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Old 13th March 2013, 11:43 PM   #101
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It's called current feedback because the current through the feedback resistor directly drives the TIS stage via the common base summing junction stage - some of it is of course shunted to ground by the lower resistor in the feedback leg. There is no gain stage interposed between the feedback and the TIS as in VFA's. Thus, CFA I believe is an apt name.
The characteristics of a "current feedback" amplifier are due to the fact that the first stage is basically a common emmiter amplifier with major loop feedback applied to its emitter.

Recall that the gain of a common emmiter amplifier is simply Rc/Re, where Rc is the collector load and Re the effective emitter resistance or impedance as the case may be.

Because the major loop feedback network is connected to the first stage's emitter, it modifies the gain of the first stage and, consequently, the forward path gain of the amplifier.

In other words, reducing the thevenin impedance of the feedback network reduces the amplifier's forward path gain, and conversely.

Further, because "current feedback" amplifiers actually employ shunt derived series applied major loop negative fedback, they're, in fact, voltage feedback amplifiers and were refered to as such long before someone decided to call them "current feedback" amplifiers.

A true current feedback amplifier must necessarily possess series derived shunt applied mojor loop negative feedback.
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Old 13th March 2013, 11:47 PM   #102
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On the contrary diyAudio is full of current feedback themes and still rising. Your book will completely ignore one of the most important topics, greatly improved to date, solid state amplifier designs based on modern semiconductors can offer, so misleading. Definitely not buying something that crippled.
On the contrary, Douglas is correct: "current feedback" amplifiers are inferior (in audio applications) to voltage feedback amplifiers which use a differential amplifier for their first stage.
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Old 13th March 2013, 11:56 PM   #103
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Default Of course he is rigth!

i cannot believe someone dare to disagree with him.

Hummmmm... cannot believe...he is Douglas Self...our teacher!

Carlos
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Old 14th March 2013, 12:03 AM   #104
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Hello Carlos and michaelkiwanuka

I agree with you and Douglas, I've try current feedback and I did not like it, so I came back to voltage feedback.

Bye

Gaetan
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Old 14th March 2013, 12:05 AM   #105
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Michael, stop talking about resistors and forward loop gain because that is what is causing the confusion.

Think about the current flows in the feedback network and up through the common base summing junction stage. Therein lies the correct answer.

You have an entirely different situation with a VFA. The feedback current in these topology amps does not directly drive the TIS and is only related to the TIS input current in as much as the LTP transistor beta effects it. In a VFA the feedback voltage is the key quantity and it steers the LTP current into the TIS input or away from it as the case may be.

You can test this proposition simply by changing the values of the feedback network in a VFA, but keeping the ratio the same. Within reason, you will not affect the performance of the VFA in frequency response, rise/fall times or bandwidth. In a CFA, the result will be very different - you will change the rise fall times and the bandwidth of the amplifier.
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Old 14th March 2013, 12:27 AM   #106
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Michael, stop talking about resistors and forward loop gain because that is what is causing the confusion.
There is no such thing as "forward loop gain", and, no, there is no confusion on my part.

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Originally Posted by Bonsai View Post
Think about the current flows in the feedback network and up through the common base summing junction stage. Therein lies the correct answer.
The current flowing through the feedback network is irrelevant, because it is the output voltage being sampled (shunt derived negative feedback) and NOT the output current.

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Old 14th March 2013, 12:33 AM   #107
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On the contrary diyAudio is full of current feedback themes and still rising. Your book will completely ignore one of the most important topics, greatly improved to date, solid state amplifier designs based on modern semiconductors can offer, so misleading.
I'd be interested to hear some evidence to back up these assertions.

The fact that there is much talk on a topic does not mean a lot. I seem to recall much discussion of the Mayan calendar recently.

Can you point me to a CFA that gives Blameless performance?
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Old 14th March 2013, 12:36 AM   #108
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You can test this proposition simply by changing the values of the feedback network in a VFA, but keeping the ratio the same. Within reason, you will not affect the performance of the VFA in frequency response, rise/fall times or bandwidth. In a CFA, the result will be very different - you will change the rise fall times and the bandwidth of the amplifier.
See below:

Audio Power Amplifier Design book- Douglas Self wants your opinions
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Old 14th March 2013, 01:54 AM   #109
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Michael, I am not going to get into an argument over this because it seems you cannot discuss principles but instead rely on semantics. My statement on the differences between the two topologies stands.
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Old 14th March 2013, 02:00 AM   #110
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I'd be interested to hear some evidence to back up these assertions.

The fact that there is much talk on a topic does not mean a lot. I seem to recall much discussion of the Mayan calendar recently.

Can you point me to a CFA that gives Blameless performance?
Douglas, we could well ask you the same question about your assertions. . .

Show us a VFA that has the same performance as a CFA?

You want ultra low distortion - I want wide bandwidth

You say PSRR is Important - I think fast rise fall times are important

and so on it goes.

See where this will get us? No where - and we are not even talking about the subjective aspects.

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