OTA based circuit problems

[Fluffyboii]

I am once again here because I build another lm13700 based circuit and it is once again not working.

Ignore the 6V peak to peak warning at input, description says it should work up to 10V peak to peak.
I am feeding it with around +/-4.6V square wave. I did try lowering it, it doesn't matter.

At the node 13 I see same square wave around 1V peak to peak. The weird thing is at the output of IC1-D (node 14) there is a positive voltage offset of around 10V and same 1V peak to peak signal on top of it. I know that it must be coming from either Q1 or IC1-B. Since turning resonance pot thats connected to Q1 doesn't change anything it must be from IC1-B instead. And indeed I read around -10.3V at IC1B output.
The CV input side is almost identical with the last circuit I build and it gets -10.7V on the node 5 and 16 (Amp bias inputs of OTA) I think it is working fine.
I obviously checked the IC1-B and 1C1-C connections because both of them seem to have that negative voltage at the output. IC1-C seems to not have the same negative ofset though.

I did felt that the op amp was getting a bit warm. Not like it is shorting and magic smoke will come out kind of warm but I can feel it getting slightly warmer which makes me think there is something pulling one of the outputs low. I checked all of its connections with my multimeter and measured the resistances. I also obviously tried with different Op Amps and LM13700s. I tried using my multimeter to measure more voltages instead of using oscilloscope but it gave me illogical results like it measured +/-12V at my power supply output but measured -13.5 +9.5V at circuit power terminal. I think it had bad batteries but changing the batts did not change anything. Yesterday it wasn't like this. The whole circuit is acting a bit different than what I observed yesterday so Op Amp may be already damaged.

I will write OTA based problems here to not open more forum pages about similar topics, I am sorry for the trouble. OTA based circuits seem to be pain to get working right.

 

[Fluffyboii]

I also put this on breadboard since first circuit failed to work and I don't want to check it for the billionth time now. LM13700 gets too hot to touch in 2 seconds on this one.

 

[rjenkinsgb]

Have you included the buffer amps? The two triangle symbols, one following each OTA?

I'd guess they are supposed to be non-inverting buffers, so input signal to +in and output connected back to -in..

 

[Fluffyboii]

Have you included the buffer amps? The two triangle symbols, one following each OTA?

I'd guess they are supposed to be non-inverting buffers, so input signal to +in and output connected back to -in..

I think those are the darlington transistor pairs of the LM13700.
In the comments of that circuits instructable: https://www.instructables.com/Diode-Ladder-VCF-With-NO-PCB/#discuss
There is this schematic:

And the person who draw this schematic from the other image says his LM13700s are keep burning after overheating like mine.
Any idea why the first circuit I built on perf board is not working.

 

[rjenkinsgb]

OK, that makes sense.

I think the 13700 is a "current input" device. It may need resistors in series from the diode ladder to the top 13700 inputs, to regulate the current? Try 10K and see if that stops the overheating?

The only thing I can think of on the other circuit is check across each opamp input to see if any are shorted with power off, and verify the are all the right way around??

 

[danadak]

No its a transconductance amp, V to I. The input diodes are linearizing diodes.

Regards, Dana.

 

[Fluffyboii]

No its a transconductance amp, V to I. The input diodes are linearizing diodes.

View attachment 138543


Regards, Dana.

Does that mean the drawing is correct.

 

[danadak]

The OTA output, a current is converted back to V with a resistor.

The schematic shows a buffer, the internal Darlington connected emitter follower
output inside LM13700 used for buffering.

So yes, schematic overall looks OK. Note pin 9 is terminated thru R to -12V, But hard
to read on schematic so would have been easy to connect it to +12 where it
would not function as a emitter follower buffer. I had to mag up the pic and still
was not sure what 12V author was going for.


Regards, Dana.

 

[rjenkinsgb]

OK, I was remembering all the datasheet examples that have the inputs connected to an offset pot to ground.

The circuit is still wrong / badly designed though, as from the datasheet the IC is not guaranteed to work with the input within 3V of a supply, and typical limit is within 1.5V (min +/- 12V and typical +/- 13.5V when using +/- 15V supplies).

Two diode drops, so around 1.2V below V+, is not enough for guaranteed proper operation. Adding another two diodes in series to where the 12V connection is shown at the top of the diode ladder would help, but three would be needed to strictly stay within datasheet specs.

I can't see that causing the device to get hot though, just possibly not work as it should..


I'd double check all the pin voltages with the IC unplugged & make sure they match what they should be from the schematic - nothing showing much voltage other than power pins and the diode connections?

 

[Fluffyboii]

OK, I was remembering all the datasheet examples that have the inputs connected to an offset pot to ground.

The circuit is still wrong / badly designed though, as from the datasheet the IC is not guaranteed to work with the input within 3V of a supply, and typical limit is within 1.5V (min +/- 12V and typical +/- 13.5V when using +/- 15V supplies).

Two diode drops, so around 1.2V below V+, is not enough for guaranteed proper operation. Adding another two diodes in series to where the 12V connection is shown at the top of the diode ladder would help, but three would be needed to strictly stay within datasheet specs.

I can't see that causing the device to get hot though, just possibly not work as it should..


I'd double check all the pin voltages with the IC unplugged & make sure they match what they should be from the schematic - nothing showing much voltage other than power pins and the diode connections?

I got that second circuit working on perf board. I think I installed one of the transistors under the diode ladder in reverse and that killed one of my LM13700s. The only problematic part is resonance. It seems to do something and increase the magnitude of the output signal and slightly change its shape when I turn the pot right side but unless it is at utmost left side the effect is same and it doesn't change with pots movement.

 

[Fluffyboii]

Update on the first circuit. It turns out that the 2n3904 was placed backwards. I fixed that up and checked the circuit again. While I was observing that this did not change anything at all the circuit started self oscillating randomly by itself. I knew that it was from the feedback loop and I was able to adjust it from 40Khz up to 200Khz until it get lower in volume and died. This oscillation was visible at all outputs. I tried giving the CV a voltage from outside and it did nothing. CV off set pot also seemed to do nothing so it is not very suprising. But the oscillation stopped by itself at some point and it turned into its original state. So I get +/-30mV at HP side and at others there is 5mV something oscillating but the frequency is higher than input. From this I can tell that my first assumption of CV side working fine and problem being elsewhere was wrong.

 

[Fluffyboii]

No its a transconductance amp, V to I. The input diodes are linearizing diodes.

View attachment 138543


Regards, Dana.

The problem with the first circuit is caused by the part that drives the Amp Bias of the LM13700. Transistor log converter thing is just super unreliable. Maybe it needs transistor pairing but I don't know how to do that with PNP and NPN transistor pairs. I used my tester device to measure Beta values and Base Emitter voltage difference value to pick the closest ones yet it failed.