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Cap

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zesla

New Member
Hello guys,

just wanted to make a search via the net but unfortiunataly I had no clues and the needed keywords.

My problem is why those 100nF caps are there mounted in parallel with an electrolyt cap while the net result (Cx+Cy) is equal to the bigger one?
the other question is how to calculte thier values? (I am familier with Xc=1/2pixFxC) but tell me about the F and the XC for a circuit?

I have seen those 100nF caps used on the op-amps and the amplifier supply rails too, what about those ones?

Thanks
 

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audioguru

Well-Known Member
Most Helpful Member
The 100nf capacitors have a very low impedance at high frequencies where transients and possible oscillations can happen.
The electrolytic capacitors are good at low frequencies but are poor at high frequencies.
 

zesla

New Member
Thanks but what are those HIGH frequncies you are refering to??!
How to identify that freq you meantion to choice the suitable cap?
 

zesla

New Member
I have another question regarding to that 56W power op-amp:
As you can see the Rf1 is 20K and the RB is 1k the ratio is 20, So what if I use 200K and 10K or any other two resistors but remain the ratio? makes any sense?
 

audioguru

Well-Known Member
Most Helpful Member
The datasheet has a part that talks about Supply Bypassing.
The LM3875 has plenty of gain at a frequency as high as 2.5MHz so you must use capacitor types and values that are rtecommended.

If you increase the value of the feedback resistors then the input bias current in them will cause a DC voltage at the output that you do not want. Also, the input capacitance of the IC will cause all kinds of problems when the feedback resistor values are too high.
 

zesla

New Member
The datasheet has a part that talks about Supply Bypassing.
The LM3875 has plenty of gain at a frequency as high as 2.5MHz so you must use capacitor types and values that are rtecommended.

If you increase the value of the feedback resistors then the input bias current in them will cause a DC voltage at the output that you do not want. Also, the input capacitance of the IC will cause all kinds of problems when the feedback resistor values are too high.

Even if I keep the ratio of the Rf/Rb equal to 20 nut just use two higher reistors?
 
Last edited:

audioguru

Well-Known Member
Most Helpful Member
I told you that if you increase the values of the feedback resistors then the input bias current in them will cause a DC voltage at the output of the amplifier that you do not want.
 

zesla

New Member
I told you that if you increase the values of the feedback resistors then the input bias current in them will cause a DC voltage at the output of the amplifier that you do not want.

That's strange, incrasing the resistance will reduce the current through the resistor so why in the case it acts in oposite?!
 

audioguru

Well-Known Member
Most Helpful Member
That's strange, increasing the resistance will reduce the current through the resistor so why in the case it acts in oposite?!
The bias current in a resistor causes a voltage drop that the amplifier amplifies. A high value resistor has a higher voltage drop than a lower value resistor. The bias current is the same in both resistors.
 

zesla

New Member
The bias current in a resistor causes a voltage drop that the amplifier amplifies. A high value resistor has a higher voltage drop than a lower value resistor. The bias current is the same in both resistors.

Is your saying uinversal for all op-amps even for those smal ones like TL07x? If so I would consider it for those op-amps too. What is the solution yet using the higher resistors?
 

zesla

New Member
By the way regarding those smal caps, So If I have an square wave oscillator in the circuit oscilating at 100Khz so what about their values?
 

RCinFLA

Well-Known Member
Hello guys,

just wanted to make a search via the net but unfortiunataly I had no clues and the needed keywords.

My problem is why those 100nF caps are there mounted in parallel with an electrolyt cap while the net result (Cx+Cy) is equal to the bigger one?
the other question is how to calculte thier values? (I am familier with Xc=1/2pixFxC) but tell me about the F and the XC for a circuit?

I have seen those 100nF caps used on the op-amps and the amplifier supply rails too, what about those ones?

Thanks

Any load that has impulsing spurts of current can create stabilities issues to the feedback control system of a regulator.

Digital CMOS logic, for example, is basically switching capacitance around from logic '0' to logic '1'. Nearly all the current drain of CMOS digital logic occurs only during the logic transitions. This creates high current spikes at logic slew rate duration on these digital state transitions.

100 nF caps are often sprinkled around power pins on digital I.C.'s to provide the instantaneous current impulses, smoothing the resultant supply voltage ripple.
 

audioguru

Well-Known Member
Most Helpful Member
Is your saying uinversal for all op-amps even for those smal ones like TL07x?
every opamp is different. Simply look at its datasheet.
The TL07x opamps have Jfet input transistors that have [n]no[/b] input bias current, just a tiny leakage current.

regarding those small caps, So If I have an square wave oscillator in the circuit oscilating at 100Khz so what about their values?
A 100kHz square wave has harmonics in its output up to 2MHz. A 0.1uF ceramic capacitor with short leads is good up to 2MHz.
 
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