operational amplifier (op amps) It is an integrated circuit with the following properties.
(a) Very high voltage gain - up to 105 although it decreases with frequency.
(b) very high input resistance
(c) very low output resistance
(d) The amplifier output signal is proportional to the difference between two input signals.
A commonly used op amp is of type 741 shown in fig.o4. Of the eight pins on the op amp we are only interested in five.
(i) The inverting input P - input here will be changed in sign.
(ii) The non inverting input Q - inputs here will not be changed in sign.
(iii) The output F.
(iv) The positive supply Vs.
(v) The negative supply -Vs.
Op amps are used as amplifiers, in measuring instruments, in radio and TV for amplification of weak signals received at aerial, and in loudspeakers. It is also used in certain type of computers.
An inverting amplifier is shown in fig. o5. The voltage to be amplified is fed into the inverting input of the op amp. The voltage gain of this amplifier is given by,
Vo/Vi = -R2/R1 (o4)
Notice that the voltage gain in this circuit depends only on the values of the external resistors. The gain of the amplifier falls off rapidly above certain frequency.