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Original question:

THS3491: unity-gain stable

THS3491: Minimum value of feedback resistor

Christian Budnik
Prodigy 10 points
Part Number: THS3491
Other Parts Discussed in Thread: LMH6703

I use the THS3491 on the ti demo board in a noninverting amplifier configuration. The gain is set to 6dB (2V/V).

In order to get the lowest possible noise figure my attempt is to lower the feedback resistor value.

It seems that at least on the demo board there is a limit of about 470Ohm for Rf (R4). If Rf is lower than it starts to oscillate at around 250MHz.

Is it possible to overcome that behaviour and gain somehow phase margin? Are there other better siuted part.

My requirements are:

Gain = 2V/V = 6dB

Uout = 4Vpp at 100Ohm

Noninverting amp.

3dB bandwidth 200MHz

Noise figure of the amplifier = 18dB

  • 0
    TI__Expert 6050 points

    Hi Christian,

    Unfortunately, lowering the resistor values to as low as 470 ohms will likely cause instability as the phase margin is below 45 degrees. With current feedback amplifiers, lowering the Rf and Rg values will cause a large gain peaking in the frequency response which will lead to instability. Also, there is no way to improve the phase margin without significantly reducing the bandwidth which affects another one of your application restraints. 

    Can you share how you are measuring the NF of the amplifier?

    I would recommend looking into the LMH6703. The datasheet recommends using Rf of 560 ohms but it may be possible to lower the Rf and still have a phase margin larger than 45 degrees based on a few simulations I have put together, which I have attached below. Also, I used the following resource (link) to calculate the NF. Using an Rf of 300 ohms for the LMH6703, the NF should be about 18dB. 

    LMH6703_stability_E2E.TSC

    Please let me know if you have any questions.

    Thanks,

    Nick

  • 0 in reply to Nick Nauman
    Prodigy 10 points

    Hi Nick,

    i measure the NF with the r&s fsw noisefigure option and an external r&s noise source.

    Best regards

    Christian