Isolate the output of a simple low pass

[spider_dula]

Hey, I am a 2nd yr electrical eng. student, I have a simple low pass filter to convert a square wave to triangular wave.

here is the circuit: (the capactitor and resistor values are not actual... its just for illustrative purpose)

After I have converted it to triangular, I would like to use THIS VERY SAME output voltage to appear across a resistor R2.

but the problem is if i connect the R2 like that as shown ... it will change the the triangular wave shape, because the resistance seen by the capacitor changes.

My question is : Is there any way to ISOLATE the output so that I can use it on the resistor R2 without it affecting the low pass filter ?

 

[BobK]

Yes, you can use an op-amp in voltage follower confguration.

Bob

 

[duke37]

What you have is an integrator which can be made with an op-amp and will give a low output impedance.
The output voltage will not be much different if you make R2 larger, say100k. This is what Bob's solution does.
Stick with R2 1k, the waveform should be the same but of half ampltude. To bring the time constant back to the original, double C1.

Duke

 

[spider_dula]

Thanks for the reply BobK and thanks duke37 for clarifying what BobK said. I did consider using an op amp integrator but to keep things simple i decided to omit the op amp, but like you mentioned that the output impedance is low using the Op Amp.

I didnt quite get what is the importance of low output impedance, my concepts are a bit hazy so if you could help me with that I d be really grateful.

 

[BobK]

Low output impedance means a load will not reduce the output voltage as much. Thik of it as a resistor in series with the voltage source. Of course, it is relative. An opamp might have an output impedance of hundreds of Ohms, while a high power audio amp is going to be a fraction of an Ohm.

Bob

 

[spider_dula]

Hey thanks for explaining that BobK , I just realised that my square wave is from 0 to 5 V , so an op amp integrator will integrate the 0 voltage as 0 and I will not get a triangular wave but something like a saw tooth.

One way would be to offset the square wave so it become -2.5 to 2.5, but any idea how to get that done practically ?