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OPA847 output impedance for cascade

Merlin Cooper68095
Prodigy 10 points
Other Parts Discussed in Thread: OPA847

Dear all,

 

I wish to receive a clarification of the paragraph on page 10 of the OPA847 datasheet which states:

 

"If the signal source is actually the low impedance output of

another amplifier, RG should be increased to be greater than

the minimum value allowed at the output for that amplifier

and RF adjusted to get the desired gain. It is critical for stable

operation of the OPA847 that this driving amplifier show a

very low output impedance through frequencies exceeding

the expected closed-loop bandwidth for the OPA847."

 

My circuit takes the output of a low-gain transimpedance stage based on the OPA847 and amplifies the voltage with a gain of -40V/V in a second inverting gain stage also using an OPA847. What exactly would be a large enough value of RG to place at the output of the first OPA847? I was thinking RG= 50R with a feedback of 2K in the inverting gain stage to give -40V/V. However, this has not proved to operate well in practice.

 

Thanks.

 

 

  • 0
    TI__Genius 17905 points

    Merlin,

    The first stage transimpedance amplifier is basically seeing a load equal to the Rg of the second amplifier, since the inverting pin is a virtual ground. The key datasheet graphs to consider when choosing the load (Rg of second amp) are the Harmonic Distortion (HD) versus Load graphs at the top of p. 5 of the current datasheet (Rev December 2008) and the Output Voltage/Current Limitations graph on p. 7. The HD versus Load graphs show that the harmonic distortion, in particular, second harmonic, improves as the load resistance increases. These graphs should help you choose a load (Rg) value in line with your target HD performance. The Output Voltage/Current Limitations graph shows that too low a resistance will limit the output swing due to the limited output current sourcing and sinking ability of the amplifier. To avoid the output voltage swing being limited by the output current, a load of at least ~100ohm should be used. Once you choose the Rg value for the second amplifier, you will have to scale the Rf resistor value to get the desired gain.