2
\$\begingroup\$

I built the standard Wien bridge oscillator circuit using the UA741CN op amp IC, however I am not getting any oscillation. Schematic

I am powering this circuit with two 9V batteries. I'm using the values R1=R2=10KΩ, C1=C2=100nF, R3 = 1KΩ, R4 = 470Ω. The oscilloscope just shows a flat line, no matter what values I try. I also tried hooking up a 8 Ohm speaker to V_OUT and circuit ground, since the oscillation amplitude and expected frequency are sufficient to make the wave audible. But there is only static.

I have tried a number of things to troubleshoot the circuit:

  1. Using different resistor values, consistent with the circuit requirements.

  2. Built a working comparator circuit using the op amp chip to ensure that the chip is good.

  3. Checked all voltages.

I'm very new to electronics, so I might be missing something simple. I am wondering if somebody could provide a step-by-step guide that would eliminate any common problems with this circuit. enter image description here

\$\endgroup\$
  • 1
    \$\begingroup\$ @SpehroPefhany yes my mistake, I calculated about 1.5Hz due to mental fatfinger. \$\endgroup\$ – user133493 Dec 17 '17 at 3:44
  • 2
    \$\begingroup\$ Is the op amp powered from +9V and -9V? Consider adding antiparallel diodes in parallel to the "R3", and making R3 slightly higher resistance than twice R4, for amplitude stability. Try waiting, too, because the oscillation isn't self-starting, it has to build from noise. \$\endgroup\$ – Whit3rd Dec 17 '17 at 7:54
  • 1
    \$\begingroup\$ on the face of it, 1k/470 should give enough gain to start oscillation, but if all components are at the wrong end of tolerance, you could easily be gain starved. Try 10k/10k, with antiparallel diodes across R3 to limit oscillation. It will make a mess of the sine wave, but at least it should start. BTW, an 8ohm load on the 741 will kill the gain and guarantee failure to oscillate. \$\endgroup\$ – Neil_UK Dec 17 '17 at 11:44
  • 2
    \$\begingroup\$ The power supply is two 9V batteries, connected in series. Ground is the midpoint of this series. So this gives me the +/- 9V referenced to ground that is required by the chip. That's how I understand the circuit, but perhaps my understanding is in error. \$\endgroup\$ – tavr Dec 17 '17 at 17:00
  • 1
    \$\begingroup\$ Changing the R3/R4 resistances to 10K/4.7K results in V_flatline = -7V. In contrast to -4V with the original values. \$\endgroup\$ – tavr Dec 17 '17 at 18:22
1
\$\begingroup\$

After much struggle and consternation, I got oscillation, by putting a 100 Ω resistor between output (pin 6) and ground. Perhaps someone could explain why the oscillation doesn't happen without loading the output.

\$\endgroup\$
  • \$\begingroup\$ where did you connect the ground clip of your scope probe? what is the oscillation freq? is your scope set to dc ĉoupling? is probe set to 1X? \$\endgroup\$ – Genzo Jan 14 '18 at 3:56
  • \$\begingroup\$ Ground clip is connected on the other side of the 100 Ohm resistor (i.e. to circuit ground). 220Hz. Yes. Yes. \$\endgroup\$ – tavr Jan 14 '18 at 6:42
0
\$\begingroup\$

Long inductive jumpers are about 10nH/cm and this, when coupled with positive feedback, can cause unstable parasitic oscillations at the LC resonant frequency that were limited by loading the output with high current. It is much neater and better performance to use AWG 24 solid wire short jumpers cut to length to lay flat with the shortest distance. You end up with a small jar of odd length jumpers.

The Wein Bridge oscillator is very unstable as it is critically dependant on R tolerances. You can overcome this by increasing the gain up to 50% and adding some soft limiting. Example.

Furthermore the high Q makes the oscillator start very slow at the critical gain ratio.
Hence, it is rarely used outside the student lab. enter image description here

For what it's worth, batteries are sources of noise. Just put one an a speaker and listen.

\$\endgroup\$
0
\$\begingroup\$

The output of a 741 opamp cannot drive 100 ohms. Its datasheet shows 2k minimum. Get rid of the breadboard with the messy long wires all over the place and use a compact soldered stripboard or pcb layout.

\$\endgroup\$

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.