Active Circuit Elements. High-Gain DC Amplifiers

1 Active Circuit Elements. High-Gain DC Amplifiers 

The most commonly employed active elements are high-gain DC amplifiers, function generators, and function multiplier-dividers. We shall consider only amplifiers. 

The design of amplifiers and the function of their components are beyond the scope of the text. Interested readers may consult the references at the end of this chapter. Here we picture the amplifier simply as a black box (Fig. 3.28) and discuss only those amplifier characteristics conceptually important for analog computer solutions. Designating the input and output potentials relative to the ground by and eo, respectively, we first define the gain A of the of amplifier by the ratio15 

The sign of this ratio -will be discussed in connection with Fig. 3.29. 

Let us now consider a circuit consisting of a high-gain DC amplifier, an input resistor i i, and a feedback resistor f o (Fig. 3.29). A potential, say e, is applied to the input of the amplifier through the resistor R. Since ig = 0 according to property (3), Kirchoff s law applied to the input of the amplifier gives 

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