1
\$\begingroup\$

I've connected the circuit of Fig 3.4.6 in this link which is shown below:

Here is the circuit that I've connected:

Where in my circuit:

  • R3 = 100 kOhm
  • R4 = 47 kOhm
  • R1 = R2 = 5.1 kOhm
  • C1 = C2 = 100 nF
  • Op amp is of type 741
  • Diodes are of type 1N4001

I chose R3 and R4 based on this circuit from here:

This circuit is the same as the one in the first figure except that there are no diodes.

When I connected the output of the circuit to an oscilloscope, here is all what I could see:

I tried connecting a potentiometer in place of R4 and varying its value but with no noticeable changes in the output.

Note: I've simulated my circuit using Multisim but using different diode types (1N4148), and the results were good.

How can I get the circuit to operate properly?

\$\endgroup\$
  • \$\begingroup\$ I think you need to get your multimeter out and check the values of those resistors. The yellow-blue-brown-gold resistor looks out of place to me. \$\endgroup\$ – Bimpelrekkie Apr 13 '16 at 19:37
  • \$\begingroup\$ No. It's a 47 kOhm resistor. \$\endgroup\$ – ammar Apr 14 '16 at 14:26
3
\$\begingroup\$

You are using your op amp in a single-supply configuration without providing a virtual ground. The simplest fix is to get another 14 volt supply, and connect this as a negative supply to pin 4. If you're not willing to do this, try

schematic

Your output will now have an offset of about 7 volts, which may or may not be a problem, especially since any noise on the 14 volt line will (except for the filtering provided by C1) show up on the output as well. This is why I say it's easier to provide a second power supply. YMMV, of course.

Also, make sure to provide a 0.1 uF ceramic cap between V+ and ground for each op amp. Connect the cap right next to the pins of each IC - don't use jumpers.

\$\endgroup\$
  • \$\begingroup\$ There are simpler solutions (one opamp only) for single supply oscillators. Try googling - and you will find the circuits, for example:analog.com/media/en/technical-documentation/application-notes/… \$\endgroup\$ – LvW Apr 14 '16 at 8:32
  • \$\begingroup\$ Thank you. I connected the negative supply today in the lab and the circuit worked well. But maybe I need some adjustments. I may ask about them in different questions. \$\endgroup\$ – ammar Apr 14 '16 at 14:28
  • 1
    \$\begingroup\$ @ammarx - Any time. We live to serve. In the future, though, learn to use the schematic entry option (the icon that inludes a diode, 7th from the left when you open a question or answer window, or simply hitting ctrl-m). It's much clearer than a picture of your breadboard. \$\endgroup\$ – WhatRoughBeast Apr 14 '16 at 14:34

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.