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-   -   Cantilevered-Cascode Buffer (CCB) aka LinuxBuffer (http://www.diyaudio.com/forums/analog-line-level/220622-cantilevered-cascode-buffer-ccb-aka-linuxbuffer.html)

linuxguru 29th September 2012 02:56 AM

Cantilevered-Cascode Buffer (CCB) aka LinuxBuffer
 
The purpose of this thread is to discuss the Cantilevered-Cascode Buffer aka LinuxBuffer, a simple high-performance voltage-follower/buffer. Anything related to this buffer and variants may be posted here. Please also post links here to previous postings on the subject. The intent is to declutter the Discrete Opamp Open Design thread.

RNMarsh 29th September 2012 02:58 AM

Ok. Pls show circuit here, too. And, your changes.

linuxguru 29th September 2012 03:40 AM

1 Attachment(s)
Here's the basic schematic of the CCB:

RNMarsh 29th September 2012 04:42 AM

Got performance FFT data? And, can it be made to drive 30 Ohms... how? For driving low Z headphones.
And, what is the explanation on how it works?

Thx - Richard

linuxguru 29th September 2012 06:04 AM

A brief explanation of how it works was posted earlier at the Discrete Opamp Open Design thread: the main idea is to operate the lower JFET at nearly constant Id, Vds and Vgs; almost all of the load current swing occurs in the lower NPN, outside the direct signal path.

steveh49 29th September 2012 06:56 AM

It's essentially the same circuit as Steven's 'Constans'. See post #13 here and the discussion that follows:

http://www.diyaudio.com/forums/solid...utput-fet.html

Edit: also just found a JFET version (post 6): http://www.diyaudio.com/forums/solid...tml#post803157

Ken Newton 29th September 2012 12:53 PM

Quote:

Originally Posted by steveh49 (Post 3183299)
It's essentially the same circuit as Steven's 'Constans'. See post #13 here and the discussion that follows:

http://www.diyaudio.com/forums/solid...utput-fet.html

Edit: also just found a JFET version (post 6): http://www.diyaudio.com/forums/solid...tml#post803157

These do appear to be interesting variations on a White follower, with either straight or folded cascoding.

linuxguru 29th September 2012 01:46 PM

Quote:

Originally Posted by steveh49 (Post 3183299)
It's essentially the same circuit as Steven's 'Constans'. See post #13 here and the discussion that follows:

http://www.diyaudio.com/forums/solid...utput-fet.html

Edit: also just found a JFET version (post 6): http://www.diyaudio.com/forums/solid...tml#post803157

The idea is similar, but there's a subtle difference in the topology that makes a huge performance difference. That buffer references the base of the cantilever PNP to the output (emitter/source), while mine references it to the *input* (base/gate).
While it looks similar, that buffer needs dominant-pole compensation for stability, which limits BW. Mine requires little or no compensation - BW is limited by the devices alone.

RNMarsh 29th September 2012 07:43 PM

Quote:

Originally Posted by linuxguru (Post 3183631)
While it looks similar, that buffer needs dominant-pole compensation for stability, which limits BW. Mine requires little or no compensation - BW is limited by the devices alone.

That is a hall-mark of the current-mode feedback circuits, as well. No additional Cdom added for stability. How is this circuit like a current-mode feedback buffer?

steveh49 1st October 2012 02:00 AM

2 Attachment(s)
Quote:

Originally Posted by linuxguru (Post 3183631)
The idea is similar, but there's a subtle difference in the topology that makes a huge performance difference. That buffer references the base of the cantilever PNP to the output (emitter/source), while mine references it to the *input* (base/gate).
While it looks similar, that buffer needs dominant-pole compensation for stability, which limits BW. Mine requires little or no compensation - BW is limited by the devices alone.

I've just tried them both in LTspice using transistor models that come with the software - schematic and 20kHz square wave attached. The green trace is the Linuxbuffer and the blue Constans. Using these models and 100pF load capacitance, the Linuxbuffer oscillates at a lower source impedance than the Constans. Caution may still be required.


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