# LTspice gain enhancement sallen and key highpass simulation

I have been trying to simulated a filter and amplification circuit. In one of the stages I am using a LP Sallen and Key circuit and another I am using a HP Sallen and Key circuit. I can successfully get the shape of the filter in LTspice AC analysis with 307kHz being my center frequency, but when I add gain enhancement the shape completely goes away. The first image is my calculations for my component values. The first image below is my LP filter circuit and the resulting AC analysis. The second image is my circuit when I add gain enhancement and the resulting AC analysis that exits. My calculations are at the bottom.

I am hoping to get the same or similiar shape to the filter without the gain enhancement, but would just like a higher gain. Looking at the datasheet of the LT6233 the gain bandwidth is 320MHz. Does anybody know what I might be doing wrong?

simulate this circuit – Schematic created using CircuitLab

With Q=1.5, ωo=307000*2π and H=20 we can find the component values for each:

C3=(2*Q/ωo)*((H-1)/H)=1.477*10-6
C2=1/(2*Q* ωo)=1.728(10-7)
C4=2*Q/ ωo=1.555(10-7) R3=R4=1 R1=R3*H/(H-1)=1.0526 R2=R3H=20

Then, normalizing each component to C4=1nF, we get: C4n=C4*(1nF)/C4=1nF C2n=1.111(10-10)≈0.1nF C3n=9.5nF≈0.01µF R3n=R4n=R3*C4/(1nF)=1555Ω≈1.6kΩ R1=1637Ω≈1.6kΩ R2=31.1kΩ≈30kΩ

I am not 100% confident in the source I got these notes from and after simulations I found that I needed to cut out C3

• Apologies, the first image is not showing up on its own, so you have to click to see it. Commented Aug 10, 2017 at 19:34
• Provide the details about how you calculated your component values. Also, I recollect gain and damping are coupled in the Sallen-Key.
– jonk
Commented Aug 10, 2017 at 19:50
• Ah, in you saying that I found where my error was for the high pass Sallen and Key. Thank you very much. Let me re-post with my low pass Sallen and Key and I'll post my calculations too. Commented Aug 10, 2017 at 19:58