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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 30th May 2018, 06:58 PM   #651
Hierfi is offline Hierfi  Canada
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Operational amplifiers can be identified as a VFA or a CFA from the perspective of dependent and independent variables in accordance with Ohms Law. For the discussion we can examine a VFA and CFA having an inverting input impedance of 10Mohm and 50 Ohm respectively.

When V1 is output from a theoretical zero output impedance voltage source and applied to the inverting terminals of a VFA and CFA the resulting current becomes 1/10MOhm for the VFA and V1/50 Ohms for the CFA. In both cases V1 is an independent variable that causes current to flow in accordance with ohms law. Because the current is a dependent variable in both cases, both are pure VFA’s.

In contrast, when I1 is output from a theoretical voltage source of infinite output impedance (hence infinite voltage) as thereupon a pure current source, and this applied to the inverting terminals, the output voltage becomes I1x10MOhm for the VFA and I1x50 for the CFA. In both cases I1 is the independent variable causing voltage to appear across the inverting input resistors. Because voltage is a dependent variable both can be classified as pure CFA’s.

This is to suggest that for any real world voltage source driving the inputs having non-zero or non-infinite source impedance there is no such thing as a pure CFA or VFA and that both a VFA and CFA must convert from being one to the other at some point. As the only term that is available to us is the value of the input impedance it appears logical to identify the crossover point as equal to the inverting input impedance of the CFA or VFA devices.

What this means is that a VFA can be only be characterized as a VFA when the source to which it is attached has an impedance less than 10MOhm in the VFA example above, and that a CFA can only be characterized as a CFA when the source to which it is attached has an impedance greater than 50 ohms in the CFA example.

Using such criteria it is interesting to examine a feedback network shown in the application note of the AD844. The AD844 has a 50 Ohm Ri on the inverting terminal. Figure 32 on page 14 of 20 shows a non-inverting feedback amplifier having a feedback resistor from the output to the inverting terminal of 499 Ohms and a resistor from the inverting terminal to ground of 4.99 Ohms (gain of about 100). This means that the source impedance feeding the inverting terminal is the parallel combination of these two resistors, or about 5 Ohms.

If one uses the criteria above, the amplifier in Figure 32 is dominantly configured as a VFA, not a CFA. In other words the feedback divided voltage has a source impedance of 5 Ohms, far lower than the input 50 Ohms of Vin, hence the input can be considered driven by voltage. From the above a CFA must become a VFA if the source impedance becomes 0 Ohms. This begs the question, "If the configuration in Figure 32 is not yet a VFA then when does it become one?"
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Old 31st May 2018, 03:18 PM   #652
CPaul is offline CPaul
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I find much to agree with about what you've written, Hierfi. It is the ratio of the impedance seen by the feedback input to that OF the feedback input which determines important characteristics such as whether or not the bandwidth is inversely proportional to closed loop gain. As such, no amplifier is inherently CFA or VFA, but the combination of a feedback network and an amplifier is.

Even so, I find it useful to acknowledge the following: the signal current flowing in what we call a standard VFA input stage does NOT come from the VFA output. On the other hand, a significant portion of that current in what we call a CFA input stage does come from the CFA output (all of it, actually, in a non-inverting mode.) This becomes clear if the output and network are replaced with a simple Thevenin equivalent.

In that sense, we could justify classifying amplifiers alone as either CFAs or VFAs: the signal current in CFA input stages is fed back from the output; that in a VFA input stage is not, but rather is controlled by the voltages fedback from its output and applied to excite the circuit.
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Old 1st June 2018, 07:31 PM   #653
Hierfi is offline Hierfi  Canada
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Quote:
Originally Posted by CPaul View Post

I find it useful to acknowledge the following: the signal current flowing in what we call a standard VFA input stage does NOT come from the VFA output. On the other hand, a significant portion of that current in what we call a CFA input stage does come from the CFA output (all of it, actually, in a non-inverting mode.) This becomes clear if the output and network are replaced with a simple Thevenin equivalent.
Hi CPaul. There seems little difference between the networks feeding the so-called trans-impedance node in a CFA to that of the impedance node in a VFA. Both can be considered requiring similar currents in generating the voltage feeding the output buffer. Hence the differences required to drive these nodes is variant by the current gain in the VFA connection, as being reduced at the input by this gain. This implies that the only difference between a CFA and VFA is that the VFA has current gain in driving the voltage generating node and the CFA doesn't. This doesn't seem as a convincing argument to discriminate one as being current driven and the other not.

Data sheets are normally created by manufactures to assist designers in implementing their devices. From a designers perspective, the term "CFA" can be considered as a simplistic mnemonic characterization of "a network configured of electronic parts in a specific way". This mnemonic is useful to decide between looking at lists of so-called VFA's or CFA's. How these devices are used, with feedback or otherwise, the manner of connection to variant nodes, the adherence to a manufacturers data sheet, and if the words associated with the mnemonic properly reflect the network can be "don't cases" to designers.

It is nevertheless interesting to rationalize what is meant by calling these complex arrangements as being a "voltage feedback amplifier" or "current feedback amplifier" . The inclusion of the letter "F" in "CFA" and in "VFA" seems irrelevant of inclusion to contribute in the descriptor of such devices. Both VFA's and CFA's are capable of operating to DC. This means that at some fixed DC input voltage these amplifiers produce some output voltage in accordance with a resistive network connected back to the inverting terminal. If we disconnect the output voltage from the feedback network and replace this with a DC source of equal voltage, as feeding the same resistive network, the output voltage and the DC source voltages would continue to match.


This suggests that feedback is not a relevant condition in the determination of its operation in what can then be more simply described as a voltage amplifier "VA" or current amplifier "CA". The above leads to a conclusion supporting a nodal examination as the only mechanism in the determination of the behaviour of inputs as responding to voltage or current. This seems where this thread is ultimately heading.

Quote:
Originally Posted by vzaichenko View Post
Hi All,

I'm not sure if somebody mentioned this already, but if you try to look at any node in the circuit from the impedance ratio point of view - everything becomes pretty clear.

If you drive a high-impedance node from the low-impedance output - you drive with voltage. Example - driving the base of the next stage from emitter of the previous one. Voltage drive.

If you drive a low-impedance node from the high-impedance output - you drive with current. Example - driving the emitter of the next stage from collector of the previous one. Current drive.

This is true for any kind of node - active or passive one. The driving/driven impedance ratio shows what you drive the node with.
Cheers,
Gerrit
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Old 1st June 2018, 09:56 PM   #654
CPaul is offline CPaul
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I'm not sure if you think I was saying something like, "the VFA has current gain in driving the voltage generating node and the CFA doesn't." That was not my intent.

Your point about the irrelevancy of the "F" in the amplifier names is taken; it is unnecessary. However, I disagree that that leads to the conclusion of "nodal examination as the only mechanism..." for the following reason.

Whether employed in open or closed loop configurations, negligible portions of a VFA's input stage signal current come from its output or from a signal source external to the amplifier. But such current in a CFA comes only from such sources. This is a noteworthy distinction at least because it translates into an innate feature of the CFA which an unenhanced VFA lacks: slewing current on demand. Impedance ratios won’t reveal the presence or absence of such.

Still, I agree that the impedance ratio is important - it informs regarding the question of constant bandwidth vs. constant gain-bandwidth. But it is not the sole key to distinguishing between basic CFAs and VFAs.

Finally, I'm afraid I don't understand the clause containing the phrase "don't use". But anyone who remembers Miss Emily Litella is OK in my book!
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