school: opamp homework

sammy · Post by **sammy** » Mon May 29, 2006 10:22 am

I've got this homework about opamps but I don't really understand it. As you can see on the image there is a signal coming on Ui it is a 2V ptp sinusoÃ¯dale signal but that's not really important. The normal formula for this opamp would be: Uo=-U1 * Rt/R1 but since R1 consists of 2 resistors it would have to be Uo=-U1 * Rt/ (500/(500+1000)). I say 'would have to be' because that would only be so if that 500ohm was connected to gnd which it is not. Can anybody help me find the correct formula for this opamp? Some of the symbols might be incorrect because I'm using European symbols (except for the resistors).

Post by **Skyone** » Mon May 29, 2006 3:29 pm

I figure that it's an AC circuit?

codeman · Post by **codeman** » Mon May 29, 2006 7:27 pm

hooray for op-amps! Ok, it's been a while since i've done this, so bear with me here...
for simplicity i'm going to say the 2v ptp sine wave = V2, the 1k resistor = R1 and the 500ohm resistor = R2

The inverting configuration is really easy to figure out if you follow the current path. You also know that the point at the inverting terminal is going to be a virtual ground, or 0v. The output, Vo is going to be the inverting terminal node voltage minus the drop across Rt, or

Vo = 0 - I_t*R_t = -I_t*R_t

I_t is easy to figure out, because it's just the sum of the current through the two resistors. So...

I_t = V1/R1 + V2/R2
and
Vo = -R_t * (V1/R1 + V2/R2)

someone want to check my work?

sammy · Post by **sammy** » Tue May 30, 2006 1:06 pm

I guess it's correct since I got 10/10 for that homework. Thanks!