If a voltage is applied to the inverting input of an operational amplifier and the non inverting input is earthed, we may simultaneously amplify and invert the voltage. For the op – amp to work as an amplifier the voltage
between the input terminals (+ and -) in the diagram below must be less than 150 microVolts. This can be achieved by suitable choices of
and
in the circuit below.
The resistoris part of a feedback loop. The feedback is out of phase with the input (the – input is inverting), so the feedback is negative. The maximum value of
will be 150 microVolts when the output is
(the saturated voltage), so we may taketo be zero. The voltage acrossis then
and the voltage acrossis
but of opposite sign. Almost no current flows through the op – amp so the current throughequals that throughand we can write
The voltage or closed loop gain of the amplifier is then given by