Determine the capacitor values of C3, C4, C7 and C8 to create a bandpass filter.

Question

Determine the capacitor values of C3, C4, C7 and C8 to create a bandpass filter. The cut-off frequency is 85hz for the low pass and 1200hz for the high pass. Make sure the bypass capacitor on the mosfet does not interrupt the filtered signal. The gain of the mosfet is 4.5, Vto=3.6 and k=2.13. The DC source are 12 and -12.

1H R1 2.42 Meg R2 1Meg Vdd RD2 1390 U3 rf 540n R15 316 R13 C2 260 11H R18 1Meg 1.9 Meg Vdd- U5 540n R20 565 c5 1H RD3 1390 R17 Meg U6 2.42 540n R21 R23 1Meg 315 R22 260 Vdd 11H R24 1Meg Irf540n R25 R26 565 1.9 Meg Vdd- c9

Explanation / Answer

The cutoff frequency of the band pass filter is given by the cut off frequency of the low pass and high pass filters as square root of cut off frequency of low pass filter multiplied by cut off frequency of high pass filter therefore the cut off frequency is =sqrt(85*1200)=319.37Hz.

Now capacitance C3=(1/2pi*fc)*(1/sqrt(RD2*R19) )=9.7nF

C4=(1/2pi*fc)*(1/sqrt(R18*R15) )=28.1nF

C7=1/2pi*fc)*(1/sqrt(RD3*R15) )=9.7nF

C8=1/2pi*fc)*(1/sqrt(R23*R24) )=28.1nF

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