First order RC Circuit help!

Thread Starter


Joined Aug 10, 2006
i've got this circuit:

im after the time constant which i know as

tau = RC.

in another example in tutorials it shows a similar circuit but the capacitor is around the 200 ohm resistor and hte 800ohm resistor taking the place of the capacitor.

afterwards it works out the thevenin of the circuit.

im guessing i have to do that here? but im not sure where to start.


Joined Dec 25, 2006

The tutorial that you found sounds like an attenuated low pass filter. What you have here is a high pass filter or differentiator.

Unless you want to go back and prove the circuit from first principles (-jXc, have fun!) I would use the LaPlace transform to put the equation in 1/(Ts+1) format. Then you can easily eyeball the time constant T (tau).

Really quick, it looks like the time constant would be (R1R2C)/(R1+R2). The DC gain is of course a voltage divider created by R1 and R2.

Someone else needs to verify that, and if you have Spice it would help as well.

Thread Starter


Joined Aug 10, 2006
i don't have answers either as it's just a sample paper.. don't ask why they don't supply answers.
sorry i don't know how to use spice yet, they haven't gone into details with that program. only circuitmaker

(R1R2C)/(R1+R2) sounds a lot easier than the laplace