I want to use OP amplifier AD822 to build a voltage follower which generate exactly the same output voltage with the input voltage. The power to OP is 5v. The input signal is 3.3v, the period is 20ms.

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However, the amplitude of output voltage(green) is not 3.3v but only 2.8v. Can someone tell me why there is the gap between the input and output voltage? How can I get the output voltage with the exactly the same value as input? enter image description here

Edit: Thanks to Dwayne Reid's comment. I changed the resistor from 100 to 1k and the output voltage is almost the same as the input voltage. enter image description here enter image description here

  • \$\begingroup\$ Do you use an AD822 model? Or some generic OP-Amp model? \$\endgroup\$
    – Janka
    Commented Nov 27, 2018 at 23:54
  • \$\begingroup\$ Yes, the AD822 model is provided by LTspice. \$\endgroup\$
    – Ross
    Commented Nov 27, 2018 at 23:58
  • 2
    \$\begingroup\$ I would check if the problem disappears with a higher supply voltage. \$\endgroup\$
    – Janka
    Commented Nov 28, 2018 at 0:01
  • 1
    \$\begingroup\$ Load resistor of 100R seems very low. Is the op-amp running out of current? \$\endgroup\$ Commented Nov 28, 2018 at 0:04
  • \$\begingroup\$ Yes, you are right! I change the resistor to 1k and the output voltage is almost the same as the input voltage. \$\endgroup\$
    – Ross
    Commented Nov 28, 2018 at 0:19

1 Answer 1


The AD822 datasheet specifies loading as 15 mA maximum, source or sink. At 15 mA it can only source Vcc - 800 mV, or sink to Vee within 300 mV. Since a 1K load proved much better, I would suggest a minimum load of 2.2 K. This op-amp is rated for rail-to-rail output, so anything less than that means too heavy of a load or an out-of-band signal.

Unless an op-amp is designed for high current drive, a typical op-amp is rated for a 2.2 K minimum load. As always check the datasheets, though not all datasheets make it clear what the minimum load is. If not listed as text, check performance graphs at the end of the datasheet.

To validate simulations, compare the results with the datasheet.

Link to AD822 datasheet


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