help with diode load

[rachelgl]

Hi,

In the attached circuit, i have 2 diodes used for isolating the charge on the capacitor C2 which will be a mechanically variable capacitor in reality. However, the issue I am having is that at the point "test", the input AC is not being converted to DC. If I replace the ZHCS350 diodes with virtual diodes and chage the junction voltage to 0.2V, the circuit works as i'd expect. Once I use non-ideal diodes though, i just get the same AC signal at "test" as I do at "V_in". I'm guessing this has to do with the fact that I'm using a capacitive load and the diode resistance is not high enough compared to it but I dont really know what to do to fix this.

Any help is greatly appreciated.

[JasonF]

You will want to be using larger capacitors. (2uF should be sufficient)

Because you are working at low frequencies your capacitors are not holding a large enough charge to carry from one cycle to the next. 
With your ideal diode, the capacitors are unable to discharge during the off cycle, once you add a real diode they discharge through the parasitic resistances.

[rachelgl]

Thanks for the reply!

I'm a little stuck for what to do though because my variable capacitor will be varing between say 0.3-1.5pF and adding a capacitor in parallel isn't really an option for what i'm trying to design. Can I perhaps limit the charge into the capacitor by maybe adding some resistor/inductor to increase the impeadance?