Simple OP-AMP, but what type is it?

[Lac]

Hi.

What kind of op-amp is this circuit when u1=u2=1.0V. Not and diff.op-amp I presume, since the resistor pair is not matched on both inputs. How could I calculate the output voltage from this circuit?

 

[dougy83]

In this simple case of negative feedback, you can assume that the +ve and -ve inputs of the opamp are at the same voltage and that the opamp inputs are not drawing any current (assume it's a 'perfect' opamp).

So all the current flowing through R2 is also flowing through R1. The voltage drop across each resistor this current multiplied by its resistance (V = IR). That will give you the output voltage.

 

[MikeMl]

An OpAmp tries to keep V+ = V- (the non-inverting and inverting inputs, respectively).

V- = V+ = U1.

An OpAmp has zero input current (well, almost zero), therefore I(R1) = I(R2).

I(R1) = E/R = (U2 - V-)/R1
I(R2) = E/R = (V- - UT)/R2

Substituting U1 for V- and equating the two currents,

(U2 - U1)/R1 = (U1 - UT)/R2

R2U2 - R2U1 = R1U1 - R1UT

Solving for UT,

UT = -(R2U2 - R2U1 - R1U1)/R1

UT = U1(R1+R2)/R1 - U2R2/R1

UT = U1(1 + R2/R1) - U2(R2/R1)

Which is the superposition of the inverting gain and non-inverting gain formulas.

This is one of the Quizzes I would give my junior EE job applicants. You would be surprized how many fell on their ass!

 

[dougy83]

This is one of the Quizzes I would give my junior EE job applicants. You would be surprized how many fell on their ass!

Not surprised (sorry). Actually, the first ee job I got out of uni (actually my last job) I was asked to name some circuit symbols and draw the basic circuit for buck smps, so I'm guessing there's a lot of people that forget everything as soon as the exam is over.

We learnt the opamp stuff in the "hard subject" of 1st year uni; I guess it was hard - well a new concept anyway - when compared to the grade 10 maths we were supposed to be learning at the same time.

 

[Lac]

Great, thanks! Got it figured out now. Does this configuration have a specific name?

 

[dougy83]

You can call it an "inverting amplifier" or "non-inverting amplifier" depending if the reference voltage is connected to the positive or negative input. Have a look on wikipedia, all is revealed

 

[Roff]

In this case (U1=U2), it's a voltage follower with crappy bandwidth.