Monday 22 May 2017

operational amplifier - opamp without negative supply


I connected the V- pin to ground and the V+ pin to +5V. I then connected in+ to ground and in- to +5V. I expected Vout to be a low value near ground (0V) but I'm getting about +1.7V. Is this normal?



Answer



Standard opamps can't work with input voltages close to the power supply, neither V+ or V-. Same for the output: they don't go all the way to V+ or V-. What you need is a rail-to-rail I/O opamp. (There are also opamps which are only input or output rail-to-rail!).
Also, most opamps want a dual supply, often minimum +5V/-5V. Since you're using it as a single supply opamp, your V+ - V- difference may be too low for normal operation.
Look for a single supply opamp. They often work at low power supply voltages only, and most of them will be rail-to-rail I/O.


note: actually, any dual supply opamp can be used as a single supply; since voltage is relative the opamp doesn't know whether V- is a negative voltage or ground. By single supply opamp I mean specifically an opamp which is low voltage, like I mentioned above.


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