# single supply opamp

Discussion in 'Electronic Basics' started by [email protected], Mar 28, 2005.

1. ### Guest

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

2. ### BobGuest

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