How did you simulate it ?
Did you check the ports for corresponding impedance, so that you have a correct input ?

Hi, guys

Thanks for the kindly help.

I have ideal vsin source from analoglib. how come the impedance of the port ?


James

Hi James,

The passband gain of a ladder filter designed directly as per the textbook recipe should always
be -6 dB. This is owing to the fact that the ladder filter is generally designed as a doubly-terminated
reactance network. Essentially, you will have an Rin and an Rload in the network, and if you have
normalized them to the same value, you can see for yourself that the voltage gain cannot exceed
1/2 in the passband.

As for your not seeing any ripple in the passband, I would have been happier if there was none. Probably has
to do with the choice of your filter type and coefficients.

In order to increase the gain, you will need to either add an explicit gain stage, or make some modifications,
typically adding some extra branches etc. However, I am not so familiar with ladder filters as to be able to
tell you exactly what should be done, and what the impact on the final circuit performance will be.

Perhaps someone else can.

Regards
Vivek

Hi,

this is the schematic, the attenuation in the above pic in the passband is -6, you can hardly see it . Sorry for that.

Hi,

Yes, you are right, I also worked it out after I post it.  

If this circuit is transformed to active filter with leapfrog technique, it has the same attenuation. Then there are two ways to compensate the attenuation. First is to add a new linear stage with 6db gain, the other way is to adjust the resistor values in the filter, if so, then I need transfunction of this n order filter. Does anyone know active leapfrog filter n order transfunction H(s) ?