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Identify whether op-amp feedback is voltage or current feedback?
The question of how to describe and call certain types of feedback has always bothered me. This is not just a frivolous fantasy. I know of at least one public "fire fight" that I think led to false inferences from these feedback features.
So, what exactly is feedback? One answer is that it refers to the process of detecting a certain signal that you want to influence and feeding part of it back to a previous point in the circuit so that you can exert some kind of control. Figure 1 shows four classical circuits with two signal routes: feedback and excitation of the amplifier. We say that the feedback source is either derived in parallel (voltages at both ends of the load) or derived in series (current through the load, expressed as voltages at both ends of the impedance in series with the load).
We also talk about series and parallel feedback, where the signal is connected in series or parallel with the excitation signal. In the "parallel" case, the two signals meet at the inverting input and the non-inverting input is grounded. In the "series" case, the excitation is applied to the non-inverting input and fed back to the inverting input. Note that the way I draw and specify the excitation sources Sp and Sm is somewhat ambiguous, and I do this on purpose because they are not ideal. I mean, they can be thought of as ideal voltage sources in series with impedance, or ideal current sources in parallel with impedance.
The first case we'll look at is a concatenated application (regardless of the export type). The source Sp has nonzero output, while the Sm has zero output. Thus, Sm is just a connection to ground through its intrinsic impedance Zm. The amplifier output sends current through the Rf-Rg-Zm network. Real operational amplifiers, such as the old TL072, can accept almost zero input current. As with all operational amplifiers, the signal output of the input stage is the current that ultimately controls its output voltage. In this case, the current comes from the op-amp itself and is controlled by the differential pressure between the two (excitation and feedback) input voltages. Traditionally, this is obviously voltage feedback, where a voltage signal is fed back to the inverting input and the control is carried out there. At this control point, the current entering the amplifier is negligible.
Now, parallel feedback. Let's flip the source so that Sm now has a nonzero output and Sp is zeroed out. Recall that parallel applications are called current feedback. Now I ask you: can we change the type of feedback in the circuit simply by changing the amplitude of the signal source? Assuming both sources have nonzero outputs, do we also have voltage and current feedback? If both are zero, do we have any feedback? (Of course, if the op-amp's output stabilizes around 0V, this conclusion will be strongly countered!)
Is this current feedback?
Let's move on. This current feedback (?) Put the amplifier in (the current coming in is negligible, right?) Bypass and terminate at source Sm. If the ratio of the network to the Sm impedance is large, then any effect it has is negligible. Is this still current feedback? I don't think so. Instead, it is voltage feedback, whose op-amp input voltages are now at zero and near zero, but their differential pressures remain roughly the same as long as the Sm's level is close to the Sp's level (which is important for this op-amp).
If that's not convincing enough, double the impedance of Rf, Rg, and Sm. We've just halved the current flowing through these devices. Therefore, the envisaged current feedback must also be halved. However, the output of the circuit has not changed substantially. Therefore, it is not affected by current feedback.
So, does current feedback actually happen? Of course. We REPLACE THE TL082 WITH A DIFFERENT OP-AMP WHOSE INPUT STAGE OUTPUT COMES FROM A POINT OUTSIDE THE OP-AMP THROUGH ITS inverTING INPUT. For the feedback part of the signal, this point is the output of the operational amplifier. Almost anything that connects one or more emitters to this input is eligible (SSM2019 is one option, and current-feedback amplifiers, known in the industry as CFA, are another). Almost all devices entering the emitter exit their associated collector at some point inside the op-amp, constituting the output of the input stage and determining the output of the amplifier. With a parallel application configuration, the amplifier still drives current through the network and terminates at the source Sm (in most cases). But this time, a small portion of the current is "stripped" and fed back to the inverting input of the op-amp to exert the desired control over the output of the device. This is, of course, current feedback, as it is consistent with parallel applications.
Finally, returning to the series application case, the operational amplifier's work is of course the same. In fact, the input stage output signal current still comes from outside the amplifier -- only from the op-AMP output. To see more clearly, replace the circuit seen at the inverting input with the Vinnan equivalent: a signal source with a resistor feed voltage Vout • Rg/(Rg + Rf) with Rf• Rg/(Rg + Rf). I think this is still current feedback -- the output current of the input stage of the amplifier comes from the attenuated output of the op-amp through a single resistor.
You could argue that the type SSM2019/CFA amplifier is still sensitive to differential input voltages and therefore still a voltage feedback device (the only way this is not possible is if its low inverting input impedance is zero, in which case no input voltage difference occurs). But if we agree with this argument, then we also have to admit that there is a very large but still limited impedance between the inputs of TL082. This means that a current is generated between these inputs, so TL082 is a current feedback device!
This reasoning does not hold water. Instead, the way I propose to classify circuits by feedback type is to analyze the current sources coming from the amplifier input stage. If the current source is inside the OP-AMP, then virtually no current is fed back from the output, and the OP-AMP and circuit must operate with voltage feedback. If the current source comes from an inverting input outside the OP-amp, we consider it current feedback.
