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Old 17th August 2007, 07:48 AM   #1
roender is offline roender  Romania
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Default Smart Folded Cascode power amp, SFCamp

Hello my friends,

What do you think about this amplifier and what can be improved?
My concern is stability in folded cascode current sources
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Old 17th August 2007, 11:11 AM   #2
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Q11 and Q13 work in "zener Vbe" ??
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Old 17th August 2007, 12:25 PM   #3
roender is offline roender  Romania
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R21, Q11, Q29 and R22, Q13, Q30 are components of a well known ccs, aka "two BJT CCS".
J3, R1 and J4, R2 are another two current sources used for first two ccs biasing (2sk246 has zero tempco at 0.34mA)
That first two BJT ccs form a folded cascode VAS because R11 and R22 currents are modulated by LTP currents.
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Old 17th August 2007, 12:58 PM   #4
x-pro is offline x-pro  United Kingdom
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Quote:
Originally posted by roender
R21, Q11, Q29 and R22, Q13, Q30 are components of a well known ccs, aka "two BJT CCS".
You've drawn Q11 and Q13 upside down.

Cheers

Alex
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Old 17th August 2007, 01:04 PM   #5
roender is offline roender  Romania
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sorry, my mistake

Thank you, Alex.
What do you think about this amp?
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Old 17th August 2007, 01:35 PM   #6
x-pro is offline x-pro  United Kingdom
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Quote:
Originally posted by roender
What do you think about this amp?
Looks OK. It is not in my taste thought. Too many transistiors for too little OL gain . Also I would not use 2SK170 for the input as these have too much capacitance (IMHO) . My preferred VAS is here:

VAS with CD-CB stage

In one of the versions I've used a cascoded JFET pair (selected BF244B) on the input (in a similar arrangement to your schematics) with very good results (in Creek A52/A52SE power amplifier).

Cheers

Alex
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Old 17th August 2007, 01:59 PM   #7
roender is offline roender  Romania
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95dB OLG it is ok for a moderate NFB design.
As per k170, I like the sound of Toshiba jfets.
It is important to have a low capacitance j-fet in a cascoded stage?

Thank you,
Mihai
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Old 17th August 2007, 02:19 PM   #8
Bonsai is offline Bonsai  Taiwan
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Alex,
if input diff pair is FET and it drives a cascode, surely the input capacitance is less of a problem? What's your view on this?

I've simulated this type of input circuit on a pre-amp design and get great results.
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Old 17th August 2007, 02:24 PM   #9
x-pro is offline x-pro  United Kingdom
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Quote:
Originally posted by roender
95dB OLG it is ok for a moderate NFB design.
As per k170, I like the sound of Toshiba jfets.
It is important to have a low capacitance j-fet in a cascoded stage?
95dB sounds fine. Is this figure from a simulation?

Re. JFET - I found in the past that higher capacitance in the input stage is not very good for the sound. However in your circuit you need as high transconductance as possible in a first stage otherwise you will lose the OL gain.

Cheers

Alex
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Old 17th August 2007, 03:43 PM   #10
roender is offline roender  Romania
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Quote:
Originally posted by x-pro


95dB sounds fine. Is this figure from a simulation?

Re. JFET - I found in the past that higher capacitance in the input stage is not very good for the sound. However in your circuit you need as high transconductance as possible in a first stage otherwise you will lose the OL gain.

Cheers

Alex
Yes, 95dB come from simulation.
It is a one stage amp and as you already had mention, it is nice to have a high transconductance LTP (see how big is the cureent in the front stage ...) Also, a good current mirror in the "VAS" is another trick used to enable high OLG
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