OPA855: A high pass filter

Hi Mojtaba,

Can you share what Sallen-Key configuration you used and what the issues you are facing? There is a design guide for a 2nd order Sallen-Key HPF that walks through the design steps (found here). One note to keep in mind, the OPA855 is a decompensated amplifier and needs a gain of 7V/V or greater in order to be stable. What gain do you need for this stage of your application?

Thanks,

Nick

Hi Nick,
Thank you for your reply and the link. Here is the Sallen-key HPF, I tried to set the gain to 10 by Ra and Rb but it only attenuates! Also the cutoff frequency is not as expected 100MHz.

My goal is to achive a pre-amphasis circuit by tuning the values of the following schematic to amplify the frequencies over 45 MHz to at least 100MHz.

Sallen-Key.TSC

Hi Mojtaba,

I am looking into why you are seeing attenuation for your circuit configuration. If I change the magnitude of the values in the first circuit configuration you show, the dB gain increases. Please allow me some time to explore this further and find a solution that will work for your application. For clarification, are you looking for a high pass filter that has a cutoff frequency of 45 MHz or are you looking for a bandpass filter with a passband range of 45MHz to 100MHz?

Thanks,

Nick

Hi Nick,

Thank you so much for your time and reply.

Actually, I want to design a pre-amphasis circuit which is similar to a high pass filter. I would like to have unity gain (or a low gain) up to 45 MHz, then it starts amplifying at leat up to 100MHz. Frequencies beyond 100 MHz are not a concern due to the 100 MHz input bandwidth. So both options could work for me.

Hi Mojtaba,

I am still looking into your circuit. I have found this IEEE article that has a reference of a pre-emphasis circuit (link) and I have created a simulation based on this design. I am still trying to tune the passive component values to meet your frequency requirements. I have attached the simulation file so you can look into this design as well.

OPA855_Preemphasis_E2E.TSC

Thanks,

Nick

Hi Nick,

Thank you once agian for your time and for providing the design.

I am facing challenges in achieving the required frequency values as well. How did you achieve these values in the given desing? I believe the single resistor defines the DC and low-frequency gain, while the RC will set the higher frequency gain. Is it correct that the 3dB frequency should be calculated as 1/(2PiR*C)?

It seems simple but it does not work as expected 

Thank you

Hi Mojtaba,

I am unable to share any collateral on equations used to determine the R and C values as any collateral on pre-emphasis circuits are very limited. I have not calculated the exact transfer function, but there are three parallel paths so it is not as simple as the equation (1/2*Pi*R*C). My recommendation is to tune the values in simulation to match the frequency response you are trying to attain. 

Thanks,

Nick