Smart Folded Cascode power amp, SFCamp

[roender]

Hello my friends,

What do you think about this amplifier and what can be improved?
My concern is stability in folded cascode current sources

 

[darkfenriz]

Q11 and Q13 work in "zener Vbe" ??

 

[roender]

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.

 

[x-pro]

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

 

[roender]

sorry, my mistake

Thank you, Alex.
What do you think about this amp?

 

[x-pro]

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:

http://www.diyaudio.com/forums/showthread.php?s=&threadid=104205

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

 

[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?

Thank you,
Mihai

 

[Bonsai]

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.

 

[x-pro]

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

 

[roender]

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

 

[aparatusonitus]

Also, a good current mirror in the "VAS" is another trick used to enable high OLG

According to J. Carr, for proper FC operation (soundwise and measurements) we need an active CS for FC transistors. Another good trick is to use a voltage regulator with low impedance output to bias the bases of FC transistors (place a base stoppers form 47~100R if you experience some oscillation).

 

[darkfenriz]

You/ve chosen aa rather small idle current of the second stage compared to first stage. Why?

 

[roender]

According to J. Carr, for proper FC operation (soundwise and measurements) we need an active CS for FC transistors. Another good trick is to use a voltage regulator with low impedance output to bias the bases of FC transistors (place a base stoppers form 47~100R if you experience some oscillation).

I known all that recommendations. Jonathan Carr is a big fan of FC topology
Firstly, I have an active CCS for folded cascode transistor.
The front stage will be supplied separately, from a good PSU.
Thank you for remind me that

 

[roender]

You/ve chosen aa rather small idle current of the second stage compared to first stage. Why?

Because I have a triple darlington output stage, there is no need for more current.
You see, when we use active CCS cascode, there is no limitation in "VAS" current. With standard folded cascode, when you increase current in "VAS" you also decrease OLG.
With only 2.5mA "VAS" current I have 75V/us SR which is more than enough

 

[aparatusonitus]

Look for pat.#4959622 and Fig.1b explenation, may be of interest.

 

[roender]

Look for pat.#4959622 and Fig.1b explenation, may be of interest.

The patent, which try to make LTP current equal to 2x differential folded cascode current by using a current mirror between LTP and differential folded cascode stage, it's very hard to be implemented. The circuit will easily oscillate like hell...
Also, will be impossible to apply that patent in a LTP cascoded stage with floating bias reference.