I want to build a first order low pass filter using an inverting opamp with a gain of 10 below the bandwidth, which should be about 1.7kHz. How do I calculate suitable values of the two resistors and 1 capacitor?(Only these three components will be in the filter)?
your transfer function should be something like H(s) = A/(s+ 1/w)
i think u have a cap and res. in parallel in the feedback branch, and another resistor into the inverting input
calculate the transfer function of that configuration and see if the form matches what i said above (i.e., 1 pole)
I'm too lazy to figure it out now
Divide numerator and denominator by R2 - and you can compare the general form with your specific form.
That means: You can identify the time constant T=1/wo (with wo= 3dB cut-off).
I want to build a first order low pass filter using an inverting opamp with a gain of 10 below the bandwidth, which should be about 1.7kHz. How do I calculate suitable values of the two resistors and 1 capacitor?(Only these three components will be in the filter)?
With R1 being the input resistor and R2 being the feedback resistor and R2 in parallel with the cap, the 3db down point is:
w=1/(R2*C)
and the midband gain is:
A=R2/R1
So for example with a R2=10k feedback and R1=1k input resistor, and a cap of 1uf, we would see the output 3db down at the frequency of about 15.9Hz and somewhat lower than that we'd see a gain of 10. So the gain would be 10 until we get close to 15.9Hz and then it would start falling.
The midband gain is R2/R1 so that's 10k/1k=10.
The cutoff frequency is 1/(2*pi*R2*C)=15.9Hz.
Note this is only a first order LP filter. Usually op amp filters are made to be 2nd order, but that's not mandatory.
I don't understand (probably, it is my fault) your reply - nevertheless, the gain is defined by the resistor ratio only. And the cut-off frequency is defined by the time constant, which can be modified via C without touchuing the gain value.
OK. I understand what you mean. What about measuring the gain at a particular frequency? I derived the voltage transfer function but how do I use it to derive the gain in dB at a particular frequency?
OK. I understand what you mean. What about measuring the gain at a particular frequency? I derived the voltage transfer function but how do I use it to derive the gain in dB at a particular frequency?
I like to inform you that the transfer function gives you the output-input-ratio (gain) with the angular frequency as a running parameter.
Since this function is complex you have to discriminate between magnitude and phase. Do you need more infos? (I don't think so).
If you use an input resistor of 936 ohms and a feedback resistor of 9362 ohms and a capacitor of 0.01uf you'll get a gain of 10 in the passband and the cutoff frequency will be 1.7kHz.
The input impedance to this amp will only be 936 ohms however, so just in case you need a higher input impedance you can use a 9362 ohm input resistor and a 93.6k feedback resistor, and a cap of 0.001uf instead. So to get higher input impedance you multiply the two resistor values by a factor X and divide the cap value by that same value.