The fundamental circuits of Figs. 7.7 and 7.10 for two-stage current and voltage amplifiers are also used for d.c. and pulse amplification. Voltage amplifiers often include an emitter-follower stage to give a high input resistance or a low output resistance, and negative feedback is commonly applied over the three stages.
These are amplifiers used for magnifying small voltages or currents so that slow variations in them can be more readily seen and measured. To ensure a good response at the very low frequencies of interest the amplifiers are usually direct-coupled.
D.C. amplifiers are used in electro-cardiographs, computers, oscilloscopes, pyrometers and in sensitive measuring instruments ...