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[SOLVED] Feedback topology identification of inverting OpAmp

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ashrafsazid

ashrafsazid

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Hi,

Can anybody please explain the feedback topology for the inverting opamp configuration. I cite the figure 8.26 from Razavi 3rd ed., an example given for voltage-voltage feedback. But I am confused about the mixing, from my understanding the voltage is sensed and the current is mixed at the input (is it voltage-current?).

Also about the input node of capacitive configuration, it is floating right? How to deal with this floating node?(Probably only with discrete circuits like SC circuits?)
 

Deduction of the gain for any topology of Op.Amp. is often straightforward, just handling equations so that it turn into a form that explicit Vo/Vi in the left side. The main assumption is the sum of currents toward each + or - inputs is zero. Anyway, I wonder why didn't you add any picture of the mentioned topology.
 

An inverting amplifier is a shunt-shunt (Here shunt is the feedback at the input, shunt is also the sensing at the output) configuration. You sense voltage at the output and feedback the error current ( difference between input current and fed back current) at the input.

An easy way to identity the type of feedback is to identity whether error voltage or current is being fed back. To identify the sensing at the output look at the feedback network. If the feedback network is directly connected to the output, then it is shunt sensing (you are sensing a voltage at the output)
 

The op amp has a very high open-loop gain so, when in a closed loop negative feedback configuration, it will always try to keep the two inputs with essentially 0V between them.
So you just determine what the external voltages have to be to keep those two voltages equal to each other.

If the plus op input is grounded, as for the common inverting op amp circuit, then the input and output resistor voltages must be such as to keep the minus op amp input at 0V (virtual ground).
This leads to the equation for the gain of Rf/Rin for a simple two resistor inverter.

Don't understand your question about capacitance.
 

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