# How can one distinguish between negative feedback types?

There are several negative feedback types, like voltage shunt, voltage series, current shunt, andcurrent series. I can distinguish between them with op-amp or box diagrams very well (it's easy I guess), but when it comes to the point when we use real transistors, resistors and capacitors I don't understand which one is which.

I checked some books, also the web, and they all have something like boxes with the gain written in them, but I couldn't find a direct way to distinguish between negative-feedback types in a given real transistor-resistor-capacitor circuit. I hope my question isn't silly.

For example this circuit. What should I check first? Then, according to the result, what should I do next? And how does one decide if it is current series?

If you could answer in the form of an algorithm it would help me understand way better.

• Personally I don't understand your question. Perhaps you can clarify with some diagrams if nobody else provides a sufficient answer. Commented Jan 10, 2015 at 12:31
• @BrianJHoskins i hope this way is more clearer for you and everyone else. Thank you for your attention in either way Commented Jan 10, 2015 at 13:04
• As you can see, I have included this example in my detailed answer (current-controlled voltage feedback).
– LvW
Commented Jan 10, 2015 at 14:13

No - I don`t think that your question is "silly". It is, indeed, in some cases not easy to identify the type of feedback. At first, you have to find out which output quantity (current or voltage) determines the feedback signal. And secondly, it is important if this feedback signal is a current or a voltage.

In most cases, it is not a problem to answer the first question (output quantity). For answering the second question (feedback quantity) it is best to find out how the feedback signal is combined with the input signal. This can be best explained using an example (opamp with feedback):

1.) For the inverting opamp amplifier the input signal is combined with the feedback signal in a common node (directly at the inv. input terminal). In such a node only two CURRENTS can be superimposed. Hence, we have voltage-controlled current feedback. This case can be transferred to a BJT amplifier, which has a feedback resistor from the collector node to the base node. In this case, signal feedback is established because the input signal is connected via a series resistor to the base node.

2.) For a non-inverting opamp amplifier the input VOLTAGE is directly superimposed with the feeedback signal using the differential input of the opamp. Hence, the voltage difference directly results from the feedback voltage. In this case, we have voltage-controlled voltage feedback. A similar case exists for a BJT amplifier (common emitter configuration) which has an emitter resistor RE. Here, the driving voltage difference Vbe directly results from the difference input voltage minus feedback voltage (developed across RE).

In some cases, it might be helpful to look at the input resistance. In most cases, it is not a problem to see if the feedback path causes an increase or a decrease of the input impedance. In the first case (increase) we have voltage feedback and in the second case (decrease) we have current feedback.

• Thank you sir! This clearifies most of my question but if there is one i need an answer like a algorithm, check this if this is like that than check this alike. but this helps also a lot so thank you. Commented Jan 10, 2015 at 12:59
• i dont know why but it took more time to sink in for me but i get it now i'll mark this as an answer. Commented Jan 10, 2015 at 16:24