The Endless Semantic Debate: Current and Voltage Feedback Amplifiers
It seems some are still agonizing over the ‘current feedback’ versus ‘voltage feedback’ definition. Clearly a case of people wanting to continue to flog a horse that was laid to rest five decades ago during the heyday of the analog computer, or they simply fail to grasp the CFA concept. I suspect there are an equal number of both types. This is the same crowd that deny the existence of CFA’s, claiming they are just ill-designed VFA’s. Here then is the question that vexes the CFA doubting Thomas’s:-
What do we call an amplifier that actually has current feedback?
Lets consider this whole current feedback thing by first making clear there are two very different issues to consider here: current output and current feedback and the voltage mode equivalents, voltage output and voltage feedback. Output mode and feedback mode are emphatically not the same thing, and anybody who makes this claim is simply playing word games to further their dogma. Quite depressing, given we are talking about an engineering discipline here – namely electronics.
You can use either a voltage feedback (VFA) or a current feedback (i.e. CFA) amplifier to control its output as either a voltage or a current.
A current output amplifier is an amplifier in which the output current is the controlled parameter. Example: a 4-20 mA industrial loop where the load resistance or impedance can be changed, but the current remains constant and related to the input reference signal (be that in itself a current or a voltage). The ability of a current output amplifier to drive a load resistance(impedance) is its compliance and is measured in volts and it therefore follows that a high compliance current output amplifier is capable of driving a high resistance load.
A current feedback amplifier is an amplifier in which the feedback from the controlled output parameter is in the form of a current.
Similarly, a voltage output amplifier is a device in which the output voltage is controlled. Example: a typical audio amplifier (be it a VFA or a CFA).
In a voltage feedback amplifier, the feedback signal is in the form of a voltage (ignoring the typically minute bias currents) and the controlled parameter, the output voltage, is related to the input reference signal (again, be that a voltage or a current).
Eagle eyed readers may then well ask: what then is an inverting amplifier using a VFA op-amp? The feedback current into the feedback summing node is Vo/Rf i.e. a current. Surely then this makes an inverting amplifier like this a current feedback amplifier?
No, it doesn’t. In a VFA, the current into the op-amps inverting input is NOT linearly related to the feedback current – its just the bias current and will be highly non-linear wrt to the output voltage, though very small – typically nA or uA in a practical amplifier. In other words, in the inverting mode, the inverting input of a VFA is still a voltage input, albeit held at some reference voltage (usually 0V). The output controlled parameter arises because of the op-amp drives the output (and hence the feedback resistor) so that its input voltages remain equal.
So, a simple question with a simple answer that does not require an endless semantic debate.
For more information on CFA and VFA amplifiers see ‘CFA vs VFA: A Short Primer for the Uninitiated‘