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I am assembling a simple follower circuit and am confused about its behavior. In the diagram below, R1 = R2 = 15 k\$\Omega\$. As expected, the voltage at the non-inverting input of the op amp is 2.5V. I expect the output at Vout to also be 2.5V. When I use a 741 for the op amp, everything works as expected and Vout is 2.5V.

However, when I use the 3130EZ op amp, I find a significant voltage drop between the inverting and non-inverting inputs (which seems to violate one of the fundamental op amp principles) and Vout is not 2.5V. I've tried varying the voltage supply to the op amp from about 5V to 15V and this causes Vout to change. I've also tried replacing the op amp and the results are the same. Any ideas why the 3130EZ would behave so differently from the 741? I'm pretty new to op amps and electronics in general, so it's possible I've missed something really obvious.

Op amp circuit

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  • \$\begingroup\$ How are you powering your opamp exactly? \$\endgroup\$ – Vladimir Cravero Aug 28 '15 at 6:03
  • \$\begingroup\$ Where is the input reference? \$\endgroup\$ – LvW Aug 28 '15 at 7:29
  • \$\begingroup\$ Also, what is a "significant voltage drop"? From memory the input offset on 3130s may be higher - 10mv rather than 2-3mv. And what is loading the output? the 741 may be capable of higher output current. \$\endgroup\$ – Brian Drummond Aug 28 '15 at 11:10
  • \$\begingroup\$ The op amp is powered with a 15V variable DC power supply. The 5V source (coming off an arduino) has a common ground with the power supply. The voltage drop across the op amp inputs varies with the power supply, but can be several volts (not mV!). There is nothing at the output right now, I am only measuring Vout with a voltmeter. \$\endgroup\$ – bax Aug 28 '15 at 15:09
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Yes, as you suspect the CA3130EZ is not unity gain stable without compensation capacitor(s). On page 3 of the datasheet states that a capacitor (called \$C_c\$) of at least 47pF be placed from pin 1 to pin 8 of the device. \$C_c\$ provides Miller compensation, and reduces bandwidth of the part. Circuits shown with unity gain also typically have a 2kOhm || 0.1uF feedback network, and a 25pF load. See the Bode plot of open loop response on page 14 for details of effects of various values of \$C_c\$ and \$C_L\$.

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I believe that the problem was that the CA3130EZ is not unity gain stable. Following Reasons for a voltage follower introducing voltage gain? I added a 100 pF capacitor across pins 1 and 8 of the op amp. This seems to have solved the problem. I'd be grateful for comments, though, as my understanding of why this worked is shaky.

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