THS3491 Amplifier Instability Issues

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The discussion revolves around amplifying a -6 dBm 75MHz signal to at least 31 dBm using THS3491 evaluation boards. The user removed an attenuation circuit but encountered instability when attempting to increase the gain beyond 10 by adjusting feedback resistors. A second approach using two amplifiers in series yielded lower than expected output (23 dBm instead of 26 dBm) with irregular waveform shapes observed on the oscilloscope. Questions were raised regarding supply voltages and physical layout, indicating potential factors contributing to the instability. The user seeks guidance on their approach and understanding of amplifier behavior.
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Trying to amplify a -6 dBm 75MHz signal but encountering amplifier stability issues
Hello all,
I'm currently trying to amplify a -6 dBm 75MHz signal to at least 31 dBm by using THS3491 evaluation boards. The boards came with an attenuation circuit which I have removed, currently there is only a 50 ohm resistor in series with the output (output-6 dBm). Initially our approach was to tweak the gain by changing the feedback resistors RF/RG but the amplifier seems to become unstable at a gain over 10. Is this normal behavior?

Our second approach was to use two gain amplifiers in series (one 8x with no output resistor into a 10x with a 50 ohm resistor). We received somewhat promising results. We expected 26 dBm output but only measured 23 dBm and the oscilloscope sine wave reading has an irregular shape. *Hopefully I can attach some images*.

I have limited experience with amplifiers, am i approaching the issue wrong?
 
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Welcome to PF.

Aryia said:
to at least 31 dBm
And what output voltage does that correspond to?

What are your supply voltages and how are you laying this out physically?

TI datasheet attached for reference...

EDIT -- the datasheet is too large to attach. Let me find a link...

https://www.ti.com/lit/gpn/ths3491
 

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