Issue with AD8039 Producing 5MHz Triangular Wave

Hi,

How does your hardware look like? Could you provide a photo? I guess on the scope the output of U1 can be seen. What is the waveform at the output of U2?

Zoltan

Hello ,

Yeah on the scope its output of U1 . The output of U2 is shown in yellow wave form  . The second image is hardware schematic.

Thanks

Hello Zoltan.

Is this a problem with biasing or any other? Thanks in advance 

Farhan

Hi,

I guess you are speaking about the output of IC3 pin 1.

I don't really understand, what is the feedback voltage. Do you apply it at X4? Why do you use this differential amplifier and feedback voltage, what is their role in your circuit?

Zoltan

Hello.

Sorry for the confusion earlier; you are correct, the output is at IC3, pin 1. I apply DC voltages ranging from +4V to -4V at X4 to attenuate the amplitude of the input signal. Normally, we set feedback voltages between 0V and +4V to adjust the input signal's amplitude.

This circuit is a class D amplifier with feedback derived from the output. My load is a transducer, and since we use multiple transducers with different resonance frequencies, the output amplitude varies. The feedback loop detects the signal peak, converts it to a DC voltage, and this DC voltage is controlled by a trimmer and fed back to the input. This is my feedback loop diagram.

Thanks.PDF

Hi,

OK, I got it, you use it as an analog multiplier. If you apply positive voltage on X4, then the transistors' base-emitter junctions are reverse biased, thefore the do not work in the active mode. This analog multiplier only works properly for negative emitter voltages, when the base-emitter junctions of both transistors are forward biased.

Zoltan

When X4 voltages are positive i get this waveform. but its shifted. Do i have to adjust R13 or i have to change base resistance. Can you guide me about Basically, I am using this stage to attenuate the amplitude of input signal. Thanks

Hi,

I can only recommend what I've said: keep the base emitter junctions forward biased all the time, so use negative voltage on the emitters. R13 does not affect this. 

Zoltan

I keep the base emitter junction forward biased but still i have phase shift .any idea why its like that. Thanks

What is the voltage on X4? If it is a negative DC voltage (say, less than -1V), you should not get phase shift.

Make a simulation of that part of your circuit in LTSpice to see what is going on.

Just for clarification R14 is connected -5v as well.I also try to give less then -1 volt at X4 but if voltages goes to negative side its no longer sine wave as shown in figure.

Hi,

The amplitude of the input signal is too large. The transistor diffamp only works fine, if the differential voltage is not much larger than  the thermal voltage (about 26mV). I think you should learn more about this kind of circuit, otherwise you'll have difficulties to use it. I recommend to take your time to do so, play with the simulations, try to find alternatives to impement what you want. There are other options to tune the amplitude. You can also use analog multiplier ICs, like the AD633. Give it a try.

Good luck!

Zoltan 

Hi,

The amplitude of the input signal is too large. The transistor diffamp only works fine, if the differential voltage is not much larger than  the thermal voltage (about 26mV). I think you should learn more about this kind of circuit, otherwise you'll have difficulties to use it. I recommend to take your time to do so, play with the simulations, try to find alternatives to impement what you want. There are other options to tune the amplitude. You can also use analog multiplier ICs, like the AD633. Give it a try.

Good luck!

Zoltan