Does that change the way the feedback divides the output voltage?
Does the -IN of a conventional amplifier have virtual ground at the -IN. That too should draw current from the output. But, it's still a voltage divider, just like the CFB.
I just don't understand why they need a different name when to me they look the same.
Does the -IN of a conventional amplifier have virtual ground at the -IN. That too should draw current from the output. But, it's still a voltage divider, just like the CFB.
I just don't understand why they need a different name when to me they look the same.
If you were 9 years old, I'd say: "Nice thinking!"
Ever heard of marketing department?
There are guys in big semiconductor companies that try to sell you a well known thing as something revolutionary new and to do that they usually first come up with a new name for something old. That's how "current feedback" came into being - as a marketing label for a new line of OpAmps...
You have made a very nice amp, indeed. I also intended to build the Hiraga, but ended up with this amp instead. But I have not heard neither Hiraga's 20W or the KSA50, so I cannot compare these with mine.
Today I would not call it 'Current Feedback', it is global negative feedback with the difference that the minus input is low impedance. It has the consequence also that this type of feedback gives very wide bandwidth, since the feedback is applied to the source of the FET's (thus coupled in common gate here).
If my memory is trustworthy, I think second harmonic and third harmonic distortion was dominant over higher distortion components, so I liked the sound of this amplifier. I even tried it without global feedback and still liked the sound of it.
The F5 is a newer design, by the way.
If my memory is trustworthy, I think second harmonic and third harmonic distortion was dominant over higher distortion components, so I liked the sound of this amplifier. I even tried it without global feedback and still liked the sound of it.
The F5 is a newer design, by the way.
Current feedback, sounds like low/no daming to me. A waterfall diagram would probably agree
I remember testing it with dummy reactive loads while feeding it a square signal and being impressed by the way it stayed pretty much on the money no matter what I connected to its output, so my guess is df is not that low. Anyway, I'll measure it just out of curiosity and report back.
Btw, a low df doesn't necessarily mean a bad sounding amp (many tube amps have low df and sound gorgeous), just that you need to be more careful with what speakers you pair it with, imo.
Current feedback, sounds like low/no daming to me. A waterfall diagram would probably agree
I don't understand...
My circuits-knowhow is not up to par to check, but if this really is a current feedback amplifier think about this;
Imaging a recording of a sinusoid sound wave, and after say 2 seconds, the soundwave stops.
The recorded signal from the microphone shows the speed of the membrane during the sound waves and after two seconds, due to the size/specs of the mic the speed of the membrane goes rapidly to zero and the recorded signal rapidly goes to zero.
Then, use that recorded signal as an input on our current amplifier. The membrane of the loudspeaker begins to move and should reach the speeds of the original microphone. So far so good. Now the signal goes to zero while the bigger and heavier mass of the loudspeaker is still moving. Due to nature of the amplifier, the current output of the amp goes to zero.
When the current goes to zero the exerted force on the loudspeaker membrane goes to zero. (F= B*i*L ?) In other words, the only damping of the loudspeaker is that of the rubber.
How is this different from a "regular" amplifier? Well, if in a normal amplifier the signal goes to zero, the voltage output of the amplifier goes to zero. This means that no matter what happens in the dynamics of the loudspeaker, the amplifier tries to keep that voltage at 0. If the membrane of the loudspeaker is moving, the resulting voltage creates a current. That current DOES create a force on the membrane which damps the motion.
Compare it to the alternator on your bike. When there is no load/lamp attached the required current is zero and the alternator turns easy (low resistance/damping). When the load is connected, so there is a flow of current, the resistance (damping) is higher.
This has little/nothing to do with the audio definition of "damping ratio". Next to that, an ideal resistive load does not have any dynamics so it would be surprising to see any affect of damping on that measurement.
Again, maybe I'm wrong due to the fact that the circuit has some tricky stuff in it, but a true current feedback amplifier should be performing as described above.
Imaging a recording of a sinusoid sound wave, and after say 2 seconds, the soundwave stops.
The recorded signal from the microphone shows the speed of the membrane during the sound waves and after two seconds, due to the size/specs of the mic the speed of the membrane goes rapidly to zero and the recorded signal rapidly goes to zero.
Then, use that recorded signal as an input on our current amplifier. The membrane of the loudspeaker begins to move and should reach the speeds of the original microphone. So far so good. Now the signal goes to zero while the bigger and heavier mass of the loudspeaker is still moving. Due to nature of the amplifier, the current output of the amp goes to zero.
When the current goes to zero the exerted force on the loudspeaker membrane goes to zero. (F= B*i*L ?) In other words, the only damping of the loudspeaker is that of the rubber.
How is this different from a "regular" amplifier? Well, if in a normal amplifier the signal goes to zero, the voltage output of the amplifier goes to zero. This means that no matter what happens in the dynamics of the loudspeaker, the amplifier tries to keep that voltage at 0. If the membrane of the loudspeaker is moving, the resulting voltage creates a current. That current DOES create a force on the membrane which damps the motion.
Compare it to the alternator on your bike. When there is no load/lamp attached the required current is zero and the alternator turns easy (low resistance/damping). When the load is connected, so there is a flow of current, the resistance (damping) is higher.
This has little/nothing to do with the audio definition of "damping ratio". Next to that, an ideal resistive load does not have any dynamics so it would be surprising to see any affect of damping on that measurement.
