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Old 24th April 2013, 01:15 PM   #4041
maxente is offline maxente  Romania
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Ok, here's what did
With output bias set to ''0'' i slowly adjust offset to 0V and no problem
Next i tried to rise up output bias to say 30mA, offsset start to swing between -16mV to +50mV, the output heatsink starts to warm a little and those two resistors 33r/10w are hot and if i replace resistors with 2,5A fuse and start to set slowly output bias again the amp burn those fuse
I think your SSA don't like this layout
Any suggestions where the problem might be, or what should i do next?

Last edited by maxente; 24th April 2013 at 01:20 PM.
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Old 24th April 2013, 02:13 PM   #4042
Belaji is offline Belaji  United States
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Quote:
Originally Posted by Lazy Cat View Post
WOW nice take, just needs to be realized. Any obstacles?
Yes a few. Time and I have a new idea on a variation of this which is not complete. My idea (which simulates correctly) is a circuit with no need for thermal feedback. Open loop thermal stability. Included with this is a low distortion output stage. I'm close...

Bela
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Old 24th April 2013, 03:52 PM   #4043
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Quote:
Originally Posted by maxente View Post
Ok, here's what did
With output bias set to ''0'' i slowly adjust offset to 0V and no problem
Next i tried to rise up output bias to say 30mA, offsset start to swing between -16mV to +50mV, the output heatsink starts to warm a little and those two resistors 33r/10w are hot and if i replace resistors with 2,5A fuse and start to set slowly output bias again the amp burn those fuse
I think your SSA don't like this layout
Any suggestions where the problem might be, or what should i do next?
Your SSA is oscillating, needs frequency compensation. That kind of calibration procedure needs oscilloscope. Please don't connect the amp directly, use rail serial resistors until you surpress oscillations.
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Old 24th April 2013, 04:52 PM   #4044
RNMarsh is offline RNMarsh  United States
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BTW -->

CFB is real.... just need to Not adapt a VFB model for it. A new model is shown and described in a complete evaluation and description of the differences between VFB and CFB here:

"Emerging Techniques For High Frequency BJT Amplifier Design: A Current-Mode Perspective." by C.Toumazou, J.Lidgey & A.Payne.

Sponsored by - 1994 First International Conference on Electronics Circuits and Systems. Cairo, Egypt.

Look it up and get a copy. No need to say more on the subject - its all there.

Thx-RNMarsh
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Old 24th April 2013, 07:10 PM   #4045
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Quote:
Originally Posted by RNMarsh View Post
Look it up and get a copy. No need to say more on the subject - its all there.

Thx-RNMarsh
Thanks Mr. Marsh.

Quote:
Originally Posted by maxente View Post
Any suggestions where the problem might be, or what should i do next?
Of course, SSA you've build was never build as a real circuit yet, although it is very similar to TSSA CF output topology, but don't worry will tamed the beast.

Your SSA has not even one compensating capacitor, that is why it is oscillating, so first you must add:
- 100 pF mica capacitor from input bases to GND, as close as possible to input transistors
- serial RC from output to GND, 10 Ohm/2W in series with 100 nF/100 V
- 1 uF/63 V capacitor from rails DC, after the fuses to GND

After this measures will be taken, please report the situation.
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Old 25th April 2013, 11:59 AM   #4046
maxente is offline maxente  Romania
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On output i a have from the beginning 10ohm+100n
I will instal 100p on input + 1uf caps on rails and report back
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Old 26th April 2013, 03:11 PM   #4047
maxente is offline maxente  Romania
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I think i am the first to build this simple amp with BIGBT
It seems that there is no way i can get rid of these oscilations
I mount 100p as close as possible to input bases, then mount 1uf+100n on bouth rails
The offset is stable at +14mV but when i tried to adjust output bias the offset jumps to 80 - 100mv and 10w resistors are very hot
Any ideas how to proceed next ?
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Old 26th April 2013, 05:39 PM   #4048
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Instructions I gave you were just a start, so proceed from there on.
Now you need to add serial RC compensation from mosfet gates to rails. Use 100 Ohm trimmer in series with 50 pF trimmer on both mosfets. For this calibration you would need oscilloscope to observe the output. Fine tuning of these trimmers will set the amp below phase margin and prevent it against instability - no oscillations.
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Old 27th April 2013, 05:51 PM   #4049
maxente is offline maxente  Romania
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I manage borrow a o-scope
I mount 100 ohm trimmer + 47p on each gate to rails
If i adjunst trimmer from the positive rail nothing happens, if I adjust the trimmer from gate to negative rail more oscilation appear
How should i proceed next?
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Old 27th April 2013, 06:48 PM   #4050
ALAZ is offline ALAZ  Iran
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hi to all
Unfortunately 2sk1058/2sj162 not availble in my city but irfp240/9240 is availble.for use this irfp what modification is Necessary for ssa?
regard
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