Question about long RC time?

Milad-D said:

The time constant at node 3 is determined by the parallel combination of R1 and R2 and C4. So its time constant is much smaller than what you have stated.

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chuckey said:

I think there is an error in the circuit. The + input of the opamp is decoupled with the 100MF. The active (-) input is connected to this point via C 3, so at any frequency where the reactance of C3 is low, the + and - inputs of the amp are shorted together. So the input is only going to respond at frequencies where the C3 reactance is high (>100K), so its a "bass amp" !! The input is coupled via a 22PF capacitor, which won't pass bass frequencies. I would try to connect the - end of C3 to earth, so the + input is solidly bypassed to RF.
Frank

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samy555 said:

To everyone: What is the purpose of having R1, R2 and C4 network?
thanks alot

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Milad-D said:

2) The capacitor C4 is used to make this node ground at high frequency.

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No, it will be charged and discharged by the parallel combination of the two resistors.

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schmitt trigger said:

Is to bias the noninverting input(+) tp about 0.521 volts DC. That is it. By coincidence it will take about 9.5 seconds to charge, but it is not relevant to the operation. It is not 220 seconds, the actual equivalent charging resistance is the parallel of 2.2M and 100k.

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I have see the circuit, and I agree with chuckey: the circuit, as designed does not work. There are way too many reasons.

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samy555 said:

The transmission frequency of mobile phones ranges from 0.9 to 3 GHz.

Please tell me in a simplified manner what is the most important of these reasons

Thank you very much

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schmitt trigger said:

At those frequencies, the selected components and the layout are vital. Ground planes, stripline tuned circuits, impedance matching, neutralizing parasitic elements....... the list goes on and on.

The photo in the the attached link shows a protoboard with long wires.

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schmitt trigger said:

"CA3130 operates at a frequency of only 15 MHz."

That is precisely my point......mis-interpretation of data sheets.

15 Mhz is not the operating frequency, it is only the unity gain crossover frequency....google the term.

And the spec sheet shows that would be only true with a compensation capacitor of zero pF...the schematic shows 47 pF, which would lower that value to 4 Mhz.

But even then, let's assume that the CA3130 could indeed work at 15 Mhz.....the phone's carrier frequency is 900 to 3000 Mhz, orders of magnitude higher. It still won't detect anything at the frequencies that a mobile phone is transmitting. It will pick up powerline harmonics, AM and short wave radio, SMPS noise.....but no cell phone transmissions.

Now...C4 is to clean the DC bias applied to the non inverting input. But its value is waaaaaaay too large, 1 uF would be enough.

There are many other mistakes like that.

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"CA3130 operates at a frequency of only 15 MHz."

That is precisely my point......mis-interpretation of data sheets.

15 Mhz is not the operating frequency, it is only the unity gain crossover frequency....google the term.

And the spec sheet shows that would be only true with a compensation capacitor of zero pF...the schematic shows 47 pF, which would lower that value to 4 Mhz.

But even then, let's assume that the CA3130 could indeed work at 15 Mhz.....the phone's carrier frequency is 900 to 3000 Mhz, orders of magnitude higher. It still won't detect anything at the frequencies that a mobile phone is transmitting. It will pick up powerline harmonics, AM and short wave radio, SMPS noise.....but no cell phone transmissions.

Now...C4 is to clean the DC bias applied to the non inverting input. But its value is waaaaaaay too large, 1 uF would be enough.

There are many other mistakes like that.

Click to expand...