Opamp as differentiator designing

[featherheart9]

In the book opamps and linear integrated circuit by gayakwad
it is given that if we have to design differentiator using opamp
the following methods should be followed
1.select fa equal to highest frequency of input signal to be differentiated.then assuming a value of C<1microfarad calculte Rf.
2.choose fb=20 (fa) and calculate values of R1 and Cf so that R1 C1=RfCf

so my question is if we are using it for input signal of say 10 Hz to 1kHz then why we should equate fa=1kHz.
because it will work as differentiator in the range fa to fb .so fa should be 10 Hz nd fb should be 1kHz..
please explain why we use this method for designing

 

[audioguru]

The circuit is just a very simple highpass filter. 1kHz is reduced 3dB (0.707 times the level of high frequencies) and lower frequencies are reduced 6dB (0.5 times the level) per octave. There is no low frequency limit but its sensitivity is reduced as the frequency gets lower.

If fa is 1kHz then fb is 20kHz. If fa is 10hz then fb is 200hz.
The lousy old 741 opamp works poorly at 20kHz but it is simple to calculate the value of CF to reduce higher frequencies.

If R1 C1=RfCf then the circuit passes 1kHz or 10Hz but cuts higher and lower frequencies.

 

[featherheart9]

The circuit is just a very simple highpass filter. 1kHz is reduced 3dB (0.707 times the level of high frequencies) and lower frequencies are reduced 6dB (0.5 times the level) per octave. There is no low frequency limit but its sensitivity is reduced as the frequency gets lower.

If fa is 1kHz then fb is 20kHz. If fa is 10hz then fb is 200hz.
The lousy old 741 opamp works poorly at 20kHz but it is simple to calculate the value of CF to reduce higher frequencies.

If R1 C1=RfCf then the circuit passes 1kHz or 10Hz but cuts higher and lower frequencies.

srry but i dot understand

 

[audioguru]

srry but i dot understand

We like to talk about electronic circuits here. What do you like to talk about? Fishing?

 

[MrAl]

In the book opamps and linear integrated circuit by gayakwad
it is given that if we have to design differentiator using opamp
the following methods should be followed
1.select fa equal to highest frequency of input signal to be differentiated.then assuming a value of C<1microfarad calculte Rf.
2.choose fb=20 (fa) and calculate values of R1 and Cf so that R1 C1=RfCf

so my question is if we are using it for input signal of say 10 Hz to 1kHz then why we should equate fa=1kHz.
because it will work as differentiator in the range fa to fb .so fa should be 10 Hz nd fb should be 1kHz..
please explain why we use this method for designing

Hi,

It looks like they want you to be able to get a 1v output for a 1v input for the max frequency.
An ideal differentiator would be different but the output amplitude would be too high to be of practical use for normal signals like 1kHz, so there has to be some scaling too. What that technique seems to achieve is a scaled output that works from 0 up to the frequency you need as max (like 1kHz) with the values found that way.

The most critical parts are C1 and Rf because they make up the actual differentiator. The other parts are just there to limit the behavior of the circuit in a real environment that will have noise from various sources. That means you could probably get away with a lower value cap for CF if you wanted to, but because it is mostly going to limit higher frequency response at a frequency 20 times higher than you need, you should be ok with the value calculated. The factor 20 is a sort of fudge factor just so that we dont pass frequencies out of band too well which would disturb the following circuit.

Also, the 741 does not have to actually handle 20 kHz for a 1kHz top end design. That is just a fudge factor used to limit noise. The 741 only has to work up to around 1kHz if your top end is 1Hz, as long as it does not start clipping or distorting too much.

Note also the bottom frequency does not appear in the equation. It's mostly about the top end that matters, and what we want to limit going through the circuit.

We could look at this in more detail if you like including simple ways to alter the scaling factor.

 

[featherheart9]

Hi,

It looks like they want you to be able to get a 1v output for a 1v input for the max frequency.
An ideal differentiator would be different but the output amplitude would be too high to be of practical use for normal signals like 1kHz, so there has to be some scaling too. What that technique seems to achieve is a scaled output that works from 0 up to the frequency you need as max (like 1kHz) with the values found that way.

The most critical parts are C1 and Rf because they make up the actual differentiator. The other parts are just there to limit the behavior of the circuit in a real environment that will have noise from various sources. That means you could probably get away with a lower value cap for CF if you wanted to, but because it is mostly going to limit higher frequency response at a frequency 20 times higher than you need, you should be ok with the value calculated. The factor 20 is a sort of fudge factor just so that we dont pass frequencies out of band too well which would disturb the following circuit.

Also, the 741 does not have to actually handle 20 kHz for a 1kHz top end design. That is just a fudge factor used to limit noise. The 741 only has to work up to around 1kHz if your top end is 1Hz, as long as it does not start clipping or distorting too much.

Note also the bottom frequency does not appear in the equation. It's mostly about the top end that matters, and what we want to limit going through the circuit.

We could look at this in more detail if you like including simple ways to alter the scaling factor.

thank you.
it helped alot

 

[featherheart9]

We like to talk about electronic circuits here. What do you like to talk about? Fishing?

We like to talk about electronic circuits here. What do you like to talk about? Fishing?

well actually it you only who is talking about fishing.
and i got the answer..see the post by MrAI
although i think you were trying to say the same.. but it was not quite understanble
thank you