30 MHz amplifier to drive isolation transformer

Hi,

Do you happen to have a specific part number for the transformer? Additionally, are you actually loading the secondary of your transformer? If not, you're only presenting the magnetizing inductance to the two amplifiers. In that case, you have setup an LC resonator tank with the 22pF caps.

Additionally, how much signal swing are you intending to get out of the amplifiers? Supposing that each could hit the full rail, you would present a differential voltage of 10V into 100ohm (assuming terminated output of the secondary) results in a peak current of 100mA. This exceeds the linear output current for the parts (though it is less than short-circuit peak current).

I would like to suggest to try using a 50ohm resistor where the primary winding is now (removing the transformer) and see how well the design behaves. If things begin to behave, then that would lead me to place further scrutiny on the transformer portion of the design.

Cheers,

-David

David,

Thanks for your answer, and sorry for slow reaction - I have been very busy on that circuit.

I haven't been accurate in my description, true, I am indeed loading the transformer output with 50 Ohm. And yes, even before plugging the transformer, I had a first try with a single 100 Ohm load instead of primary. That was definitely unstable at gain 25, which is not quite I would have expected from the specifications. On input, there is a 50 Ohm impedance too. The level was rather low, like 20 mVrms in, which is 500 mV out, far from rails.

Anyway, I have noticed some needed changes that improve the behavior :

- bias current is huge (11 uA) but quite nicely matched (offset typ 20 nA). I do not care about small DC offsets, so did not bother about impedance matching - how wrong ! It seems to be very touchy on this point, and matching both inputs of the opamp strongly reduced the observed oscillation...

- even with matched input impedance, and with a resistive load only, I haven't been able to get stable output whatever I put in the feedback. So I had to add a small RC pole on the load (like 10Ohm 10pF in series). I am a bit surprised, as this is never mentioned anywhere in datasheet, nor from simulations.

I also gave up with differential input configuration, and turned back to single-ended with a single amplifier - not absolutely sure it was needed, but I considered I had to go back to simpler configuration at some point, to have it under control.

With a small capacitive load and matched inputs, I get decent behavior from gain 10 to 25. But I still have some less than optimal behavior just below cutoff, and have the feeling it could still be improved. I am a bit disappointed too about PSRR, I have pairs of bypass caps (100nF 0805 and 10nF 0603), still I keep several mV on output ripple that come from my supply ripple of 20mV at 1MHz - where has the expected 60dB PSRR (for 1 MHz) gone ?

Anyway, if anyone has some experience to share with this opamp I would be glad to here it, in the meantime I will satisfy myself with what I got...

Thank you Harry, this is just the kind of answer I was sort of expecting.... I had the feeling this differential architecture was ill-formed, but could not understand exactly how/where/why !

Is this what you are suggesting (See PNG) ? I have just copy-pasted nearby components, the 10Ohm and 4.7pF values are random and probably have to be tuned - right ? Would this remove the need for the capacitive load ? However I have found a small capacitive load to be necessary even with a very basic single ended / single opamp design (Rf // Cf and Rg only) - is that expected ?

My PCB has 2 layers and already tons of modifications made in last couple of days , I am already thinking about version 2.

Thanks for other post on PSRR too, I'll have a careful look...