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THS4271: Driving a high frequency ac ripple through an LC bias tee

Anoop Joshi
Expert 1125 points
Part Number: THS4271

Hi,

I was considering a design as shown below where the THS4271 amp drives the AC input (80MHz, 50mVpp sine wave) to an LC bias tee composed of a 2.2uH inductor and 100nF capacitor. DC is assumed to be driven by an ideal DC power supply via the inductor, and AC by the THS4271 amp via the capacitor.

The output of the bias tee (ie the AC+DC summed output) is connected to a time variable resistor represented by "VCR_1K" below. The resistance profile of this variable resistor is assumed to vary sinusoidally at 500MHz with a peak to peak value of 6 ohmspk-pk and average value=9ohms.

Can the THS4271 amp be expected to drive this AC load successfully resulting in say at least 40mVpp ripple amplitude at the bias tee output, especially considering the fast transient current requirements posed by the sinusoidally varying variable load?

NOTE: The end application intended is a power supply ripple rejection test circuitry, wherein the DC supply with AC ripple is to be applied at the DUT supply. The voltage variable resistance is intended to mimic the DC and transient current requirements of the DUT. Normally we use standard 50E output resistance signal sources to drive the AC input of the bias tee, but with fast switching transient current requirements of the DUT it poses a lot of limitations in satisfying the transient current requirements especially because we remove the DUT supply decaps for this test.

The intent here is to attempt to use a buffer stage to do away with the 50E output resistance and hopefully be able to supply the fast transient current requirements better with the lower output impedance of the buffer.

Thanks,

Anoop

  • 0
    TI__Genius 15975 points

    Hi Anoop,

    If the intended application is to get 40mVpp ripple amplitude onto a variable resistor with 3 to 15-ohms, then the THS4271 should be able to achieve it. I think the key care-about for selecting a low impedance output buffer for PSRR application is to make sure the buffer is able to source or sink current as required by the DUT. As long as the DUT does not consume more than 160mA during the positive half of 40mV and 80 mA during the negative half of 40mV from the THS4271, you should be ok.

    Best Regards,

    Rohit

  • 0 in reply to Rohit Bhat
    Expert 1125 points
    Hi Rohit,

    Thanks for the details. Is the fact that the amp is driving the ripple to the DUT through a 100nF cap as shown in the circuit diagram I had attached in the original post a cause for concern?

    Thanks,
    Anoop
  • 0 in reply to Anoop Joshi
    TI__Genius 15975 points
    Hi Anoop,

    The 100nF cap is ac coupled at the amplifier output and should not be a concern for resistance higher than 5-ohms. But, I believe the amplifier could become unstable if the resistance gets lower than 5-ohms, because the 100nF cap would look like it is directly tied to GND at the THS4271 output.

    So, would it be possible for you to add a 5 to 10-ohm series resistor in-between the THS4271 output and 100nF cap, and instead increase the input swing at the THS4271 to make the PSRR measurement?

    Best Regards,
    Rohit
  • 0 in reply to Rohit Bhat
    Expert 1125 points
    Hi Rohit,

    I will incorporate the option for that, thanks

    Thanks,
    Anoop