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Weird Amplifier Problem

Jay9848
Prodigy 10 points
Other Parts Discussed in Thread: THS3001

Hi,

I am building a simple three stage amplifier with the THS3001 with passive RC/CR filter in between each amplifier. I have the board running except that when I turn off the input I notice that the board is amplifying a strange signal at 3-5MHz. I've tracked down the problem to the power supply pins of the THS3001 which somehow has a signal of 5MHz going in. I also double checked the power supply and I am pretty sure the power supply is not the culprit. I currently have a 0.1uF cap connected to the power supply and ground. Is this the culprit? If so will adding a 6.8uF cap fix this problem? Thanks.

  • TI__Expert 4115 points

    Jay,

    Usually for oscillation, we look at loop gain problems that will happen near the bandwidth of the amplier; so roughly 200MHz to 400MHz for the THS3001, or single transistor oscillations that happen near the ft of the transistors, which is about 1GHz or a bit more for the process being used. Given the frequency it should not be either of these and there could be some power supply interaction. Decoupling is always needed for op amps.  0.1uF on each power pin to ground as near as possible to each device and a larger bulk capacitor like 6.8uF within an inch or so that can be shared between multiple devices is our typical recommendation and should be sufficient. Aside from that, I have no recommendation other that you will need to trouble shoot the circuit to try to isolate the problem.

     

    Respectfully,

    Jim