Inverting configuration of op-amp, output wave forms

[INGLE.SHUBHAM]

Hello guys i facing a problem of inverting configuaration of opamp



in this configuration,the current start flowing through Ri, Rf to Vout, due to high input impedance no current flows to the input terminal of op-amp,
so my question was if there is no current flowing in the inverting terminal of the op-amp then how the output waveform is shifted 180 degree out of phase
by mathematical calculation im able to find the inverted output signal, at the output pin of op-amp,
but by practically im getting bit difficult to analyse how the signal is getting shifted 180 degree

please suggest how i can analyse the circuit properly

 

[KlausST]

Re: inverting configuration of op-amp, output wave forms

Hi,

how to explain inverting OPAMP circuit?
Here my attempt:

* At noninverting input there is GND
* the OPAMP tries to regulate the output in a way that the same voltage (GND, 0V) is at the inverting input, too.
* this is called "virtual GND" at the inverting input. Just imagine as if there is GND.
* now (if there is GND), then the input current through Ri: I_in = V_in / Ri.
* now the OPAMP input is very high ohmic ... one consdieres, that the OPAMP_input_current = 0
* because of this, all input current through Ri must flow through Rf.
* this means the voltage across Rf: V_Rf = Rf x I_in
* current flow trough both resistors is in the same driection, this means the voltage is in the same direction, too. If input voltage is positive, then at Ri: left side positive, right side negative.
The same is at Rf: left side positive, right side negative.
At Rf the left side is virtual GND, therefore the right side needs to be negative. --> inverting circuit.

Now combine the formula for Ri and Rf:

I_Ri = V_in / Ri --> = I_Rf = (-V_out / Rf)
solve it to:
V_out = - V_in * Rf / Ri
A = -Rf / Ri

Klaus

 

[INGLE.SHUBHAM]

Re: inverting configuration of op-amp, output wave forms

then how will non inverting configuration works, even in this configuration the left side is positive and right side is negative, then why the signal is in phase.
please explain this configuration too.

 

[KlausST]

Re: inverting configuration of op-amp, output wave forms

Hi,

I personally don´t like the phrase "phase shift by 180 degree" in this case, because phase shift (in my eyes) has something to do with time delay.
With an inverting OPAMP circuit we don´t have considerable time delay.
The signal voltage just has opposite sign.
+ at input becomes - at output
- at input becomes + at output

Therefore you may see it statically.
Let´s imagine just two resistors in a string. Let´s name them Ri and Rf. (Don´t think of an OPAMP)
Let´s assume both have same value. Let´s say 1kOhms
Now connect the free end of Ri to +1V.
Now assume you have a variable voltage supply connected to the free and of Rf.
What voltage is necessary, that the center of both resistors become 0V?

You can use this method, because the OPAMP input is high impedance, it doesn´t (considerably) influence the voltage at the center node. The OPAMP just "senses" the input voltage and adjusts/regulates the output voltage.

Klaus

- - - Updated - - -

Added:

I just recognized you wrote "non inverting". The above explanation still was for inverting.

**
Now for non-inverting:
From Post#2:

* the OPAMP tries to regulate the output in a way that the same voltage (GND, 0V) is at the inverting input, too.

--> it regulates the output voltage in a way, that the inverting input gets the same voltage than the non-inverting input.
That´s the key.

Lets assume there are the two resistors Rg and Rf. Rg is connected to GND, RF is connected to the output. The center node is connected to the inverting input.
Now V_in is at the noninverting input.
The OPAMP regulates the inverting input to the same voltage. Means the voltage across Rg is the same as V_in.
I_Rg = V_in / Rg.
the same current is through Rf. The voltage across Rf is: V_Rf = I_Rg x Rf.
To calculate the output voltage: you need to add the the voltage across Rf to the voltage at the inverting input. V_out = V_in + V_in * Rf / Rg
= V_in * ( 1+ Rf / Rg)
A = 1 + Rf / Rg

Klaus