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Old 19th June 2004, 12:44 AM   #1
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Default Question(s) about CFP.

Why don't consider a CFP like an emitter follower were the first transistor set up the voltage and the second supply the current needed by the load. In this case the "timing" is correct assuming that the second transitor suck_out the current need by the base of the second BJT to the collector of the first one. Is it clear ? I need help to convert Microcap schematics to jpg or gif files.
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Old 20th June 2004, 06:15 PM   #2
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With a schematic, it will be clearer.
What i would say, by "timing" is that for me it doesn't look like feedback, but maybe i am wrong ?
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Old 20th June 2004, 06:57 PM   #3
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Sorry , i am not yet "fluently" in this forum, so i hope this time the shematic will be here !

Click the image to open in full size.
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Old 20th June 2004, 07:41 PM   #4
PMA is offline PMA  Europe
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Default Re: Question(s) about CFP.

Quote:
Originally posted by Justcallmedad
I need help to convert Microcap schematics to jpg or gif files.
Edit - copy visible part of window to clipboard - or copy to WMF file. Paste to painbrush and save, you have created the bmp image.

Or push printscreen and paste to painbrush.
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Old 21st June 2004, 02:48 AM   #5
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Default And your question about the CFP is.....?

I'm all ears.

Jocko
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Old 21st June 2004, 05:19 AM   #6
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Thanks PMA, for schematics coversion.

The question about CFP is that this configuration is very linear and except "degeneration" in R1, i don't understand why this is called Complementary FEEDBACK Pair ? But maybe i am wrong and i would like more explanations. Even in the case of it will be true, i don' think that this "feedback" pair has the drawbacks associated with feedback, to the fidelity of the reproduced sound. Has anyone tested (listen), this configuration (input, VAS,output)?
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Old 21st June 2004, 09:49 AM   #7
sreten is offline sreten  United Kingdom
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Quote:
Originally posted by Justcallmedad
Thanks PMA, for schematics coversion.

The question about CFP is that this configuration is very linear and except "degeneration" in R1, i don't understand why this is called Complementary FEEDBACK Pair ? But maybe i am wrong and i would like more explanations. Even in the case of it will be true, i don' think that this "feedback" pair has the drawbacks associated with feedback, to the fidelity of the reproduced sound. Has anyone tested (listen), this configuration (input, VAS,output)?
D Selfs books cover using the configuration in the input and output.

The two transistors are in a very tight feedback loop, the
"amplified" output of Q1 across R1 drives Q2s base, Q2s
output being the reference for Q1 input, so 100% feedback.

This sort of feedback is known as "local" feedback
and should not be confused with feedback loops.

R1 does not "degenerate" Q1 or Q2.
R1s current is only approximately constant
in that its wired across Q2s base emitter.

sreten.
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Old 21st June 2004, 01:47 PM   #8
Eva is offline Eva  Spain
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These are my thoughts :

CFP behaves in different ways depending on how it's driven :

- If it's driven from a current source or voltage source connected between Q1:B and Q1:E then it works as an open loop circuit

- If it's driven from a current source connected between Q1:B and the other side of Rload then it works also as an open loop circuit

- If it's driven from a voltage source connected between Q1:B and the other side of Rload then it works as a closed loop circuit with current feedback

The lower the drive impedance the more feedback it has, and when driven from high impedances [like the output of a VAS at audio frequencies] it behaves just like an obfuscated darlington but with much worse turn-off characteristics than an actual darlington [bigger current tail and phase shift at high frequencies so it's prone to blown devices and parasitistic oscillation]
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Old 21st June 2004, 04:15 PM   #9
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Muchas gracias Eva por las esplicaciones.

I Will continue in English for others people interested in this forum .

Could you tell me more :

- About the turn-off time, why ?, and is it really a problem for audio amplification ? about wich magnitude vs darlington.
- Even in the case when driven by a low impedance source the collector of Q2 act immediately before the base of Q2 (for me feedback is a problem at the moment where you have a delay, when you "compare" input signal and output (no ?).
- I would like more explanations about phase shift and parasitic oscillations.

I am really interested in working with this circuit, and i didn't find a lot of literature on it, even D. Self (like suggested by sreten)don't explain very much in detail this configuration.

For the moment in simulation it works nice, but real world is "crual" especially in audio ! I would learn more on it, before design the complete circuit, so if people has worked on it, tested, listen, they are welcome.
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Old 21st June 2004, 04:55 PM   #10
Eva is offline Eva  Spain
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Bipolar transistors are not ideal switching devices. If you abruptly increase Ib, there will be some delay [100s of nanoseconds] before Ic starts to increase and it will take up to some microseconds to reach its steady state value

Also, when there is an Ic and an Ib flowing in steady state, if Ib is abruptly removed, there will be a delay in the order of 1-10uS before Ic starts to decrease and it will take some more microseconds to fall completely

These timings are orientative numbers for power bipolar transistors

In a CFP, the 'current boost' transistor suffers from all those delays and thus the current it provides may be more than 180 out of phase from the driving signal at high frequencies. This causes cross-conduction and unstability problems

Turn-off times and delays may be dramatically reduced by providing a negative base current to the power device at turn-off [for switching circuits optimum value is Ic/2], but the CFP circuit is very poor on that aspect since R1 usually provides a very small current

To see this obscure behavior at high frequencies, just get an oscilloscope, build some CFPs with standard transistors and test them at 100Khz or even at 10Khz of square wave. Simulation software is not very precise simulating bipolar transistors behavior at high frequencies since charge storage and removal phenomena [the thing that causes those delays] are hard to model and depend on the construction and the structure of the die of each particular device
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