Amplitude instability in Wien Bridge - Page 1
Forum section: electronics / projects

author: 6SN7WGTB
author_rank: Regular Contributor
timestamp: October 29, 2021, 10:30:18 am
content: I have recently got into SMD components, having been a 'through hole' constructor for tens of years.I'm just building an SMD version of a published Curve/Signature tracer.The circuit has a number of elements:1. 1kHz sine wave generator using Wien bridge circuit (1k5, 100nF pairs) and the obligatory small filament lamp with trimpot in the inverting FB loop2. Buffer amplifier on the output of that - which feeds...3. A peak detector (so you can connect a DMM if you don't have voltage measurement on your 'scope), and...4. A constant current source to the DUT, and then H and V outputs to 'scope.All opamps are TL072. Supply +/-15V. The supply is stable (linear PSU), the PCB has 100µ filter caps, and the opamp has 0µ1 decoupling adjacent.There appear to be two problems:I. when I adjust the trimpot to get an unclipped sine wave, as soon as it 'unclips' it starts oscillating in amplitude. Frequency is pretty stable.II. The TL072 (which is combined Wien and buffer) seems to get hot - i.e. 55degC.Questions I have are:(a) Due to my inexperience, have I missed something obvious about translating a through-hole design to SMD?(b) The 100n capacitors are ceramic, but NOT NP0/C0G - is this contributing to the issue?(c) Why's the TL072 getting hot???!!! The current draw for the whole circuit (3 opamps, constant current source to DUT) is around 10mA.I am flummoxed by this - I have ruthlessly checked for obvious faults such as tracking (solder, flux etc.), incorrect components. It seems such a simple circuit and such a low frequency, I cannot understand what is wrong.The relevant part of the schematic is below. You'll see it shows a quad opamp and I am using a dual for these two bits.Thanks in advance.

author: 2N3055
author_rank: Super Contributor
timestamp: October 29, 2021, 10:43:05 am
content: Which TL072 are you using exactly? TI new TL072 are vastly different Mosfet devices compared to original TL072...

author: RoGeorge
author_rank: Super Contributor
timestamp: October 29, 2021, 10:43:18 am
content: (c) When something gets hot for apparently no reason, I will suspect something might self-oscillate. Try probing the circuit with an oscilloscope, and test for any high frequency oscillations anywhere from kHz to hundreds of MHz range.

author: Kleinstein
author_rank: Super Contributor
timestamp: October 29, 2021, 10:58:40 am
content: The lamp and 150 ohms pot are quite some load for the OP. So it is normal to have the OP get warm a bit. With around 10 mA at +-15 V this is 300 mW and enough to heat up an OP (SO8).X7R or similar capacitors for the oscillator are not a good idea. The capacitors for the oscillator should be C0G or film type. The film type caps are one of the few parts where it can still make sense to have THT parts. 1.5 K and 100 nF is quite low in impedance for a FET OP. One could go higher in impedance (like 10 n and 15 K or even 1.5 n and 100 K) and than use C0G capacitors.The amplitude control loop can in theory are oscillate, at low frequency. Not all lamps are made the same and With higher amplitude the gain for the amplitude control loop is higherst. The oscillator may get stable with a lower amplitude.Close to the negative rail the TL072 may show phase reversal and this way cause additional trouble. So the amplitude should be more like < 10 V peak.

author: 6SN7WGTB
author_rank: Regular Contributor
timestamp: October 29, 2021, 11:21:40 am
content: TL072C(will reply to other posts in a mo)

author: 6SN7WGTB
author_rank: Regular Contributor
timestamp: October 29, 2021, 11:26:47 am
content: Yes the load has occurred to me - given the original was through-hole.The current was for the whole circuit, but you're right that the lion's share must be in the WB.I need to replace the X7R - I can see this is counter-productive aside other issues.Also a very good idea re 'swapping' the relative magnitudes of the RC. In fact may have some lower value NP0...Reducing the amplitude does not make it stable - oddly the other way around - if I overdrive it to clip the 'square' wave I get appears more stable.I'm getting 24V p-p at the output of the buffer (not the WB obv) with the gain at max.Will also have a probe around as a good suggestion by @RoGeorge

author: mag_therm
author_rank: Frequent Contributor
timestamp: October 29, 2021, 03:03:53 pm
content: I use an old Heathkit IG-18 audio gen here. It has transistor Wein, decade switches to select frequency and is quite accurate in frequency, considering its age.But it has always had your problem of instability in amplitude.The feedback pot usually has to be reset each time another frequency is selected.I think the problem is that the lamp resistance and perhaps more importantly, the thermal time constantis a non linear relationship to the lamp current.Leaving the lid off for adjustment. I was thinking of re-purposing the "0nes" selection pot on the front panel to be the stability pot.Except that sometimes it is used to tune a Bass and Stratocaster.Edit Nov 6 after repairing the Heathkit IG-18 instability yesterday:The IG_18 is not a Wein, it is a split T notch across a diff amp long tail pair.I have replaced the 100 uF 30 V stability capacitor and re-adjusted according to manual procedure. It now seems to stabilize after about 2 seconds at all freq.The lamp measures about 134 Ohm cold.So Thanks 6SN7 for the thread prompting this repair.

