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Chapter 8 Oscillator and Power Amplifier SJTU Zhou Lingling 1 Outline • Oscillator • Power amplifier SJTU Zhou Lingling 2 Oscillator • Basic principles of sinusoidal oscillator. • The Wien-bridge oscillator • The phase shift oscillator SJTU Zhou Lingling 3 Basic Principles of Sinusoidal Oscillator • The oscillator feedback loop • The basic structure of a sinusoidal oscillator. • A positive-feedback loop is formed by an amplifier and a frequency-selective network. SJTU Zhou Lingling 4 Basic Principles of Sinusoidal Oscillator • Feedback signal xf is summed with a positive sign • The gain-with-feedback is A( s） Af ( s) 1 A( s) ( s) • The oscillation criterion L( j0 ) A( j0 ) ( j0 ) 1 SJTU Zhou Lingling 5 Basic Principles of Sinusoidal Oscillator • Nonlinear amplitude control To ensure that oscillations will start, the Aβ is slightly greater than unity. As the power supply is turned on, oscillation will grown in amplitude. When the amplitude reaches the desired level, the nonlinear network comes into action and cause the Aβ to exactly unity. SJTU Zhou Lingling 6 A Popular Limiter Circuit for Amplitude Control SJTU Zhou Lingling 7 A Popular Limiter Circuit for Amplitude Control Transfer characteristic of the limiter circuit; When Rf is removed, the limiter turns into a comparator with the characteristic shown. SJTU Zhou Lingling 8 Oscillator Circuits • Op Amp-RC Oscillator Circuits The Wien-Bridge Oscillator The phase-Shift Oscillator • LC-Tuned Oscillator Colpitts oscillator Hareley oscillator • Crystal Oscillator SJTU Zhou Lingling 9 The Wien-Bridge Oscillator A Wien-bridge oscillator without amplitude stabilization. SJTU Zhou Lingling 10 The Wien-Bridge Oscillator • The loop gain transfer function 1 R2 R1 L( s ) 3 sCR 1 sCR • Oscillating frequency 0 1 RC • To obtain sustained oscillation R2 R1 2 SJTU Zhou Lingling 11 The Wien-Bridge Oscillator A Wien-bridge oscillator with a limiter used for amplitude control. SJTU Zhou Lingling 12 The Phase-Shift Oscillator The circuit consists of a negative-gain amplifier and three-section RC ladder network. Oscillating frequency is the one that the phase shift of the RC network is 1800 SJTU Zhou Lingling 13 The Phase-Shift Oscillator A practical phase-shift oscillator with a limiter for amplitude stabilization. SJTU Zhou Lingling 14 The LC-Tuned oscillator Colpitts Oscillator A parallel LC resonator connected between collector and base. Feedback is achieved by way of a capacitive divider Oscillating frequency is determined by the resonance frequency. 0 1 SJTU Zhou Lingling C1C2 L( ) C1 C2 15 The LC-Tuned oscillator Hartley Oscillator A parallel LC resonator connected between collector and base. Feedback is achieved by way of an inductive divider. Oscillating frequency is determined by the resonance frequency. 0 1 SJTU Zhou Lingling C1C2 L( ) C1 C2 16 Crystal Oscillators A piezoelectric crystal. (a) Circuit symbol. (b) Equivalent circuit. SJTU Zhou Lingling 17 Crystal Oscillators Crystal reactance versus frequency (neglecting the small resistance r, ). A series resonance at s 1 LCs A parallel resonance at p 1 SJTU Zhou Lingling Cs C p L( ) Cs C p 18 Crystal Oscillators A Pierce crystal oscillator utilizing a CMOS inverter as an amplifier. SJTU Zhou Lingling 19 Power Amplifier • Small-signal approximation and models either are not applicable or must be used with care. • Deliver the power to the load in efficient manner. • Power dissipation is as low as possible. SJTU Zhou Lingling 20 Classification of Power Amplifier • Power amplifiers are classified according to the collector current waveform that results when an input signal is applied. • Conducting angle. SJTU Zhou Lingling 21 Classification of Power Amplifier Collector current waveforms for transistors operating in (a) class A, (b) class B SJTU Zhou Lingling 22 Classification of Power Amplifier class AB class C SJTU Zhou Lingling 23 Class B Output Stage A class B output stage. Complementary circuits. Push-pull operation Maximum powerconversion efficiency is 78.5% SJTU Zhou Lingling 24 Transfer Characteristic SJTU Zhou Lingling 25 Crossover Distortion SJTU Zhou Lingling 26 Power Dissipation • The load power 2 1 Vˆo PL 2 RL • Maximum load power PL max 2 1 Vˆo 2 RL 2 Vˆo VCC V CC 2 RL SJTU Zhou Lingling 27 Power Dissipation • Total supply power 2 Vˆo Ps VCC RL • Maximum total supply power Ps max 2 2 Vˆo 2 VCC VCC RL RL Vˆ V o CC SJTU Zhou Lingling 28 Power Dissipation • Power-conversion efficiency Vˆo 4 VCC • Maximum power-conversion efficiency max Vˆo 4 VCC 78.5% Vˆo VCC SJTU Zhou Lingling 29 Power Dissipation • Power dissipation 2 2 Vˆo 1 Vˆo PD VCC RL 2 RL • Maximum Power dissipation PDN max PDP max 2 2 Vˆo 1 Vˆo VCC RL 2 RL 2 Vˆo VCC 2 2VCC 0.2 PL max 2 RL SJTU Zhou Lingling 30 Class AB Output Stage A bias voltage VBB is applied between the bases of QN and QP, giving rise to a bias current IQ . Thus, for small vI, both transistors conduct and crossover distortion is almost completely eliminated. SJTU Zhou Lingling 31 A Class AB Output Stage Utilizing Diodes for Biasing SJTU Zhou Lingling 32 A Class AB Output Stage Utilizing A VBE Multiplier for Biasing SJTU Zhou Lingling 33