything built..

the CCS didn't change anything wrt the 560 ohm resistor.

the simulated distortion is dominated by the input FETS.
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Old 14th March 2003, 02:47 PM   #10
jam is offline jam  United States
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I suppose you could cap couple the input and use a voltage divider to pull up the gate of the fet to avoid building a split supply.

Regards,
Jam
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