if i have the following circuit , what the expected behaviour of the following caps ? , it will be charge and discharge very quickly since we have a low inpedance form op-amp?


simulate this circuit – Schematic created using CircuitLab


1 Answer 1


I expect this will oscillate or otherwise not behave as you intend. The TL081 is supposedly unity gain stable, but it is unlikely stable at unity gain and with such a large capacitive load.

Even if the opamp were ideal, your circuit doesn't make sense. A perfect ideal opamp would drive its output to whatever it takes. The cap would just cause lots of current whenever it changes its output voltage.

If your intent is to low pass filter the output, put a resistor in series with the opamp output before the cap. A better answer for a low pass filter using these components is to put the resistor followed by cap to groun on the input of the opamp.

  • \$\begingroup\$ Actually , this circuit is taken from sample and hold circuit , i know that it's basic , but this question will help me to understand how this circuit work. \$\endgroup\$ Jan 20, 2014 at 17:33
  • \$\begingroup\$ @user: That's not a sample and hold. It could be if there was some kind of electronic switch between the opamp output and the cap. \$\endgroup\$ Jan 20, 2014 at 17:36
  • \$\begingroup\$ yes you are right , but this is quesion will help me to understand , what i want to do .. \$\endgroup\$ Jan 20, 2014 at 17:40
  • \$\begingroup\$ @OlinLathrop , could i ask you why we have a buffer in s/h circuit , we cans simply connect the analog input to the switch without buffer !! \$\endgroup\$ Jan 20, 2014 at 18:01
  • 1
    \$\begingroup\$ @user: A buffer can present a higher impedance to the analog signal, thereby altering it less. \$\endgroup\$ Jan 20, 2014 at 19:15

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