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Current feedback - Voltage feedback, how do I see the difference?
Current feedback - Voltage feedback, how do I see the difference?
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Old 24th March 2013, 04:17 PM   #501
michaelkiwanuka is offline michaelkiwanuka  United Kingdom
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Originally Posted by Bonsai View Post
Michael, you have COMPLETELY missed the point!
No, Bonsai, you've completely missed the point.

Oh, and what, precisely, is the point I've missed?
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Old 24th March 2013, 04:18 PM   #502
michaelkiwanuka is offline michaelkiwanuka  United Kingdom
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This is a joke yes?
No.
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Old 24th March 2013, 04:30 PM   #503
Waly is offline Waly
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"A CFA is a VFA, where the feedback network loading of the amp, at the input (series) node, can't be neglected."
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Old 24th March 2013, 04:55 PM   #504
michaelkiwanuka is offline michaelkiwanuka  United Kingdom
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In other words, the feedback network affects the forward path gain of a so-called "CFA", but this does not alter the fact that all so-called "CFAs" are in fact deeply compromised (for audio applications) VFAs
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Old 24th March 2013, 04:59 PM   #505
RNMarsh is online now RNMarsh  United States
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Current feedback - Voltage feedback, how do I see the difference?
Are the formula's for gain the same? They should be if a CFA is a VFA.

They are sufficiently different to describe a fundamentaly different mode of gain operation.

Thx-RNMarsh

VA-CM formulas.jpg

Last edited by RNMarsh; 24th March 2013 at 05:07 PM.
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Old 24th March 2013, 05:05 PM   #506
Waly is offline Waly
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Originally Posted by michaelkiwanuka View Post
but this does not alter the fact that all so-called "CFAs" are in fact deeply compromised (for audio applications) VFAs
To borrow your own language, this is nonsense.
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Old 24th March 2013, 05:16 PM   #507
Waly is offline Waly
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Originally Posted by RNMarsh View Post
Are the formula's for gain the same? They should be if a CFA is a VFA.

They are sufficiently different to describe a fundamentaly different mode of gain operation.
In general no, because of the loading effect, affecting the open loop gain, which in turn may affect (if not large enough) the closed loop gain. However, in certain conditions (both external - gain and internal - topology), the CFA closed loop voltage gain expression can be the same as for a VFA. Look at the LM6172, a CFA in disguise. Of course, then the main characteristic of a CFA, that is the gain independent bandwidth, is lost. You can't have cake and eat it too.
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Old 24th March 2013, 05:26 PM   #508
jan.didden is offline jan.didden  Europe
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Originally Posted by Waly View Post
"A CFA is a VFA, where the feedback network loading of the amp, at the input (series) node, can't be neglected."
Did you actually look into that instead of just following your unreliable gut? Do you realise that the current into the low impedance node is NOT determined by its low impedance?

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Old 24th March 2013, 05:43 PM   #509
michaelkiwanuka is offline michaelkiwanuka  United Kingdom
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Originally Posted by Waly View Post
Of course, then the main characteristic of a CFA, that is the gain independent bandwidth, is lost.

This so-called "gain indepedent bandwidth" is an electronic sleight of hand because for low closed loop gains a so-called "CFA" sacrifices forward path gain, and, therefore, loop gain to obtain it.

So, attempting to increase negative feedback (loop gain) by reducing the amplifier's closed loop gain accomplishes virtually nothing in terms of overall linearity because the amplifier's first stage gain is simultaneously reduced.

So, of what utility are so-called "CFAs" in audio applications?
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Old 24th March 2013, 05:50 PM   #510
michaelkiwanuka is offline michaelkiwanuka  United Kingdom
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Originally Posted by RNMarsh View Post
Are the formula's for gain the same? They should be if a CFA is a VFA.

They are sufficiently different to describe a fundamentaly different mode of gain operation.

Thx-RNMarsh

Attachment 338158

For a first order analysis, the values of the feedback components appear in the equation for the forward path gain in so-called "CFA", whereas this is not the case in a VFA.

However, this doesn't change the fact that so-called "CFAs" are really compromised VFAs.

Read the section entitled "over compensation" in the figure you posted.

Last edited by michaelkiwanuka; 24th March 2013 at 05:53 PM.
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