Wien Oscillator not working. - Page 1
Forum section: electronics / beginners

author: Zielsziek
author_rank: Contributor
timestamp: January 18, 2025, 07:36:25 pm
content: Hello,I have built on a breadboard a curve tracer that uses the sine wave generated by a Wien oscillator. The breadboard version works fine. I have built a smd version for which I made a PCB. The smd version is not working despite being mostly identical to the breadboard version. The only difference is the switchable filter in the smd version. It is not my design, it is Mr. Carlson's, except the Sine wave generation part.At pin 1 of the TL074 I am getting nothing, only random noise, not even a square wave. At this point I am scratching my head in despair as I do not know how to tackle this situation. I have to admit it is my first time working with smds and oscillators.Here is what I have checked:1. Placement of the components and solder joints. (No solder bridges on TL074, all pins are soldered fine, no shorts)2. Split supply generated by LM358 (it is fine, the TL074 is receiving proper power)3. Any mistakes made in the tht to smd transition. I only found an extra 100k resistor which i just removed.What boggles me is that I used the same LM358 and TL074 for the breadboard version which works fine as I am writing this. I have not changed any component values. I am using the same 1N4148 diodes.Thank you for taking your time to read this. I have attached the relevant info below.

author: Zero999
author_rank: Super Contributor
timestamp: January 18, 2025, 11:33:09 pm
content: The limiting circuit isn't quite right. It should look something like the attached, but it should still oscillate, just with a badly distorted output. Assuming the connections are good, have you considered the TL74 could be fake?

author: Zielsziek
author_rank: Contributor
timestamp: January 19, 2025, 08:31:03 am
content: Hello. I have checked all of the connections. I have reflowed the board.I did not consider the possibility of a fake IC as it has never happened to me before. In order to confirm this I will use some bodge wires to the THT TL074 that I have. I did consider a possible defective TL074, which I swapped for another identical part, but the replacement comes from the same supplier, a local trusted one. I do admit I usually order my parts from somewhere else.Thank you for posting the updated filter circuit and taking your time to reply to me!

author: Kleinstein
author_rank: Super Contributor
timestamp: January 19, 2025, 09:48:32 am
content: The capacitors have usually quite some tolerance. With different values for C3 or C4 the required gain to make the circuit oscillate changes. This can especially a problem if C3 and C4 are X7R or similar types as they are temperature dependent and change there value after soldering. In addition such capacitors are somewhat nonlinear. This may add distortion.Normally one would want a trimmer to do a coarse adjustment of the bridge gain. This way one can adjust how strong the diodes force a stable amplitude and balance between amplitude stability and distortion.There is a certain chance that the virtual ground part oscillates with capacitive load at the output or the OP-amp.

author: Zielsziek
author_rank: Contributor
timestamp: January 19, 2025, 12:53:00 pm
content: Thank you for this advice. Could this have been the asymmetry of the rails that I observed during testing certain components? For example, one rail would be at -14.99 and the other 14.89 something? I will continue to improve the circuit.

author: Kleinstein
author_rank: Super Contributor
timestamp: January 19, 2025, 02:37:14 pm
content: Asymmetry in teh supply is not issue: even +25 and -5 V would work somewhat.What is the output voltage from the not working oscillator ?The output voltages at the other amplifiers could also give a hint if the amplifier maybe totally fake or possibly reversed.

author: Zielsziek
author_rank: Contributor
timestamp: January 20, 2025, 09:03:54 am
content: I have only checked the first output, pin 1 which gives nothing. I will check the other outputs when I will arive home. Thanks!

author: Zielsziek
author_rank: Contributor
timestamp: January 20, 2025, 05:25:52 pm
content: Ok, I have made the measurements a couple of seconds before writing this post using an oscilloscope and all of the pins (except the power ones) show just random noise. I have rechecked every connection and previously reflowed the ic. The pads are all making proper contact. I think this is a fake IC which is surprising to me as it is the first time I encounter such an issue. I also believe it might have been static electricity that killed these ICs since they came only in a regular plastic bag that contained the cut off part of the roll(this last part came to memory recently)I have ordered replacements from the trusted supplier. Thank you all for helping me!

author: Zero999
author_rank: Super Contributor
timestamp: January 20, 2025, 07:24:38 pm
content: What are the DC voltages?Try replacing the ICs with known good ones.You could also try building a simple circuit such as a Schmitt trigger oscillator or amplifier with a gain of 2. Stick with low frequencies first, no more than 1kHz.

author: TheoB
author_rank: Regular Contributor
timestamp: January 20, 2025, 08:29:00 pm
content: Take a 1k resistor and inject some voltage on the + input of the opamps. Due to feedback, the - input should follow with the same voltage. Do the same on the - input. The output should go to the rail. At least you can see if the basics work.And remove D1,D2 and R3. The orientation is wrong. Removing R3 increases the gain. It should start to oscillate even if distorted. Then you know whether it's a gain issue.And why did you connect the 30V input supply directly to the opamps? That defeats the purpose of the reverse polarity protection of D1.

author: jimmc
author_rank: Frequent Contributor
timestamp: January 20, 2025, 09:15:11 pm
content: The Wien bridge has an attenuation of 3 at zero phase shift, so the maintaining amplifier needs a gain of>3 to oscillate.If the resistors and capacitors are not matched then the gain will need to be a bit higher.As shown the gain is 1+(R4//R5)/R6 = 3.3 which may not be enough.Two suggestions1 Try swapping C3 and C5 (less loss if C5 < C3)2 Try removing R4 to raise the gain to 4.

author: Zero999
author_rank: Super Contributor
timestamp: January 20, 2025, 10:53:58 pm
content: Note that, a Wien bridge oscillator will always need some form of limiting (diode, feedback resistor with a positive/negative temperature coefficient, AGC) in order to avoid the output saturating. It doesn't matter if the component values are all perfectly matched or not. It will still saturate unless it's limited.If the gain feedback resistors are exactly 1:2, then there won't be sustained oscillation (there might be an oscillation when power is applied, but it will decay) because the open loop gain of the op-amp isn't infinite, so the gain is slightly under 3. If the feedback is slightly over 1:2 then it will oscillate, and the output will increase, until the output saturates. The op-amp's open loop gain isn't closely controlled and will drift, making it impossible to set the gain to exactly 3.The simulation shows what will happen with perfectly matched components and an op-amp with an open-loop gain of 100k. I've set the initial voltage on C1 to 1V to show that it will result in a damped oscillation when the power is first applied.

author: Zielsziek
author_rank: Contributor
timestamp: January 21, 2025, 05:21:06 pm
content: Hello again everybody.I have decided to use the voltage injection method ( I wish I thought about this earlier)I was surprised to see that the op amp does literally nothing !!!!The reverse polarity diode misplacement is and oversight of mine.The replacement parts are on their way from TME.eu (It will take about 4-5 days)From now on I will stockpile some parts that I need. I will also try to update the design using what I have learned here. Hopefully I will be able to find some trimmer capacitors.Thank you all for helping me!

author: Grandchuck
author_rank: Super Contributor
timestamp: January 23, 2025, 09:16:04 pm
content: Here is a 1 kHz Wien oscillator with field effect transistor control of the loop gain. The gain is high enough for the oscillator to self start. Without gain control, the amplitude would keep increasing and a distorted output would result. With gain control, the output settles into a steady amplitude of about 3.5 volts peak to peak with low distortion.