Hi there Jim,
Thanks for the reply.
Yes, the vertical scale is simply the output voltage amplitude with 1vac input. You can use RMS or Peak values.
It was calculated using Nodal analysis where we sum the currents into each node and then solve the resulting set of equations simultaneously. This is done in the Laplace domain, and then later the variable 's' is replaced with 'jw' and then the magnitude is calculated by taking the square root of the sum of the imaginary part squared and the real part squared. This is all done in an algebraic program that can handle most of the algebra so all i have to do is present it with the set of equations and go from there.
In the Laplace domain the equation simplifies to 6th order which means some of the components might be redundant.
Two of the caps you mention are in series with the filter, so there is no way we will see DC being passed as in a normal LPF. With a normal LPF the DC offset would also be passed. However, if the application never has to deal with the DC offset or there is none, then in the application it would be considered to be a LPF anyway rather than a BP filter.
I will remove the three caps you talk about and see what happens. Once we remove the two series caps i would expect to see at least some DC being passed because then there is no DC blocking cap to stop it
I'll try to get back here later tonight or tomorrow sometime with the new graph.
BTW the equation for Vout is somewhat complicated until we plug in the values then it comes out simpler. It turned out to be first degree in the numerator and 6th degree in the denominator.
[LATER]
Here's the new plot with those three caps removed and R11=100 ohms.
Definitely LP now
With 1vdc input now we see 0.5 vdc output.
In case you are interested, here is the transfer function in one form for that setup:
Vout/Vin=1.0e2/(4.2336e-50*s^5+1.2096e-39*s^4+8.256e-29*s^3+1.392e-18*s^2+3.38e-8*s+2.0e2)
and we see now it is down to 5th order.