An operational amplifier (or op amp) is a complex circuit of resistors, capacitors and transistors integrated as one circuit onto a single piece of silicon. Hence the expression integrated circuit (IC). The most common form of op amp has two inputs and one output (Figure 5.1.1). The output is the amplified difference of the two inputs:

Out = K(INI - IN2) where K = gain

Most op amps are voltage devices so that Va = K(VI - V2). (In Chapter 7, Operational amplifiers 2, we will look at other types of operational amplifier which output current - these are the transconductance amplifiers.)

The gain figure K varies between device types but is typically of the order 100000 to 10000000 (see Table 5.1.1 for a comparison). So, if an op amp has a gain of 100000 (= 105 or 100dB) then in theory, if the difference between +Vin and -Vin were only lOJ.LV, then the output would be 1V.