# How to recieve the TIME derivative of an input AC signal?

#### amsscorpio

Joined Sep 28, 2011
2
I have already looked up differentiators op-amp circuits using the LM741 and have also simulated the simple circuit on a breadboard but have not received the desired output.

I am inputting a 1.5 volt sine wave with a 1 kHz frequency. The time derivative is 1 kHz *1.5 cos with a 1 kHz frequency. That should be the output I should be receiving but instead I simply get an inverse and an amplification as my output of the same signal. A cosine is simply a sine wave with a 90 degree ( pi/2) phase shift.
Does someone know why I am not receiving the signal that I need?

#### Papabravo

Joined Feb 24, 2006
19,024
Well if you're not looking with a dual channel scope, how do you know what you are looking at? Also if your input is
Rich (BB code):
y  = sin (ωt)  then
y' = ω cos(ωt)
where ω = 2*pi*f

#### amsscorpio

Joined Sep 28, 2011
2
Well if you're not looking with a dual channel scope, how do you know what you are looking at? Also if your input is
Rich (BB code):
y  = sin (ωt)  then
y' = ω cos(ωt)
where ω = 2*pi*f
Yeah I am checking my input and output simultaneously through an oscilloscope and all that seems to happen is that the original sine signal gets inverted and no phase shift seems to occur. Although there was a weird gain value that I noticed...

Question: Would you say that that simple differentiator op-amp circuit is reliable?

#### Papabravo

Joined Feb 24, 2006
19,024
With a single amplifier stage there will be an inversion. You have to work out the scaling factor from first principles and it will be frequency dependent. The best thing to do is choose components that will attenuate the output at a higher frequency by at least a factor of 10.

Is an analog differentiator reliable? Thats a harder question. It is more sensitive to noise, temperature, and power supply variations. Let me ask a counter question. What happens if you integrate instead? You get the phase shift you want and there are few noise and stability problems.