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Old 23rd March 2013, 02:23 PM   #481
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Quote:
Originally Posted by Bonsai View Post
Nonsense Michael. Absolute nonsense.

1. CFA - the feedback quantity is current.

2. VFA - the feedback quantity is voltage.

3. The terrm CFA and has been used in technical literature since a least the mid 1980's and commercially available CFA opamps since 1987

Period.
Bonsai, you must be a pretty young fellow to have a memory that stops at the 80-ies

jan
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Old 23rd March 2013, 02:38 PM   #482
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Ok, maybe it goes back even further!

;-)

(CFA or my memory - I don't know which at the moment)
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Old 23rd March 2013, 03:02 PM   #483
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Originally Posted by godfrey View Post
IMHO an emitter follower has input to the base and output from the emitter, a common emitter has input to the base and output from the collector, and a common base has input to the emitter and output from the collector. Local feedback/ local degeneration is often used with common emitter gain stages.
You can have outputs both from the emitter and the collector.
If the resistor in the emitter is equal to the resistor in the collector, it's a phase splitter. This scheme often seen in tube amps has been used with bipolars by Norman Thagard for a bridge configuration of power amps.

In my opinion, it is sometimes preferable not to be too strictly tied to the terminology EC, BC, CC. In an input differential long-tail pair, I think it is correct to consider each bipolar as working in two or three modes simultaneously.
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Old 23rd March 2013, 04:19 PM   #484
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Originally Posted by michaelkiwanuka View Post
Not true. VFA refers to the fact that the transfer function of the feedback network is itself a voltage.

Moreover, the error signal that drives the forward path in both so-called "CFAs" and traditional VFAs is a voltage:

Verror=Vin-Vfeedback
The feedback network is a passive one, typically 2 resistors and maybe a capacitor for CFA. For VFA it can be more elaboate. The "transfer function" is ambiguous. There are 4 that come to mind, Vout/Vin, Vout/Iin, Iout/Vin, Iout/Iin, orIin/Iout, etc. The transfer function is simply an output/input ratio, it can be V/V, I/V, etc.

I explained 8 years ago how the error signal is indeed a current. If the gain is 2 noninverting, with a 10V rail and ground rail, and the Vin is 6 volts, of course the output cannot reach 12V. The 2 input terminals are at the same potential due to the emitter follower buffer. But the current at the inverting input node is not zero since the feedback current does not equal the current in the grounded resistor. If the input is reduced to 4V, then the grounded resistor current is reduced and the output slews to 8V to make sure the feedback current matches the shunt current. The current at the inv input drops to zero, this being the error signal. See my schematic from 2006.

The VFA under same conditions will exhibit a large voltage difference across the input terminals. The differential input Vd, would be 1V minimum if 6V is at the noninv input. The output can only slew to 10V (rail value), the feedback network drops half this 10V value, and the inv input is at 5V. The error signal is Vd. Easy enough. Any app note fron Natl Semi, Tex Instr, Analog Devices, etc. explains it in detail.

Also, the schematic you erferred to does not resemble commercial CFA available today. Again, my 2006 schematic and posts explain in detail. Regarding Dr. Cherry, we covered that in 2004-06 exhaustively. I ask readers to scroll back and I will elaborate if needed. The schematic you gave has no current mirrors, which are important to operation of CFA.

Please refer to my 2006 schematic and raise your differences based on that and what I've posted to date. Thank you.

Claude
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Old 23rd March 2013, 04:45 PM   #485
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Originally Posted by Claude Abraham View Post
The "transfer function" is ambiguous.
No it isn't.

1) For a voltage feedback amplifier, the transfer function of the foward path is Voltage/Voltage.

2) For a transimpedance amplifier, the transfer function of the forward path is Voltage/Current.

3) For a transadmittance amplifier, the transfer function of the forward path is Current/Voltage.

4) For a current amplifier, the transfer function of the forward path is Current/Current.

There is no ambiguity. Period.

See Table 1 here:

http://users.ece.gatech.edu/mleach/e...k/fdbkamps.pdf

Quote:
Originally Posted by Claude Abraham View Post
The schematic you gave has no current mirrors, which are important to operation of CFA.
Current mirrors are NOT essential to the operation of so-called "CFAs". There are plenty of so-called "CFAs" manufactured by the IC industry that do not use current mirrors.

This has already been rebutted. See:

Current feedback - Voltage feedback, how do I see the difference?

See also fig 15 on pg 8 here:

http://www.ti.com/lit/an/snaa004/snaa004.pdf

Last edited by michaelkiwanuka; 23rd March 2013 at 04:50 PM.
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Old 23rd March 2013, 04:46 PM   #486
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Originally Posted by Bonsai View Post
Ok, maybe it goes back even further!

;-)

(CFA or my memory - I don't know which at the moment)
The first patent I am aware of is from 1985: "Reduced settling time for wideband amplifiers". That was befor marketing got involved.
It's really interesting to look at the various circuits, and indeed current mirrors are not essential at all.

jan
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Old 23rd March 2013, 05:14 PM   #487
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It is always the current !

Some guys here needs help.
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Old 23rd March 2013, 06:00 PM   #488
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The first (SS complementary current-feedback ampilifier) CFB (CFA) amp that I remember having seen is from 1966 or 1967.
The first patent (I've seen) is from 1974.
BTW: if some here take the trouble to analyze a CFB (CFA) circuit in inverting mode, it is certainly easier to see that everything is about current and not voltage. Remember that in a CFB (CFA) the input Z on the Inverting input is close to zero (ideally it should be zero.)
After analyzing the inverting mode, you can analyze the non inverting mode.

Stein

Last edited by stinius; 23rd March 2013 at 06:03 PM.
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Old 23rd March 2013, 06:42 PM   #489
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Originally Posted by stinius View Post
The first (SS complementary current-feedback ampilifier) CFB (CFA) amp that I remember having seen is from 1966 or 1967.
The first patent (I've seen) is from 1974.
BTW: if some here take the trouble to analyze a CFB (CFA) circuit in inverting mode, it is certainly easier to see that everything is about current and not voltage. Remember that in a CFB (CFA) the input Z on the Inverting input is close to zero (ideally it should be zero.)
After analyzing the inverting mode, you can analyze the non inverting mode.

Stein
Yes ideally the inverting input Z is zero. Yet, if you analyse, you will see that the current into it is not infinite, what you might expect. In reality, that input Z DOES NOT determine the current into it....

BTW Do you have a number for that 1974 patent?

jan
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Old 23rd March 2013, 07:04 PM   #490
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Originally Posted by jan.didden View Post
Yes ideally the inverting input Z is zero. Yet, if you analyse, you will see that the current into it is not infinite, what you might expect. In reality, that input Z DOES NOT determine the current into it....

BTW Do you have a number for that 1974 patent?

jan
Of course it is not infinite, who has stated that?

U.S. Patent 3852678

BTW: the 1967 circuit is designed by Eckes.

Another great resource is Roy Gosser (ADI)

Stein
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