LT spice help..difference between observations and results..

[himanshul]

Hello guys,

I have simulated two circuits, one is an RC filter and other is a Capacitance multiplier.
My aim of experiment is to validate a decrease in Iout (load current) of CM circuit as compared to load current in RC filter circuit.

Theoretically,
Iout (CM circuit) < Iout (RC filter)
Vp-p (CM circuit) < Vp-p (RC filter)

My results:
1.RC filter
Vout (dc) = 0.0458716 ( .op 300ms)
Iout (dc) = 0.00458716

Vout (ac) = 43.7 ~ 48 mV (.tran 300ms)
Iout (ac) = 4.80 ~ 4.37 mA

Vp-p (ripple) = 0.00437018 (.meas)

2.CM circuit

Vout (dc) = 0.323751
Iout (dc) = 0.0323751
Vout (ac) = 322 ~ 327 mV
Iout (ac) = 32.65 ~ 32.15 ma

Vp-p (ripple) = 0.00521016 (.meas)

From the above results its quite clear that implementing CM circuit causes Iout to increase which particularly against the aim of the CM circuit. As the CM circuit works by decreasing the load current which according to load POV can be visualised as increase in capacitance.
We can also see the peak to peak ripple voltage in both the circuits, Vp-p for CM circuit should be less than Vp-p ripple of an RC filter, but again the simulations results are quite opposite.

What's wrong here??

 

[KlausST]

Hi,

in my eyes in both simulations the part values make no sense.

CM circuit:
* The voltage divider ratio is quite uncommon
* the voltage divider resistance is way too high for the load

Usually one want the voltage drop low for the ouput voltage to be high.
In your case the DC input voltage is 5V and the ouput voltage is just tiny 0.3V. You waste about 93% of the energy.

Usually one wants the RC behaviour dominated by the resistors R1 and R3. In your case it is dominated by R2 and the gain of Q1.

I don´t know if this is what you want to achieve.

******
RC filter:
* The ratio R1 to R2 is quite uncommon.

Usually one wants high ouput voltage. And usually one wants R1 to dominate the RC time constant. But in your case the time constant is dominated by R2.
You waste more than 99% of the energy.

Maybe this is useful in your case. But I can´t imagine a real circuit where this makes sense.

****
Give some informations why your values are that unusual.

Klaus

 

[FvM]

For a meaningful comparison, both circuits should be dimensioned for a similar Vout (and similar Iout respectively), e.g. Vout = 4 V. R1 needs to be adjusted in both circuits, R3 would be probably omitted.