OPA4187: closed loop gain

Hi Hiroyuki,

there are several issues:

R2 and C1 form a high pass filter which heavily dampens 8kHz. To transmit 8kHz you would need to change this filter.

Also, 330R presents a very low ohmic load which can make issues to the driver.

Even when improving this high pass filter issue, you will notice that the OPA4187 is still not able to amplify 8kHz with the full gain. The reason can be found in the open loop gain plot:

From this plot you can see that the open loop gain - which is the maximum gain the OPA4187 can offer - is nearly equal to the gain you have set. This means that you don't have any linearizing gain reserve at 8kHz and that the OPA4187 begins to break down in the frequency response.

You will have a better chance to get what you want when splitting the gain equally onto two gain stages. This will decrease the gain per stage and increase the gain reserve:

hiroyuki_opa4187.TSC

Alternatively, you could take a much faster OPAmp. Then doing that with a single gain stage would be possible.

Kai