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Current Feedback Amplifiers, not only a semantic problem?
Current Feedback Amplifiers, not only a semantic problem?
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Old 15th January 2019, 11:01 AM   #2471
jan.didden is offline jan.didden  Europe
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Best so far! Although I fail to see what this effort brings you; but even if someone reads it, no or very little harm will be done so it is mostly harmless.

Looking forward to your next copy-paste effort!

Jan
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Old 15th January 2019, 11:08 AM   #2472
Hierfi is online now Hierfi  Canada
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Originally Posted by CPaul View Post
Hierfi, I see sense in using the terms CFA and VFA to distinguish between circuits with and without the following characteristic: In a CFA, the major signal (as opposed to bias) current(s) flowing through the active device(s) connected to the feedback input is (are) equal to the difference between the output and ground feedback network signal currents. This relationship, along with Middlebrook’s D.I.T., makes it reasonable to allude to current feedback in this circuit. (This should not be construed as meaning that there is no voltage feedback in a CFA.) This classification could be extended all the way back to circuits designed 90 or so years ago. In a VFA, there is no such relationship, and no justification for claiming current feedback.

This is separate from the question of the relative and absolute amounts of current and voltage feedback in a circuit.
Thanks Chris. A VFA doesn't seem remotely close to being identifiable as a CFA.
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Old 15th January 2019, 02:33 PM   #2473
Hierfi is online now Hierfi  Canada
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Originally Posted by Hans Polak View Post
In case you might wonder wether this works, here is an example for Rg=100 Ohm and Rf=500 Ohm giving A=6 and I made Z(in-)=25 Ohm.
I have also added a DC voltage of 1V at the input, thereby making the DC voltage visible at Vth2.
Hans
Hi Hans. Below is the Thevenin equivalent of your example. The only difference is that you have introduced a V1 voltage to ground to the floating Ohmic cell that was identified in my post of #2462. I have simply changed RTh3 and VTh3 in accordance with your values. Given that an Ohmic cell still exists there isn't an independent variable, hence this still leads to the conclusion that a CFA cannot be identified as functioning either as a c.f. or v.f. amplifier.
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Old 15th January 2019, 02:46 PM   #2474
Hans Polak is offline Hans Polak  Netherlands
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Quote:
Originally Posted by Hierfi View Post
Hi Hans. Below is the Thevenin equivalent of your example. The only difference is that you have introduced a V1 voltage to ground to the floating Ohmic cell that was identified in my post of #2462. I have simply changed RTh3 and VTh3 in accordance with your values. Given that an Ohmic cell still exists there isn't an independent variable, hence this still leads to the conclusion that a CFA cannot be identified as functioning either as a c.f. or v.f. amplifier.
Your drawing is completely correct, although somewhat less readable in this compressed form.

LTspice cannot function without a connection to GND and the 1V DC was added as an extra means to control the correct working.
But now you can see in your Vth3 that I(Zin-) is part of it, which was not at all obvious in your previous posting.
This is the most essential part of them all when you want to draw conclusions which you did concerning VF or CF.


Hans

Last edited by Hans Polak; 15th January 2019 at 02:51 PM.
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Old 15th January 2019, 03:07 PM   #2475
Hierfi is online now Hierfi  Canada
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Quote:
Originally Posted by Hans Polak View Post
Your drawing is completely correct, although somewhat less readable in this compressed form.

LTspice cannot function without a connection to GND and the 1V DC was added as an extra means to control the correct working.
But now you can see in your Vth3 that I(Zin-) is part of it, which was not at all obvious in your previous posting.
This is the most essential part of them all when you want to draw conclusions which you did concerning VF or CF.

Hans
Sure Hans. Your example made that clear.
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Old 15th January 2019, 03:32 PM   #2476
scott wurcer is online now scott wurcer  United States
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Originally Posted by jan.didden View Post
Looking forward to your next copy-paste effort!
Me too, my first algebra teacher was Fr. Hilbert (Jesuit of course) brilliant man. We made the obvious joke in class, last I heard he was still teaching at a Navajo reservation in his 90's.
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