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# MT-211 Mini-Tutorial

Category: Hardware

Hi

I have a question about an equation described on page 2 of the mini-tutorial where it says: "The corner frequency of the filter is set by the capacitor value and the value of R5 (F0 = 1/(2πR5C1))." Assuming that the values ​​of R5=10K and C1=1uF, I got 15.90 Hz.
I understand that this corner frequency is low and the signal will be almost entirely attenuated at the output assuming 1 kHz at the input, but... this is not happening in the simulation. I simulated this circuit on LTSpice, its attached below.
Could someone explain me this equation with details? Where could I be going wrong?

PrecisionFullWaveRectifier_avaliable_ripple.asc

• Hi  ,

I made some adjustments with your values wherein R2 = 0.5 × R1, R4 = 2 × R3, and R1 + R2 + R3 = R4, then I adjusted the capacitor from 1uF to 1pF because the input has a frequency of 1kHz so we need to get the fc higher than 1kHz. If we will calculate the corner frequency for 1uF and 1pF, the equations will be:

1uF: fc = 1/2pi*30k*1uF = 5.305Hz

1pF:  fc = 1/2pi*30k*1pF = 5.305MHz

Hence, I used a 1pF capacitor for this circuit. You can check the attached file for the updated circuit and results.

(Updated)PrecisionFullWaveRectifier_avaliable_ripple.asc

Hope it helps. If you have further questions, feel free to ask again. Thanks!

Regards,

Mae

• Hi Mae

Thanks for the clarification !

I found that the circuit you uploaded is a slightly different from mine, but that doesn't affect the explanation.

In fact, what I need is to obtain a DC level at the rectifier output with the lowest possible ripple. If the value of the filter capacitor is too low (1pF, for example) it will not filter well enough and there will be only (or very close) the positive pulses at the output.

Therefore, I need to find a reasonable value for this capacitor for good ripple filtering. I think that by doing some simulations (cmd .step) with other values to capacitor, I will be able to find a value that suits my project.

Regards,
Leonardo

• Hello,

It seems that the above question was already answered by   so I'm temporarily closing this thread. In case there will be some additional questions, feel free to reply again on this thread. Thanks!

Best regards,
Paul