<P> The input stage consists of a cascaded differential amplifier (outlined in blue) followed by a current - mirror active load . This constitutes a transconductance amplifier, turning a differential voltage signal at the bases of Q1, Q2 into a current signal into the base of Q15 . </P> <P> It entails two cascaded transistor pairs, satisfying conflicting requirements . The first stage consists of the matched NPN emitter follower pair Q1, Q2 that provide high input impedance . The second is the matched PNP common - base pair Q3, Q4 that eliminates the undesirable Miller effect; it drives an active load Q7 plus matched pair Q5, Q6 . </P> <P> That active load is implemented as a modified Wilson current mirror; its role is to convert the (differential) input current signal to a single - ended signal without the attendant 50% losses (increasing the op - amp's open - loop gain by 3 dB). Thus, a small - signal differential current in Q3 versus Q4 appears summed (doubled) at the base of Q15, the input of the voltage gain stage . </P> <P> The (class - A) voltage gain stage (outlined in magenta) consists of the two NPN transistors Q15 / Q19 connected in a Darlington configuration and uses the output side of current mirror Q12 / Q13 as its collector (dynamic) load to achieve its high voltage gain . The output sink transistor Q20 receives its base drive from the common collectors of Q15 and Q19; the level - shifter Q16 provides base drive for the output source transistor Q14 . </P>

Characteristics of an ideal and practical op-amp (ic 741)