For all the reasons mentioned above, we should correct the view that series applications are always voltage feedback and parallel applications are always current feedback. Whether it is voltage feedback or current feedback depends on the amplifier of the circuit, not its circuit topology.
So, what exactly is feedback? One answer is that it refers to the process of detecting a certain signal that you want to influence and feeding part of it back to a previous point in the circuit so that you can exert some kind of control. Figure 1 shows four classical circuits with two signal routes: feedback and excitation of the amplifier. We say that the feedback source is either derived in parallel (voltages at both ends of the load) or derived in series (current through the load, expressed as voltages at both ends of the impedance in series with the load).
We also talk about series and parallel feedback, where the signal is connected in series or parallel with the excitation signal. In the "parallel" case, the two signals meet at the inverting input and the non-inverting input is grounded. In the "series" case, the excitation is applied to the non-inverting input and fed back to the inverting input. Note that the way I draw and specify the excitation sources Sp and Sm is somewhat ambiguous, and I do this on purpose because they are not ideal. I mean, they can be thought of as ideal voltage sources in series with impedance, or ideal current sources in parallel with impedance.
The first case we'll look at is a concatenated application (regardless of the export type). The source Sp has nonzero output, while the Sm has zero output. Thus, Sm is just a connection to ground through its intrinsic impedance Zm. The amplifier output sends current through the Rf-Rg-Zm network. Real operational amplifiers, such as the old TL072, can accept almost zero input current. As with all operational amplifiers, the signal output of the input stage is the current that ultimately controls its output voltage. In this case, the current comes from the op-amp itself and is controlled by the differential pressure between the two (excitation and feedback) input voltages. Traditionally, this is obviously voltage feedback, where a voltage signal is fed back to the inverting input and the control is carried out there. At this control point, the current entering the amplifier is negligible.
Now, parallel feedback. Let's flip the source so that Sm now has a nonzero output and Sp is zeroed out. Recall that parallel applications are called current feedback. Now I ask you: can we change the type of feedback in the circuit simply by changing the amplitude of the signal source? Assuming both sources have nonzero outputs, do we also have voltage and current feedback? If both are zero, do we have any feedback? (Of course, if the op-amp's output stabilizes around 0V, this conclusion will be strongly countered!)
Is this current feedback?
Let's move on. This current feedback (?) Put the amplifier in (the current coming in is negligible, right?) Bypass and terminate at source Sm. If the ratio of the network to the Sm impedance is large, then any effect it has is negligible. Is this still current feedback? I don't think so. Instead, it is voltage feedback, whose op-amp input voltages are now at zero and near zero, but their differential pressures remain roughly the same as long as the Sm's level is close to the Sp's level (which is important for this op-amp).
If that's not convincing enough, double the impedance of Rf, Rg, and Sm. We've just halved the current flowing through these devices. Therefore, the envisaged current feedback must also be halved. However, the output of the circuit has not changed substantially. Therefore, it is not affected by current feedback.
So, does current feedback actually happen? Of course. We REPLACE THE TL082 WITH A DIFFERENT OP-AMP WHOSE INPUT STAGE OUTPUT COMES FROM A POINT OUTSIDE THE OP-AMP THROUGH ITS inverTING INPUT. For the feedback part of the signal, this point is the output of the operational amplifier. Almost anything that connects one or more emitters to this input is eligible (SSM2019 is one option, and current-feedback amplifiers, known in the industry as CFA, are another). Almost all devices entering the emitter exit their associated collector at some point inside the op-amp, constituting the output of the input stage and determining the output of the amplifier. With a parallel application configuration, the amplifier still drives current through the network and terminates at the source Sm (in most cases). But this time, a small portion of the current is "stripped" and fed back to the inverting input of the op-amp to exert the desired control over the output of the device. This is, of course, current feedback, as it is consistent with parallel applications.
Finally, returning to the series application case, the operational amplifier's work is of course the same. In fact, the input stage output signal current still comes from outside the amplifier -- only from the op-AMP output. To see more clearly, replace the circuit seen at the inverting input with the Vinnan equivalent: a signal source with a resistor feed voltage Vout • Rg/(Rg + Rf) with Rf• Rg/(Rg + Rf). I think this is still current feedback -- the output current of the input stage of the amplifier comes from the attenuated output of the op-amp through a single resistor.
You could argue that the type SSM2019/CFA amplifier is still sensitive to differential input voltages and therefore still a voltage feedback device (the only way this is not possible is if its low inverting input impedance is zero, in which case no input voltage difference occurs). But if we agree with this argument, then we also have to admit that there is a very large but still limited impedance between the inputs of TL082. This means that a current is generated between these inputs, so TL082 is a current feedback device!
This reasoning does not hold water. Instead, the way I propose to classify circuits by feedback type is to analyze the current sources coming from the amplifier input stage. If the current source is inside the OP-AMP, then virtually no current is fed back from the output, and the OP-AMP and circuit must operate with voltage feedback. If the current source comes from an inverting input outside the OP-amp, we consider it current feedback.
For all the reasons mentioned above, we should correct the view that series applications are always voltage feedback and parallel applications are always current feedback. Whether it is voltage feedback or current feedback depends on the amplifier of the circuit, not its circuit topology.
