Operational Amplifiers Concepts

Thread Starter

pijolysss

Joined Nov 30, 2017
17
Hello world!

Im reading a book of electronics and i found some cases that i cant totally understand it,
I have some theorical doubts in some cases of Operational Amplifiers.

In pic1(I think it is an "inverting amplifier"):
Why the Transmission Coefficient(Ku) is: Ku= -R2/R1 ??
In pic2:
If we multiply R3*2, what will happen with the Transmission Coefficient(ku)? I mean, it will "Increase" or "Decrease"? Why?

Thank you very much to all! :)
 

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GopherT

Joined Nov 23, 2012
8,012
Both of your images are inverting amplifiers. The one with R3 is simply compensated. The value of R3 should be selected during the design process as the parallel resistance of R1 and R2.

Using an R3 in a design is most important in op amps that have bipolar input transistors. Op amps with jfet inputs don't need them in most cases.
 

OBW0549

Joined Mar 2, 2015
3,544
In pic1(I think it is an "inverting amplifier"):
Why the Transmission Coefficient(Ku) is: Ku= -R2/R1 ??
That follows as a consequence of two facts:
  • In an ideal op amp with infinite open-loop gain, Va = Vb; and
  • In an ideal op amp, no current flows into or out of its inputs.
The rest is just basic circuit theory and a little algebra.

In pic2:
If we multiply R3*2, what will happen with the Transmission Coefficient(ku)? I mean, it will "Increase" or "Decrease"? Why?
Nothing will happen to the circuit voltage gain; since in an ideal op amp, no current flows into or out of its inputs, the voltage on the (+) input of the op amp is the same as it would be if the (+) input were connected directly to ground. Therefore, the circuit behaves the same as the circuit in pic1.
 

Thread Starter

pijolysss

Joined Nov 30, 2017
17
That follows as a consequence of two facts:
  • In an ideal op amp with infinite open-loop gain, Va = Vb; and
  • In an ideal op amp, no current flows into or out of its inputs.
The rest is just basic circuit theory and a little algebra.


Nothing will happen to the circuit voltage gain; since in an ideal op amp, no current flows into or out of its inputs, the voltage on the (+) input of the op amp is the same as it would be if the (+) input were connected directly to ground. Therefore, the circuit behaves the same as the circuit in pic1.

In pic1:
I dont understand it. What I mean is why always Ku(transmission coefficient) is -R2/R1 in that amplifier.

In pic2:
So, you mean that even if the value of R3 is multiplied by 2, the coefficient transmission doesnt variate? it wont increase or decrease?

Thanks for replys,
 

OBW0549

Joined Mar 2, 2015
3,544
But... in pic2, Why it doesnt variate? I didnt understand so much the explanation (sorry, Im new in electronics)
Is there any current flowing through R3? No, because the inputs of an ideal op amp don't draw any current. So if there is no current flowing through R3, is there any voltage across it? No. Therefore, the voltage at the (+) input of the op amp is the same as if it were connected to ground, just as it is in pic1.

Notice I said ideal op amp. In a real-world, non-ideal op amp, things are slightly different; but even then, R3 does not affect the voltage gain of the circuit. You'll have to study about input bias current to understand the function of R3.
 

Thread Starter

pijolysss

Joined Nov 30, 2017
17
Thank you ver
Is there any current flowing through R3? No, because the inputs of an ideal op amp don't draw any current. So if there is no current flowing through R3, is there any voltage across it? No. Therefore, the voltage at the (+) input of the op amp is the same as if it were connected to ground, just as it is in pic1.

Notice I said ideal op amp. In a real-world, non-ideal op amp, things are slightly different; but even then, R3 does not affect the voltage gain of the circuit. You'll have to study about input bias current to understand the function of R3.
Now I got it! Thank you very much for your reply, and your patience :)
Regards!
 
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