Electronic – Inverting op amp used with low frequency oscillator

operational-amplifier

I am designing a low frequency oscillator. An image of the schematic is below.
enter image description here

The oscillator part works fine. My intention is to scale the triangle wave at the output of U1 to +/-2.5 V by using U4. In the simulation this works correctly; the gain of U4 is of course -(68/100) = -0.68, or 0.68. In the simulation the triangle wave at the output of U4 is approx. +/- 2.5 V.

However, on the breadboard, Vtri at the output of U4 appears as a triangle wave equal in magnitude to its original wave. If I increase or decrease R9 it has no effect on the gain. I am not sure why. Perhaps the way I am trying to couple the output of U1 to U4 is not working the way I want? I also tried AC coupling the output of U1 to a noninverting amp, but this didn't work either.

I thought maybe somehow my 074 was damaged; I popped a 324 into its place and I got the same result. I am not sure why.

Can anyone suggest a way for me to amplify the output of U1 by 0.68, preferably with my 074 op amp?

Thank you.

Best Answer

The circuit should work as diagrammed. I think you have a breadboard wiring error. There's 4 opamps in a TL074, and you appear to only be using 3. Try using U3 instead of U4. This will force you to rewire the breadboard, perhaps correcting the wiring error in the process.