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Chapter 14 Feedback and Oscillator Circuits Feedback Concepts The effects of negative feedback on an amplifier: Disadvantage • Lower gain Advantages • Higher input impedance • More stable gain • Improved frequency response • Lower output impedance • Reduced noise • More M linear li operation ti Electronic Devices and Circuit Theory, 10/e Robert L. Boylestad and Louis Nashelsky 2 Copyright ©2009 by Pearson Education, Inc. Upper Saddle River, New Jersey 07458 • All rights reserved. Feedback Connection Types • • • • Voltage-series feedback Voltage sh nt feedback Voltage-shunt Current-series feedback Current-shunt feedback Electronic Devices and Circuit Theory, 10/e Robert L. Boylestad and Louis Nashelsky 3 Copyright ©2009 by Pearson Education, Inc. Upper Saddle River, New Jersey 07458 • All rights reserved. Voltage--Series Feedback Voltage For voltage-series F lt i feedback, f db k th the output voltage is fed back in series to the input. The feedback gain is given by: Af ≅ 1 R1 + R 2 = β R2 Electronic Devices and Circuit Theory, 10/e Robert L. Boylestad and Louis Nashelsky 4 Copyright ©2009 by Pearson Education, Inc. Upper Saddle River, New Jersey 07458 • All rights reserved. Voltage--Shunt Feedback Voltage For a voltage-shunt feedback amplifier, the output voltage is fed back in parallel with the input. The feedback gain is given by Af = − Electronic Devices and Circuit Theory, 10/e Robert L. Boylestad and Louis Nashelsky Ro Ri 5 Copyright ©2009 by Pearson Education, Inc. Upper Saddle River, New Jersey 07458 • All rights reserved. Current--Series Feedback Current For a current current-series series feedback amplifier, a portion of the output current is fed back in series with the input. To determine the feedback gain: I A Af = o = = Vs 1 + βA − h fe h ie ⎛ − h fe 1 + ( − R E )⎜⎜ ⎝ h ie + R E Electronic Devices and Circuit Theory, 10/e Robert L. Boylestad and Louis Nashelsky ⎞ ⎟⎟ ⎠ 6 ≅ − h fe h ie + h fe R E Copyright ©2009 by Pearson Education, Inc. Upper Saddle River, New Jersey 07458 • All rights reserved. Current--Shunt Feedback Current For a current-shunt feedback amplifier, a portion of the output current is directed back in parallel with the input. The feedback g gain is given by: I Af = o Is Electronic Devices and Circuit Theory, 10/e Robert L. Boylestad and Louis Nashelsky 7 Copyright ©2009 by Pearson Education, Inc. Upper Saddle River, New Jersey 07458 • All rights reserved. Summary of Feedback Effects Electronic Devices and Circuit Theory, 10/e Robert L. Boylestad and Louis Nashelsky 8 Copyright ©2009 by Pearson Education, Inc. Upper Saddle River, New Jersey 07458 • All rights reserved. Frequency Distortion with Feedback • If the feedback network is p purely y resistive,, then the ggain with feedback will be less dependent on frequency variations. In some cases the resistive feedback removes all dependence on frequency variations. • If the feedback includes frequency dependent components (capacitors and inductors), then the frequency response of the amplifier ifi will i be affected. ff Electronic Devices and Circuit Theory, 10/e Robert L. Boylestad and Louis Nashelsky 9 Copyright ©2009 by Pearson Education, Inc. Upper Saddle River, New Jersey 07458 • All rights reserved. Noise and Nonlinear Distortion • The feedback network reduces noise by cancellation. cancellation The phase of the feedback signal is often opposite the phase of the input signal. • Nonlinear distortion is also reduced simply because the gain is reduced. The amplifier is operating in midrange and not at the extremes. Electronic Devices and Circuit Theory, 10/e Robert L. Boylestad and Louis Nashelsky 10 Copyright ©2009 by Pearson Education, Inc. Upper Saddle River, New Jersey 07458 • All rights reserved. Bandwidth with Feedback Feedback increases the bandwidth of an amplifier. Electronic Devices and Circuit Theory, 10/e Robert L. Boylestad and Louis Nashelsky 11 Copyright ©2009 by Pearson Education, Inc. Upper Saddle River, New Jersey 07458 • All rights reserved. Gain Stability with Feedback Gain G i calculations l l ti with ith feedback f db k are often ft based b d on external t l resistive elements in the circuit. By removing gain calculations from internal variations of β and gm, the gain becomes more stable. stable Electronic Devices and Circuit Theory, 10/e Robert L. Boylestad and Louis Nashelsky 12 Copyright ©2009 by Pearson Education, Inc. Upper Saddle River, New Jersey 07458 • All rights reserved. Phase and Frequency Considerations At higher frequencies the feedback signal may no longer be out of phase with the input. The feedback is thus positive and the amplifier, itself, becomes unstable and begins to Electronic Devices and Circuit Theory, 10/e Robert L. Boylestad and Louis Nashelsky 13 Copyright ©2009 by Pearson Education, Inc. Upper Saddle River, New Jersey 07458 • All rights reserved. Oscillator Operation The feedback signal must be positive. The overall gain must equal one (unity gain). If the feedback signal is not positive or the gain is less than one, the oscillations dampens out. Electronic Devices and Circuit Theory, 10/e Robert L. Boylestad and Louis Nashelsky If the overall gain is greater than one, the oscillator eventually saturates saturates. 