An Op Amp Tutorial

An Op Amp Tutorial(Based on material in the book Introduction to Electroacoustics and Audio Am-plifier Design, Second Edition - Revised Printing, by W. Marshall Leach, Jr.,published by Kendall/Hunt,°c 2001.)Anopamphas twoinputs andoneoutput. The circuit isdesignedsothat theoutputvoltageis proportional to the di fference between the two input voltages. In general, an op amp canbe modeled as a three-stage circuit as shown in Fig. 1. The non-inverting input is v .TheI1inverting input is v . The input stage is a di fferential amplifier (Q and Q ) with a currentI2 1 2mirrorload(Q −Q ). Thedi ffamptailsupply isthe dccurrentsourceI . Thesecond stage3 5 Qis a high-gain stage having an inverting or negative gain. A capacitor connects the outputof this stage to its input. This capacitor is called the compensating capacitor. Other namesfor it are lag capacitor, Miller capacitor, and pole-splitting capacitor. It sets the bandwidthof the circuit to a value so that the op amp is stable, i.e. so that it does not oscillate. Theoutput stage is a unity-gain stage which provides the current gain to drive the load.Figure 1: Op amp model.If we assume that Q and Q are matched, that Q and Q are matched, that base1 2 3 4currents can be neglected, and that the Early e ffect can be neglected, we can write thefollowing equation for i :O1i =i −i = i −i = i −i (1)O1 C1 C3 C1 C4 C1 C2But i +i =I and i =I /2+i .ThusweobtainC1 C2 Q C1 Q c1i =2i −I =2i (2)O1 C1 Q c11Open-Loop Transfer ...