author: mikerj
author_rank: Super Contributor
timestamp: October 29, 2021, 04:14:38 pm
content: The resistance/voltage curve for an incandescent lamp is typically steepest at low voltages relative to the lamps rating. You may be able to calm things down by putting a suitable value resistor in series with the lamp, maybe a few tens of ohms in this case.

author: RoGeorge
author_rank: Super Contributor
timestamp: October 29, 2021, 04:22:28 pm
content: Quote from: mikerj on October 29, 2021, 04:14:38 pmMeasured once the R for different miniature light bulbs, mine were looking something like this:https://www.eevblog.com/forum/projects/resistance-of-incandescent-light-bulbs/msg3720658/#msg3720658

author: 6SN7WGTB
author_rank: Regular Contributor
timestamp: October 29, 2021, 05:32:06 pm
content: Great minds - tried a 47R in series but didn't solve the issue. And if I put too much in them I suspect I'll then be into changing the trimpot as won't be able to get the (ca. 3x ?) ration of lamp to R correct.I think my immediate steps are to replace the RC with 150k/1n and use COG/NP0 caps.What I think my SMD inexperience has done is give me a dissipation that is a bit too high for the opamp in SMD, plus that's cascading to heat up the very adjacent SMD wrong-spec caps. Plus plus probably not ideal RC relative magnitudes.So, all in all, I didn't do a very good job! And it all worked so well on the breadboard...Every day is a school day, and I'm very appreciative of the most helpful replies here.

author: Renate
author_rank: Super Contributor
timestamp: October 29, 2021, 09:13:55 pm
content: Huh? Are you trying to make that light bulbglow?You should be using a 12 V grain of wheat or maybe even a higher voltage, not 6 V.

author: 6SN7WGTB
author_rank: Regular Contributor
timestamp: October 30, 2021, 08:38:24 am
content: Not sure where you got the impression the light was required to glow?It's a 6V3 15mA GoW bulb - as it has the highest cold resistance of any I have. Next was 12V 60mA. I think the original circuit uses a 12V 40mA.I see RS have 24V 20mA, which would be even better if I buy a pack of 10 bulbs.

author: RoGeorge
author_rank: Super Contributor
timestamp: October 30, 2021, 09:36:34 am
content: I think you'll need lower voltage bulbs than 24V, because when a filament is cold it acts just like a piece of wire.When cold, the filament resistance can be 10-100 times lower than when it is white-hot, so a high voltage filament will remain cold and it will be just like a constant resistor of lower values, almost like a short circuit. Only when the filament starts to turn red-hot it acts as some sort of current stabilizer, which is the desired effect for the Wien bridge. Another thing that is used in that circuit is the thermal inertia, so if the filament is mostly cold it won't stabilize too well, especially at very low frequencies.Another thing is the incandescent light bulb might be replaced with a JFET controlled by the output amplitude, so it won't need that much current when stabilizing the amplitude.

author: Kleinstein
author_rank: Super Contributor
timestamp: October 30, 2021, 09:41:51 am
content: To some degree one could use 2 light bulbs in series to get a higher resistance. The thermal intertia is wanted and needed. For the low frequency end this could be a limiting factor.The alternative with JFETs is a bit more effort, but can be more stable in the amplitude and lower power.

author: 6SN7WGTB
author_rank: Regular Contributor
timestamp: November 05, 2021, 08:36:40 pm
content: UPDATE:I now have this working and stable - thanks to all for the feedback, no pun intended.Changes made:1. 'Flipped' the 1k5 and 100n for 15k and 10n2. Have used a 12V 60mA bulb - which measured with a 'wider' transition from 'cold' to 'hot' resistance in the 0-2V region3. Reduced the gain of the buffer amp from 10 to 5 by changing the 100k feedback R to 47kThe buffer amp gain reduction was necessary as with the originally specified 10x/100k the sine wave could not be stabilised with the Wien gain adjustment to avoid clipping at high outputs.And the SMD TL072C doesn't seem to be getting hot either.Curve Tracer works very well.

author: RoGeorge
author_rank: Super Contributor
timestamp: November 05, 2021, 10:08:00 pm
content: Well done.Just curious for the filament curve trace, how long did you wait for the filament to heat at each measured point? (or what frequency, in case your curve tracer was using AC)

author: Renate
author_rank: Super Contributor
timestamp: November 05, 2021, 10:44:32 pm
content: I thought that 12V 40mA was the most standard GoW. It might pay to look for some.

author: 6SN7WGTB
author_rank: Regular Contributor
timestamp: November 06, 2021, 08:58:15 am
content: Very good questions:1. I did let it stabilise between each reading - probably 10-15s. I also took it 'back down' the curve to see if any appreciable differences - not really as the thermal inertia was evidently small;2. I used DC, but you have catalysed me to try it with 1kHz AC to match the output of the Wien is AC.