14 Copyright ©2009 by Pearson Education, Inc. Upper Saddle River, New Jersey 07458 • All rights reserved. Types of Oscillator Circuits Phase-shift oscillator PhaseWien bridge oscillator Tuned oscillator circuits Crystal oscillators Unijunction oscillator Electronic Devices and Circuit Theory, 10/e Robert L. Boylestad and Louis Nashelsky 15 Copyright ©2009 by Pearson Education, Inc. Upper Saddle River, New Jersey 07458 • All rights reserved. Phase--Shift Oscillator Phase The amplifier must supply enough gain to compensate for losses. The overall gain must be unity. unity The RC networks provide the necessary phase shift for a positive feedback. The values of the RC components also determine the frequency of oscillation: f= 1 2πRC 6 more… Electronic Devices and Circuit Theory, 10/e Robert L. Boylestad and Louis Nashelsky 16 Copyright ©2009 by Pearson Education, Inc. Upper Saddle River, New Jersey 07458 • All rights reserved. Phase--Shift Oscillator Phase The amplifier must supply enough gain to compensate for losses. The overall gain must be unity. unity The RC networks provide the necessary phase shift for a positive feedback. The values of the RC components also determine the frequency of oscillation: f= 1 2π 6RC more… Electronic Devices and Circuit Theory, 10/e Robert L. Boylestad and Louis Nashelsky 17 Copyright ©2009 by Pearson Education, Inc. Upper Saddle River, New Jersey 07458 • All rights reserved. Wien Bridge Oscillator The amplifier must supply enough gain to compensate for losses. The overall gain must be unity. • The feedback resistors are R3 and R4. • The phase-shift components are R1, C1 and R2, C2. Electronic Devices and Circuit Theory, 10/e Robert L. Boylestad and Louis Nashelsky 18 Copyright ©2009 by Pearson Education, Inc. Upper Saddle River, New Jersey 07458 • All rights reserved. Tuned Oscillator Circuits Tuned oscillators use a parallel LC resonant circuit (LC tank) to provide the oscillations. There are two common types: Colpitts—The resonant circuit is an inductor and two capacitors. Colpitts Hartley Hartley—The resonant circuit is a tapped inductor or two i d inductors and d one capacitor. i Electronic Devices and Circuit Theory, 10/e Robert L. Boylestad and Louis Nashelsky 19 Copyright ©2009 by Pearson Education, Inc. Upper Saddle River, New Jersey 07458 • All rights reserved. Colpitts Oscillator Circuit The frequency of oscillation is determined by: fo = 1 2π LC eq where: C eq = C1C 2 C1 + C 2 Electronic Devices and Circuit Theory, 10/e Robert L. Boylestad and Louis Nashelsky 20 Copyright ©2009 by Pearson Education, Inc. Upper Saddle River, New Jersey 07458 • All rights reserved. Hartley Oscillator Circuit The frequency of oscillation is determined by: fo = 1 2π L eq C where: L eq = L 1 + L 2 + 2M Electronic Devices and Circuit Theory, 10/e Robert L. Boylestad and Louis Nashelsky 21 Copyright ©2009 by Pearson Education, Inc. Upper Saddle River, New Jersey 07458 • All rights reserved. Crystal Oscillators The crystal appears as a resonant circuit. The crystal has two resonant frequencies: f i Series resonant condition • RLC determine the resonant frequency q y • The crystal has a low impedance Parallel resonant condition • RL and d CM determine d t i th the resonantt ffrequency • The crystal has a high impedance The series and p parallel resonant frequencies q are very y close, within 1% of each other. Electronic Devices and Circuit Theory, 10/e Robert L. Boylestad and Louis Nashelsky 22 Copyright ©2009 by Pearson Education, Inc. Upper Saddle River, New Jersey 07458 • All rights reserved. Series Resonant Crystal Oscillator • RLC determine the resonant frequency • The crystal has a low impedance Electronic Devices and Circuit Theory, 10/e Robert L. Boylestad and Louis Nashelsky 23 Copyright ©2009 by Pearson Education, Inc. Upper Saddle River, New Jersey 07458 • All rights reserved. Parallel Resonant Crystal Oscillator • RL and CM determine the resonant frequency • The crystal has a high impedance Electronic Devices and Circuit Theory, 10/e Robert L. Boylestad and Louis Nashelsky 24 Copyright ©2009 by Pearson Education, Inc. Upper Saddle River, New Jersey 07458 • All rights reserved. Unijunction Oscillator The output frequency is determined by: fo = 1 R T C T ln[1 (1 − η)] Where η is a ratingg of the unijunction transistor with values between 0.4 and 0.6. Electronic Devices and Circuit Theory, 10/e Robert L. Boylestad and Louis Nashelsky 25 Copyright ©2009 by Pearson Education, Inc. Upper Saddle River, New Jersey 07458 • All rights reserved. Unijunction Oscillator Waveforms The unijunction oscillator (or relaxation oscillator) produces a sawtooth waveform. Electronic Devices and Circuit Theory, 10/e Robert L. Boylestad and Louis Nashelsky 26 Copyright ©2009 by Pearson Education, Inc. Upper Saddle River, New Jersey 07458 • All rights reserved.