Again, maybe I'm wrong due to the fact that the circuit has some tricky stuff in it, but a true current feedback amplifier should be performing as described above.
AndrewT
Dont know if this helps but, for a current feedback amplifier it is the current flowing out of the negative input which is responsible for the internally developed voltage (transimpedance), subsequently buffered to the output.
Under dc bias conditions there is little current (the error current) flowing from the negative input through the voltage divider (only that required to maintain the desired bias conditions). As input signal appears this error current increases in response (as mentioned the negative input is a low impedance) and it is this very error current which via the internal transimpedance action responds with increased output voltage.
So at least for analysis purposes its more natural to set up equations as Vo=A*Verror for Voltage Feedback and Vo=Z*Ierror for Current Feedback.
As you know there are performance differences and characteristics for each topology.
Thanks
-Antonio
Dont know if this helps but, for a current feedback amplifier it is the current flowing out of the negative input which is responsible for the internally developed voltage (transimpedance), subsequently buffered to the output.
Under dc bias conditions there is little current (the error current) flowing from the negative input through the voltage divider (only that required to maintain the desired bias conditions). As input signal appears this error current increases in response (as mentioned the negative input is a low impedance) and it is this very error current which via the internal transimpedance action responds with increased output voltage.
So at least for analysis purposes its more natural to set up equations as Vo=A*Verror for Voltage Feedback and Vo=Z*Ierror for Current Feedback.
As you know there are performance differences and characteristics for each topology.
Thanks
-Antonio
By describing a feedback being voltage or current we are referring to the physical quantity in the load at the output of an amplifier or a stage of an amplifier.
A simple way to tell the type of physical quantity being feedback is to change the load and see how the feedback at the input of the circuit changes along.
If we short the load, hence forcing the output Voltage zero, and if the feedback quantity become zero as well, it is voltage feedback amp. If we open circuit the load, hence forcing the output Current zero, and if the the feedback quantity becomes zero as well, it is current feedback amp.
A simple way to tell the type of physical quantity being feedback is to change the load and see how the feedback at the input of the circuit changes along.
If we short the load, hence forcing the output Voltage zero, and if the feedback quantity become zero as well, it is voltage feedback amp. If we open circuit the load, hence forcing the output Current zero, and if the the feedback quantity becomes zero as well, it is current feedback amp.
Nattawa,
Agreed but I think AndrewT was asking why the electronics industry has adopted the term Current Feedback Amplifier for this topology (and created this confusion).
Thanks
-Antonio
Let's apply Natt's "test".
Does a Current Feedback Amplifier behave the same as a "normal" amplifier? Or does it behave differently?
I have not done the "test" but I will guess that the "test" will show what many who have replied have been saying.
Both types of feedback are actually of the voltage type.
CFB seems to be an invention intended to confuse rather than clarify.
Does a Current Feedback Amplifier behave the same as a "normal" amplifier? Or does it behave differently?
I have not done the "test" but I will guess that the "test" will show what many who have replied have been saying.
Both types of feedback are actually of the voltage type.
CFB seems to be an invention intended to confuse rather than clarify.
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Andrew, in VFA's, the feedback quantity is voltage. take a look at a conventional diff amp. It's. Voltage feedback circuit.
CFB on th other hand converts the input voltage (through a buffer) and delivers a current into a summing node.
These are two very different structures.
Ther is no integrating TIS in a CFB, and the result is you can get very high slew rates and wide bandwidths which are very advantageous in a lot of applications.
CFB on th other hand converts the input voltage (through a buffer) and delivers a current into a summing node.
These are two very different structures.
Ther is no integrating TIS in a CFB, and the result is you can get very high slew rates and wide bandwidths which are very advantageous in a lot of applications.
I don't think this is a valid test, Nattawa.
I think what makes a CFA a CFA is:-
1. The input voltage is converted into a current after a unity gain buffer stage - see the input stage of a typical CFA
2. The buffer output is converted into a current (thru a low value resistor) which feeds an input current into a low impedance summing junction, where the feedback current is also terminated - this arrives via the feedback resistor
3. The error output (the difference between the two) from the summing junction is in the form of a current
4. This current then drives a trans impedance stage (TIS) configured around a current mirror, usually referenced to the amplifer rails. There are simpler ways to do this, but the morror
TIS gives the best performance because it raises the OLG
5. Importantly, there is no integrator involved in a CFA, unlike a VFA, where you have Cdom, or, more advanced techniques like MIC, which is still a form of integrator.
6. On a VFA, the slew rate and closed loop -3dB bandwidth are limited by the LTP current and Cdom (note SR and bandwidth can be set independently)
7. CFA's have none of these limitations - no integrator means you can make them fast, or even damn fast. See some of the integrated CFA's from AD, TI et al for axamples - 100's of V/us and 10's of MHz bandwidth.
Its important to realize that VFA's respond to differences in voltage between their inv and n-inv inputs, while the CFA responses to the error current generated arising at its current summing node.
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I think posts35/36are describing the structure of a CFA opamp.
I thought we were referring to an amplifier where the output is taken back to the emitter of a input stage (CFB).
It seems to me that these two structures are quite different.
Can we get a defintion/s of what CFA and CFB means for the required amp structure?
I thought we were referring to an amplifier where the output is taken back to the emitter of a input stage (CFB).
It seems to me that these two structures are quite different.
Can we get a defintion/s of what CFA and CFB means for the required amp structure?
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