First Order Active Low Pass Filter

[foobag]

http://img39.imageshack.us/img39/2991/captureucm.jpg


Attempt to solve:

omega = 1/R2C1 ; omega = 2pi*f

so C1 = 1/R2*2pi*f , where f = 1000Hz

so C1 = 0.53 nF (nanofarads)


then Ao = -R2/R1, so absolute value is 15 gain V/V?
in terms of dB its approx 23.52dB using 20 log X formula?

is this setup legit? any feedback would be deeply appreciated!

 

[lorenb]

your gain and decibel value are right for the given the capacitor value you found. I've never used that method or formula to find the capacitor value though.

I know Zc = 1/jwc... where'd you get the (omega = 1/R2C1)?

 

[foobag]

i got it from my textbook, for solving first order low pass filters. I believe its on Wiki too: http://en.wikipedia.org/wiki/Low-pass_filter#Active_electronic_realization

 

[lorenb]

ya, i guess the gain and decibel values don't having anything to do with the capacitor. The capacitor will block high frequency sources.

 

[foobag]

is anyone good with PSPICE/ORCAD?

I got to do a frequency sweep from 10 to 5000 Hz..and use Vin as sine with amplitude from 0.1 to 0.3 V.

I created the circuit using Vac and set the first parameter to 0.1, and bottom to 0.2 which i believe creates an offset with amplitude +/- 0.1? Not really sure, rest of circuit I was able to setup the op-amp, resistors, capacitors etc..

Is it ok to use Vac as I mentioned and then performing PSPICE simulation using AC sweep simulation profile?

 

[lorenb]

well, i don't know how to vary the voltage at the same time as the frequency. but i attached a graph of 10-5k frequency with 1V

 

[foobag]

i attached my PSPICE files if anyone can look it over.

http://www.mediafire.com/?clibucuzwv5qwu9

thanks!

 

[foobag]

[PLAIN]http://img13.imageshack.us/img13/9672/hw3a.jpg
Simulation Linear scale using Multiview

[PLAIN]http://img543.imageshack.us/img543/3769/hw3b.jpg
Simulation Logarithmic scale using Multiview

vs...

[PLAIN]http://img30.imageshack.us/img30/2825/hw3orcad.jpg
Same simulation linear in Orcad PSPICE

[PLAIN]http://img835.imageshack.us/img835/223/hw3orcad2.jpg
Orcad PSPICE Logarithmic scale


Why am I getting different results for basically the same circuit configuration? It seems Multisim is closer to the actual overall gain 300k/20k = 15 v/v, but orcad isn't giving me that =(

 

[lorenb]

make sure you pay attention to your y-axis units. Looks like multisim is in magnitude while orcad is in volts. if you are using .1 volts they both seem to be the same. . .

 

[foobag]

thanks lorenb that was my mistake i had a 0.1 v peak AC source...anyone know how to implement a 0.1-0.3 sine AC signal? will that really affect the graphs tho?

 

[foobag]

*bump and help on how to make a 0.1-0.3 sine AC signal?

 

[Zryn]

AC signals that do not oscillate around 0 merely have a DC offset. Put a 0.2V DC source in series with a 0.1V (amplitude) sin AC signal and you should get a 0.1-0.3 sin AC signal i.e 0.2V +/- 0.1V = 0.1-0.3