# How to identify the feedback topologies?

It is extremely important to identify the feedback topology first before starting analysis. However, I find it difficult and cannot get it right.

Is there an accurate yet easy way for me to identify one out of the following four feedback topologies?

1. Series-series
2. Series-shunt
3. Shunt-series
4. Shunt-shunt
• I was about to post this exact same question... Sadly, no one's answered it so far, so I guess it's no point posting it anyways :/ But... did YOU find out the answer, @farticle-pilter? – Rui Pimentel May 26 '14 at 2:12

The problem is that from a term "Series-Shunt" it is not clear what comes first: "in" or "out"? I have discovered that different authors handle this subject differently. For this reason I prefer, for example: Voltage-controlled current feedback.

Examples:

• Voltage-controlled voltage feedback: Non-inverting opamp,
• Voltage-controlled current feedback: Inverting opamp,
• Current controlled voltage feedback: Common emitter stage with Re feedback,
• Current- controlled current feedback: (a) Inverting OTA amplifier, (b) common emitter stage with a voltage divider between collector and signal input (base node at the middle point).

Yes there is one easy way in which you can identify the topology. Just follow these steps.

1. Identify the feedback network/element.
2. If at output side, feedback is connected to the output of the circuit directly, name it as 'voltage', or else 'current'.
3. If at input side, feedback is connected to the input given to the circuit directly, name it as 'shunt' or else 'series'.

Ex- if it comes out to be - voltage shunt feedback (named from output to input), you can also name it as shunt shunt feedback.

{at input side: shunt=current, series=voltage.

at output side: shunt=voltage, series=current }

Let me try to provide a intuitive way which i find very easy to understand

Voltage and Current leaves you with 4 possible combinations with which you can sample (at the output) and mix the feedback to input.

Now coming to sampling and mixing:-

Sampling:- At the output we take sample of what is there at output (since we want to check the behavior of output). Now we dont want to disturb the output when we take the sample. Thats why when voltage is sampled, its in parallel (since voltage is undivided in parallel) while current in series (current does not change in series). Very much like how we attach a multi meter to a circuit (we want the readings without affecting the setup)

Mixing:- Now on the mixing end we want to affect the signal that is provided to the amplifier, since that is the whole point of getting the feedback. So voltage will be in series and current will be in parallel (so that they can change the input and in effect change the output).

Series-Series ......Voltage in - Current out

Series-Shunt .......Voltage in - Voltage out

Shunt-Series .......Current in - Current out

Shunt-Shunt ........Current in - Voltage out

So i hope the above lines will make more sense to you.

All the above mentioned answers are correct but I was still unable to comprehend the type of feedback happened at a particular op-amp. Then, i got through this pdf--> http://cas.ee.ic.ac.uk/people/dario/files/E22/L3-feedback%20amplifiers.pdf

What LvW has mentioned above is accurately correct. In this pdf, the same things are explained. An example for each particular topology has been explained here. It has helped me.. Hope u too find it useful !

If i can summarize what i have learnt, it would be like...

In op-amp , we generally use either i) inverting mode or ii) non-inverting mode

i) In inverting mode, the input and feedback are given to same input node. Here the output voltage is taken (Hence, shunt feedback) and at input the current is sum of input and feedback currents (Hence, shunt connection). Therefore, it is a Current-Controlled-Voltage-Source topology.

ii) In non-inverting mode, the input is given at one node and the output is fed back at another node. Even here, just like before, the output voltage is taken (shunt feedback), but at input the voltage is fed back by the resistor(generally referred as R1) to another node, eventually decreasing the net input voltage (series connection). Therefore, it is a Voltage-Controlled-Voltage-Source topology.

With these two, the remaining two topologies are generally realised by BJT and FETs.

In a BJT, if we implement EMITTER DEGENERATION, ( adding a resistor at the emitter end of a BJT ), then it becomes a Voltage-Controlled-Current-Source topolgy. Since, the emitter current is generally almost equals to Collector Current, the output current is sampled at the emitter resistance which feeds back to input as Voltage drop (One can find it, by writing KVL for input loop).

And similarly for FET is explained there for the remaining Current Controlled Current Source.

Excuse me if something is wrong with my answer, since it is my first answer.

And i would really love if someone can help me with some more examples on these topologies. Thanks.

• you could add the link in the comment. or edit answer to improve it better. – Umar Jan 27 '17 at 7:07
• Yeah, realized it after answering. What about now ? – ayrus13 Jan 27 '17 at 7:20

Remember Only these 2 and rest you can figure out-

1 Shunt Shunt- Current(Parallel-Input), Voltage(Parallel-Output) {Voltage Sampling Shunt Mixing}

2 Series Series- Voltage(Series-Input), Current(Parallel-Output) {Current Sampling Series Mixing}

Always remember M-S( I/P Mixing and O/P Sampling)

You just have to look at the inputs and the outputs to identify the circuit...

Series-Series ......Voltage in - Current out

Series-Shunt .......Voltage in - Voltage out

Shunt-Series .......Current in - Current out

Shunt-Shunt ........Current in - Voltage out

• I think the issue may be how do you recognize a current vs a voltage input/output. You're providing definitions, but no substantive answer. – Scott Seidman Nov 10 '14 at 13:28