Cantilevered-Cascode Buffer (CCB) aka LinuxBuffer - diyAudio
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Old 29th September 2012, 02:56 AM   #1
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Default 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.
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Old 29th September 2012, 02:58 AM   #2
RNMarsh is offline RNMarsh  United States
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Ok. Pls show circuit here, too. And, your changes.
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Old 29th September 2012, 03:40 AM   #3
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Here's the basic schematic of the CCB:
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File Type: jpg ccascode_buf.jpg (59.5 KB, 311 views)
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Old 29th September 2012, 04:42 AM   #4
RNMarsh is offline RNMarsh  United States
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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
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Old 29th September 2012, 06:04 AM   #5
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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.
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Old 29th September 2012, 06:56 AM   #6
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It's essentially the same circuit as Steven's 'Constans'. See post #13 here and the discussion that follows:

const. voltage & const. current on output FET ?

Edit: also just found a JFET version (post 6): Simple Low Distortion JFET Buffer

Last edited by steveh49; 29th September 2012 at 07:06 AM.
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Old 29th September 2012, 12:53 PM   #7
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Quote:
Originally Posted by steveh49 View Post
It's essentially the same circuit as Steven's 'Constans'. See post #13 here and the discussion that follows:

const. voltage & const. current on output FET ?

Edit: also just found a JFET version (post 6): Simple Low Distortion JFET Buffer
These do appear to be interesting variations on a White follower, with either straight or folded cascoding.
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Old 29th September 2012, 01:46 PM   #8
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Quote:
Originally Posted by steveh49 View Post
It's essentially the same circuit as Steven's 'Constans'. See post #13 here and the discussion that follows:

const. voltage & const. current on output FET ?

Edit: also just found a JFET version (post 6): Simple Low Distortion JFET Buffer
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.
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Old 29th September 2012, 07:43 PM   #9
RNMarsh is offline RNMarsh  United States
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Quote:
Originally Posted by linuxguru View Post
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?
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Old 1st October 2012, 02:00 AM   #10
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Quote:
Originally Posted by linuxguru View Post
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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File Type: gif CCB&Constans-sch.gif (16.3 KB, 215 views)
File Type: gif CCB&Constans-sqwv.gif (8.9 KB, 209 views)
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