Talk:Current-feedback operational amplifier

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Soclof said "Norton operational amplifier...produce an output voltage for the amplifier that is proportional to the difference between the two input currents". So the Norton amplifier is a current-controlled voltage source (CCVS).

CFB amp feedback network

Is it true that a CFB op-amp has a range of recommended resistor values for the feedback network in order to minimise offset and/or maximise bandwidth? If yes, then is this point worth including in the article? Rohitbd 12:43, August 16, 2005 (UTC)

Article incorrect?

Actually the Bandwidth of Current Feedback Amplifiers is not larger than that of a VFA. The main difference is that the CFA does not have a constant GBW (gain bandwidth product) but rather a constant bandwidth. However it is not correct (at least the way I understand it) to claim it has a higher bandwidth than a comparable VFA. Steve110 07:20, 17 March 2007 (UTC)[reply]

agreed Kevin Aylward 18:38, 31 January 2014 (UTC)

I see some changes were made, but the main premise that CFB are higher bandwidth devices was not changed. This is simply wrong. The Bandwith of a CFA of comparable VOL is the same. The main difference to a VOA is the way these things are looked at. Steve110 22:24, 26 March 2007 (UTC)[reply]

I agree. The article needs a complete re-write, it is essentially, nonsense hint: http://www.kevinaylward.co.uk/ee/currentfeedbackmyth/currentfeedbackmyth.xht Kevin Aylward 18:38, 31 January 2014 (UTC) :-)

The most important point, which the article makes only indirectly, is that the gain-bandwidth of the current-feedback amplifier may be programmed externally, to suit the particular application. The words "not constant gain-bandwidth" suggest this, but do not make it clear.

In a voltage-feedback amplifier, you can scale all of the passives in the feedback network (for some constant k, multiply R and L by k, divide C by k), and the closed-loop transfer function is unchanged. It's only the impedance ratio that matters. (And this makes sense; the opamp's gain is dimensionless, and loop gains must be dimensionless (P/(1 + PC), and 1 is dimensionless, so PC is dimensionless), so the feedback network gain must be dimensionless as well.)

In a CFA, this is not true. Scaling the impedance of the feedback network is roughly equivalent to adjusting the compensation of a VFA. This is useful. For example, most VFB opamps are unity gain stable. This is necessary for many circuits, but wasteful when it's not necessary; if a higher gain is taken, then more closed-loop bandwidth could be obtained by decompensating the amplifier a bit. In the CFA, by choosing the absolute impedance of the feedback network, you can "compensate" the amplifier according to your exact needs. (And this makes sense too; the CFA's gain has dimensions of V/A, so the feedback network gain must have dimensions of A/V, which is an admittance.)

So the VFA might be just as fast as the CFA, but in designing the feedback network for the VFA, you will probably throw that speed away, by making the circuit more stable than it needs to be. In a CFA, this is not true. There's other advantages too; parasitics, like capacitance at the inputs, add extra poles in the transfer function of a VFA, which tend to make you go unstable. The low-impedance input of the CFA does not have this problem. (Contrary to the article's claims, Analog Devices is recommending some of their current feedback amps as photodiode front ends, for DVD players and such. At those speeds, the feedback resistor you can take is limited by bandwidth and stability issues; so if the current-feedback amplifier lets you take more gain without going unstable, then you might win on noise too, since you don't have to take as much voltage gain later. The front-end amplifier's input-referred noise is larger, but it gets multiplied by a smaller constant in the subsequent stages.) 74.61.11.168 (talk) 04:58, 4 February 2008 (UTC)[reply]

Clarification is in order

According to Bruce Carter of the Advanced Linear Products division of Texas Instruments...

"The bandwidth specification given in op amp data sheets refers only to the point where the unit gain bandwidth of the device has been reduced by 3 dB by internal compensation and/or parasitics -- not very useful for determining the actual operating frequency range of the device."

"Internally compensated, voltage feedback amplifier bandwidth is dominated by an internal 'dominant pole' compensation capacitor, resulting in a constant gain/bandwidth limitation. Current-feedback amplifiers, in contrast, have no dominant pole capacitor and therefore can operate MUCH MORE CLOSELY TO THEIR MAXIMUM FREQUENCY AT HIGHER GAIN [emphasis added]. Stated another way, the gain/bandwidth dependence has been broken."

Bruce Carter is wrong. All practical feedback amplifiers require compensation, and in most practical cases, a main dominant pole. The dominant pole compensation of a CFA is achieved by the capacitor marked as Cs in the schematic on the main CFA page in conjunction with the feedback resistor. Kevin Aylward 18:31, 31 January 2014 (UTC)

I believe the point Mr. Carter is making is that when you are using a gain significantly greater than 1, the CFA OP Amp has a significant advantage over the VFA Op Amp in terms of much greater frequency response.

This is only practically true for internally compensated VFB amps. All VFAs can be designed to have, essentially, the same BW as CFAs by choosing the correct value of compensation capacitor and incorporating an AB input stage. Kevin Aylward 18:31, 31 January 2014 (UTC)

Perhaps the article could be updated to reflect this? —Preceding unsigned comment added by 58.10.65.22 (talk) 13:03, 11 January 2009 (UTC)[reply]

Small omission

I believe the schematic diagram should show a resistor in parallel with Cs, as it is that internal resistor that determines the transimpedance gain of the I to V converter section. — Preceding unsigned comment added by 174.91.201.72 (talk) 17:09, 1 September 2011 (UTC)[reply]

Yes, there is an effective internal ro at the high gain node, which with the input GM, fixes the DC open loop gain. However, the resulting transresistance is not really relevant to the CFA description as a whole, as the node is buffered by an, ideally, zero output resistance unity gain stage. Kevin Aylward 18:49, 31 January 2014 (UTC) — Preceding unsigned comment added by Kevin aylward (talk • contribs)

Patents

US3944906 "NORTON type current responsive operational amplifier" http://www.google.com/patents/US3944906 ? --J. D. Redding 04:52, 28 August 2012 (UTC)[reply]

Error in schematic text?

I came here to learn about the current-feedback architecture, so I'm not really qualified to make changes. But I believe that pink-outlined stage in the schematic should be called, simply, a voltage buffer. The triangle has a high impedance input and a low-impedance output whose voltage tracks the input, correct? That's not a "current amplifier", as stated in the graphic. Spiel496 (talk) 18:51, 22 January 2021 (UTC)[reply]

Your reasoning is certainly correct, although it is ultimately a matter of semantics. A very good explanation by Sergio Franco here, chapter 12 simply calls it "the second buffer" and uses exactly the same schematic as for the first buffer. Which was exactly how first-generation CFOAs were built. Retired electrician (talk) 01:52, 23 January 2021 (UTC)[reply]