Differential-mode and common-mode signals are important to understand in the design of complex signal-conditioning circuits which use operational amplifiers in medical diagnostic, industrial control and military applications. Operational amplifiers mainly excel in performance to provide a high common-mode rejection ratio and high gain. By using negative feedback, its overall gain, input imledance and bandwidth are determined by external components without these parameters being affected by the op-amp’s parameter variations due to external influences like temperature and so on. Most amplifiers built using discrete transistors will not be able to match the performance characteristics of an operational amplifier like CMRR, low drift, high input impedance and stability of gain. Engineers are so used to thinking and visualising currents as differential-mode in circuit analysis that they find it hard to switch their thinking to common-mode currents and their effects in say, amplifying circuits. It will be instructive to understand and visualise Current at its most fundamental level in the presence of electric fields in ordinary conductors. Electrostatics and circuits belong to one science not two. To learn the operation of circuits, Current and the conduction process, resistors and how discussing these topics makes it easier to understand the operation of capacitors, inductors, diodes and their operation in amplifier circuits and circuit theory watch these two videos i. kzfaq.info/get/bejne/iKujiseVnJzVqJc.html and ii. kzfaq.info/get/bejne/bqiBgMKp3Ji2lqM.html Most common-mode signals originate because of sources external to the system, and the energy in them circulates annoyingly until it is dissipated in resistive portions. Therefore the need arises for amplifiers to amplify only the useful differential signal and reject the common-mode component which does not belong to the signal conditioning system. Such an amplifier is used as the first stage of an operational smplifier which opens the door to allow a signal from a sensor ‘in’, so to say and so the importance given to this stage. Topics related to these aspects are discussed in chapters 3 and 5 of textbook 4 (see last frame References in video #1) and a power point presentation with animations “Basic Action of a Differential Amplifer-Heart of the Opamp” which explains differential amplifier with a U-tube manometer analogy of differential- and common- mode signals is included in the CD alongwith this book.