single supply opamp
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Posted by on March 27, 2005, 8:44 pm
Please log in for more thread options I am simulating a single supply opamp in an inverting opamp configuration with gain set to 1. I/P is an ac sine 200mVpp 0Voffset. I am using the TLC081 TI single supply opamp spice model. Qn) When input sine wave becomes +ve wrt gnd , then o/p should drive negative. Of course that can't happen because we have a single supply system. In the simulation i see an o/p DC offset(300mV) wrt which the o/p sine wave swings up and down. Does this mean that in a single supply opamp the o/p always gets a DC offset even if input has no DC in it? I don't qualatatively understand what happens when V- input of a single supply opamp crosses the V+ input(0V always in this case). Thanks | |||||||||||||
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Posted by Bob on March 28, 2005, 9:00 am
Please log in for more thread options If the + input is more negative than the - input then the output will head toward the most negative supply that the opamp's power pins are connected to. If the most negative supply is what you're calling "gnd" then that's the most negative voltage that the output can get to. Typically, in a single-supply opamp that's hooked up in an inverting configuration, the + input is biased at the midpoint of the supply. For example, if the supply is 9V, then the + input will be set to 4.5V. If you then capacitively couple the - input (through the feedforward resistor) then the output will move up and down from its idle 4.5V point (as the input is driven with a changing signal). An interesting thing happens if you capacitively couple the output. Then, you can get an output signal that goes more negative than your most-negative supply ("gnd", in your case). This is how car audio amplifiers can get speakers to move both outward and inward, on those type of amplifiers that require one side of the speaker to be connected to the minus side of the car's 12V battery (i.e., the car's chassis). Bob | |||||||||||||
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> configuration with gain set to 1. I/P is an ac sine 200mVpp 0Voffset. I
> am using the TLC081 TI single supply opamp spice model.
> Qn) When input sine wave becomes +ve wrt gnd , then o/p should drive
> negative. Of course that can't happen because we have a single supply
> system. In the simulation i see an o/p DC offset(300mV) wrt which the
> o/p sine wave swings up and down. Does this mean that in a single
> supply opamp the o/p always gets a DC offset even if input has no DC in
> it? I don't qualatatively understand what happens when V- input of a
> single supply opamp crosses the V+ input(0V always in this case).
> Thanks
>