Everything Science Grade 12

14.4 CHAPTER 14. ELECTRONICS

The Operational Amplifier ESCGJ

The operational amplifier is a special kind of voltage amplifier which is made from a
handful of bipolar or field effect transistors. Operational amplifiers areusually called
op-amps for short. They are used extensively in all kinds of audio equipment (ampli-
fiers, mixers and so on) and in instrumentation. They also have manyother uses in
other circuits - for example comparing voltagesfrom sensors.

Operational amplifiersare supplied on Integrated Circuits (I.C.s). Themost famous
operational amplifier I.C. is numbered 741 andcontains a single operational amplifier
on an integrated circuit (‘chip’) with eight terminals. Other varieties can be bought, and
you can get a single integrated circuit with two or four ‘741’-type operational amplifiers
on it.

The symbol for an op-amp is shown in Figure14.15. The operationalamplifier has
two input terminals andone output terminal. The voltage of the output terminal is
proportional to the difference in voltage between the two input terminals. The output
terminal is on the right(at the sharp point of the triangle). The two input terminals
are drawn on the left.One input terminal (labelled with a + on diagrams) is called
the non-inverting input. The other input terminal (labelled−) is called the inverting
input. The labels + and− have nothing to do with the way in which theoperational
amplifier is connected tothe power supply. Operational amplifiers must be connected
to the power supply, butthis is taken for grantedwhen circuit diagrams are drawn, and
these connections are not shown on circuit diagrams. Usually, when drawing electronic
circuits, ‘0V’ is taken tomean the negative terminal of the power supply.This is not the
case with op-amps. Foran op-amp, ‘0V’ refers to the voltage midway between the +
and− of the supply.

−

+

∞

non-inverting input terminal

inverting input terminal

output terminal

Figure 14.15: Circuit symbol for an operationalamplifier. The amplifiermust also be
connected to the + and− terminals of the powersupply. These connections are taken
for granted and not shown.

The output voltage of the amplifier Voutis given by the formula

Vout= A (V+− V−) (14.5)

here A is a constant called the open loop gain, and V+and V−are the voltages of the
two input terminals. That said, the output voltage can not be less thanthe voltage of