Fundamental investigation of the comparative behaviour of small-signal BJTDarlington and cascode amplifiers

In-depth investigation of the fundamental physics underlying various charac teristics of the Darlington pair with common-collector/common-collector (C- C/C-C) and common-collector, common-emitter (C-C/C-E) configurations and of the cascode amplifier with common-emitter/common-base (C-E/C-B) configurat ion has been performed. Although the investigation is based primarily on ex periments, some Pspice simulations have been made, wherever necessary, to p rovide better insight into the characteristics. Both the low-frequency and high-frequency characteristics of the Darlington pair and cascode amplifier have been discussed. The effects of emitter resistance on the gain in the C-C/C-C Darlington pair and of the collector resistance on the gain in the C-E/C-B cascode amplifier and C-E amplifiers have been compared. Load resis tance seems to have identical effects on the gain in all the amplifier circ uits. The bypass capacitor has significant influence on the frequency respo nse of the devices. For example. the Darlington pair with C-C/C-E configura tion provides high gain and, for a relatively large bypass capacitor. gives a large midband width. Sinusoidal signals are less susceptible to distorti on than square wave signals. Output square waves are distorted most by the cascode amplifier and distorted least by the C-C/C-C Darlington pair. The C -C/C-E configuration of the Darlington pair is superior to the cascode ampl ifier in the sense that it provides high gain and the output square wave re mains largely undistorted even at higher frequencies.