hey guys, i do search for old post and found same topic, but my design is quite difference with it, so i decide to open new post .
first, i plan to design a hearing aid which contain a low pass filter(butterworth 2nd order) to filter off the frequency that greather than 20khz; a pre-amp to amplify the signal that received from microphone. attachement below are my circuit design. I am using tlc2274 as my op-amp for pre-amp and also filter design.
For first picture,i search from net about the design of pre-amp and i was apply it as my pre-amp . the value for those component are listed as below:
* R1 - 1 kOhm resister
* R2 - 3.3 kOhm resister
* R3 - 1 kOhm resister
* R4 - 540 kOhm resister
* C1 - 0.0054 uF capacitor
* MIC1 - an electret microphone
*actually i would like to learn about how to design a pre-amp and calculate the R and C value, is that anyone can teach me? thank!
For second picture, that is my lowpass filter that use to filter the frequency that above 20khz. I do test this circuit with function generator and oscilloscope, i observed the result shown that when my function generator generated a signal that greather than 8k and above toward 20khz, my output signal's amplitude will be keep on reduced. I assume that the filter is work.
Now, the question is, after i combine both pre-amp and lowpass filter circuit, when i try to use my handphone to make a call, i found out that my phone wil interference my circuit, which mean that it will have some "pu pu" sound heard from earphone. May i know a handphone will generate how much frequency to interface with electronic device? Or it show that my filter does not work pefectly which mean that it cant filter off the noise that cause by interference of handphone? I do search from net about frequency of phone but i cant find it..
Lastly , is that i need to improve my filter order to reduce noise? May i know where can i get the infomation for how to design a 3rd order butterworth low pass filter? (cause i only can found the filter design that up to 2nd order)
Thanks !!
first, i plan to design a hearing aid which contain a low pass filter(butterworth 2nd order) to filter off the frequency that greather than 20khz; a pre-amp to amplify the signal that received from microphone. attachement below are my circuit design. I am using tlc2274 as my op-amp for pre-amp and also filter design.
For first picture,i search from net about the design of pre-amp and i was apply it as my pre-amp . the value for those component are listed as below:
* R1 - 1 kOhm resister
* R2 - 3.3 kOhm resister
* R3 - 1 kOhm resister
* R4 - 540 kOhm resister
* C1 - 0.0054 uF capacitor
* MIC1 - an electret microphone
*actually i would like to learn about how to design a pre-amp and calculate the R and C value, is that anyone can teach me? thank!
For second picture, that is my lowpass filter that use to filter the frequency that above 20khz. I do test this circuit with function generator and oscilloscope, i observed the result shown that when my function generator generated a signal that greather than 8k and above toward 20khz, my output signal's amplitude will be keep on reduced. I assume that the filter is work.
Now, the question is, after i combine both pre-amp and lowpass filter circuit, when i try to use my handphone to make a call, i found out that my phone wil interference my circuit, which mean that it will have some "pu pu" sound heard from earphone. May i know a handphone will generate how much frequency to interface with electronic device? Or it show that my filter does not work pefectly which mean that it cant filter off the noise that cause by interference of handphone? I do search from net about frequency of phone but i cant find it..
Lastly , is that i need to improve my filter order to reduce noise? May i know where can i get the infomation for how to design a 3rd order butterworth low pass filter? (cause i only can found the filter design that up to 2nd order)
Thanks !!
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