Why the Operational Amplifier is so Useful
The operational amplifier is one of the most useful electronic components, because of it's ability to compare, add and subtract (voltages), amplifiy (or multiply) voltages, or integrate/differentiate using additional components, such as a capacitor in a circuit with an op – amp.
The operational amplifier has a very high open loop voltage gain – up to
for dc. If ac is used, this gain typically decreases with frequency.It has a very high imput resistanace, drawing close to zero current from the supply, a very low output resistance. It can act as a differential amplifier by multiplying the different between the voltages to the two input termainal, if this different is less than about 150 micro Volts.
A common op – amp is an eight pin dual (type 741) in line form, shown below.
We are interested in only five of the eight pins.
The inverting pin 2 above with input voltage
Inputs are changed in sign or polarity.The non - inverting pin 3 above with input voltage
Inputs are not changed in sign or polarity.The out put pin (6 above).
The positive supply (7 above).
The negative supply (4 above).
We treat the op – amp as an amplifier with an open loop gain
given by