tailieunhanh - Ebook Electronic circuits - Fundamentals and applications (3rd edition): Part 2

(BQ) Part 2 book "Electronic circuits - Fundamentals and applications" has contents: Oscillators, logic circuits, microprocessors, test equipment and measurements, fault finding, sensors and interfacing, circuit simulation, the 555 timer. | 9 Oscillators This chapter describes circuits that generate sine wave, square wave, and triangular waveforms. These oscillator circuits form the basis of clocks and timing arrangements as well as signal and function generators. Vin ' = Vin + 0Vout Positive feedback and In Chapter 7, we showed how negative feedback can be applied to an amplifier to form the basis of a stage which has a precisely controlled gain. An alternative form of feedback, where the output is fed back in such a way as to reinforce the input (rather than to subtract from it), is known as positive feedback. Figure shows the block diagram of an amplifier stage with positive feedback applied. Note that the amplifier provides a phase shift of 180° and the feedback network provides a further 180°. Thus the overall phase shift is 0°. The overall voltage gain, G, is given by: Overall gain, G = Vout V in Figure Amplifier with positive feedback applied By applying Kirchhoff’s Voltage Law thus Vin = Vin ' 0Vout Vout = Av × Vin where Av is the internal gain of the amplifier. Hence: Overall gain, G = Thus, G = Av × Vin ' Av × Vin ' = Vin ' 0Vout Vin ' 0 ( Av × Vin ') Av 1 0Av Now consider what will happen when the loop gain, 0Av, approaches unity (., when the loop gain is just less than 1). The denominator (1 2 0Av) will become close to zero. This will have the effect of increasing the overall gain, . the overall gain with positive feedback applied will be greater than the gain without feedback. 172 ELECTRONIC CIRCUITS: FUNDAMENTALS AND APPLICATIONS It is worth illustrating this difficult concept using some practical figures. Assume that you have an amplifier with a gain of 9 and one-tenth of the output is fed back to the input (. 0 = ). In this case the loop gain (0 × Av) is . With negative feedback applied (see Chapter 7) the overall voltage gain will be: Av 9 9 9 = = = = 1 + 0 Av 1 + ( × 9 ) 1 + G= With positive feedback applied the overall .