TL084: Why does my circuit oscillate with some 4MHz?

Hi Roger,

I would place a 100R resistor between the output of OPAmp and the base of transistor. Then you can mount a phase lead capacitor of some dozens of pF from the output of OPAmp to its -input. This can help to restore the phase margin.

Have you already simulated your circuit with TINA-TI? You could run a phase stability analysis. See these training videos:

training.ti.com/ti-precision-labs-op-amps-stability-3

Kai

Roger,

Regarding: So why does it oscillate with Rig of 1 Ohms and not with Rig of 1k?
The collector load affects the feedback loop and operating point of the transistors. TL084 is unity gain stable, but placing anything inside the feedback loop requires some frequency compensation for a robust solution. Kia's advice is a great place to start.

Hi kai!

I thank you very much for your clever tip, I was not in the vicinyty to think that way!

My problem now is that PCB is already finished and sitting there but here is the good thing, I always use sockets so I can remove the OP, bend up the legs in discussion and force a resistor+cap assembly!

I will do this in spite of the fact that two TL084 packages will have to be modified.

But this inspires me!

I do however wish that I understood what you are talking about regarding phase margin, I know what it means but I do not know how it works (BF>1@180deg is the oscillation criterium, right?)

Best regards, Roger

Roger,

Here is something practical to try, an add a snubber (pole/zero) from op amp output to ground. Start with 47 ohm and 0.33uF in seres (OK to try other values as I didn't spend a lot of time selecting these values). It is practical because you don't have to lift any pins , just tack on two components. The basic idea is to drop op amp gain over frequency twice as fast (pole) and then back off (zero) before getting to unity gain. It also lowers the impedance at the output.

Hi Roger,

you might want to have a look at this thread where we are discussing something very similar:

e2e.ti.com/.../2878407

Kai

Thank you Ronald for your tip!

This procedure of course make it much easier for me.

It makes me able to not even have to remove the OPs but to solder the snubber on outputs to ground.

Today I prepared for lifting pins and using component adaptors but thanks to you I seem to not have to.

However, I really wished I understood what you say about zero and pole, I actually know what these things are (a pole makes the gain drop and a zero makes the gain increase) but I have not come to the competence like yours such that I understand how to use them, often I do not even understand in which direction phase is "moving".

If I use my incompetence I would say that your snubber loads the OP very little at frequencies slightly above DC and 47Ohm at HF, but how this can stabilize my OP is an enigma to me.

I will try this snubber asap, thank you very much for making it more simple for me!

Best regards, Roger
PS
I pushed the "This resolved my issue"-button yesterday because I thought the tip from Kai  was very good but then I kind of regret "closing" the tread because I wish to show you my progress. Today when I see you guys talking to me in spite of this I got glad because perhaps I may show you my progress at a later date.

Hi Roger,

the snubber is also called Boucherot-network or Zobel-network. The basic idea of such a RC snubber is very simple: At high frequencies C is a short circuit and only R is appears to be present at the output. The unavoidable output resistance of every amplifier, the so called open loop output impedance and the R of snubber create a voltage divider, decreasing the amount of output voltage fed back to the -input of amplifier. By this it becomes less likely that the gain condition for maintaining oscillation (gain >=1) is fullfilled.

A disadvantage of the snubber is that high load currents can flow, if the wished signal contains lots of high frequencies. Also, the snubber must be matched to the amplifier. A power amplifier which has a very low open loop output impedance needs a much lower ohmic snubber R to create enough voltage division compared to a amplifier which has a high open loop output impendance. On the other hand, the snubber can be a very effective method for increasing the stability of an amplifier. Almost all analog audio amplifiers for driving loudspeakers have such a RC snubber at the output.

Here are some examples discussing the RC snubber:

e2e.ti.com/.../730472
e2e.ti.com/.../699771

Kai

Hi Kai!

I liked this explanation very much, it almost made me fully undrestand.
Actually, I used to think that an OP output is some 100 Ohms open-looped but gets very small when looped (due to voltage/parallell feedback).

I had some other stability problems too, a complementary power buffer (one polarity at the time, same project) oscillated but was cured by 100nF directly over output.

I have tried this with my latest oscillation problem but it has not worked, at one time I began with 100nF (Ker) and when it didn't work I increased to 1uF (polyprop) and when that didn't work i increased to 10uF (electrolythic) in pure desperation but of course it did not work.

I actually put these capacitors directly over the minus input of TL084:b to ground, thinking something like a 78-series regulator needs (or recommends to have) a 100nF over the output.

But it did not work and before your kind tips I was prepared to give up.

I will however try this snubber tonight, just stand by for results...

It did not work but I have only tried on one of the opamp and the oscialltions got better.

I recon I have to snubber both opamps to make it work, anyway here is what I tried: 100+100nF at output of TL084:b, when this didn't work I tried 10+100nF but this didn't work either exept that the amplitude got a little less.

Attaching two photos, the first is before any snubber and the next is with 10 Ohms/100nF.

Reading on oscilloscope shoud be a straight line at third DIV (1us, 1V/DIV@CH2 [10X Tek-probes is being used]).

Best regards, Roger
PS
For this polarity, the frequency is down to some 700kHz.

Hi Roger,

can you show us a complete schematic or your circuit?

Kai