Survey
* Your assessment is very important for improving the workof artificial intelligence, which forms the content of this project
* Your assessment is very important for improving the workof artificial intelligence, which forms the content of this project
Negative feedback wikipedia , lookup
Time-to-digital converter wikipedia , lookup
Electronic musical instrument wikipedia , lookup
Resistive opto-isolator wikipedia , lookup
Opto-isolator wikipedia , lookup
Electronic engineering wikipedia , lookup
Regenerative circuit wikipedia , lookup
Chapter 12 Signal generators and waveform-shaping circuits Introduction 12.1 Basic principles of sinusoidal oscillators 12.2 RC oscillator circuits 12.3 LC and crystal oscillators 12.4 Bistable Multivibrators 12.5 Generation of a standardized pulse-The monostable multivibrator Microelectronic Circuits SJTU Yang Hua Introduction (1)linear oscillators: employs a positive-feedback loop consisting of an amplifier and an RC or LC frequency-selective network. (Section 13.1-3) The two different approaches (2)nonlinear oscillators or function generators: • The bistable multivibrator(Section 13.4) • the astable multivibrator (Section 13.5) • the monostable multivibrator(Section 13.6) Microelectronic Circuits SJTU Yang Hua The basic structure of sinusoidal oscillators Amplifier circuit:realize the energy control The basic Frequency-selective network:oscillator frequency is determined structure Positive feedback loop: xi x f amplitude control :implementation of the nonlinear amplitude-stabilization mechanism Microelectronic Circuits SJTU Yang Hua 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. Microelectronic Circuits SJTU Yang Hua 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: Barkhausen criterion. L( j0 ) A( j0 ) ( j0 ) 1 A 2n Microelectronic Circuits SJTU Yang Hua 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. Microelectronic Circuits SJTU Yang Hua The implementation of the nonlinear amplitude-stabilization mechanism The first approach makes use of a limiter circuit The other mechanism for amplitude control utilizes an element whose resistance can be controlled by the amplitude of the output sinusoid. Microelectronic Circuits SJTU Yang Hua A Popular Limiter Circuit for Amplitude Control When vi is close to zero: D1 , D2is off, vo Rf R1 vI R3 R2 vA V vo R2 R3 R2 R3 R5 R4 vB V vo R4 R5 R4 R5 Microelectronic Circuits SJTU Yang Hua A Popular Limiter Circuit for Amplitude Control When vi goes positive,D1 is on, D2 is off R3 R3 L V VD 1 R2 R2 on the contrary: R4 R4 L V VD 1 R5 R 5 Microelectronic Circuits SJTU Yang Hua A Popular Limiter Circuit for Amplitude Control R3 R3 L V VD 1 R2 R2 R4 R4 L V VD 1 R5 R5 Transfer characteristic of the limiter circuit; When Rf is removed, the limiter turns into a comparator with the characteristic shown. Microelectronic Circuits SJTU Yang Hua Oscillator Circuits Op Amp-RC Oscillator Circuits The Wien-Bridge Oscillator The phase-Shift Oscillator LC-Tuned Oscillator Colpitts oscillator Hareley oscillator Crystal Oscillator Microelectronic Circuits SJTU Yang Hua The Wien-Bridge Oscillator A Wien-bridge oscillator without amplitude stabilization. Microelectronic Circuits SJTU Yang Hua Analysis of frequency-selective network for Wien-bridge oscillator (b) Low frequency: 1/wc>>R (c) high frequency: 1/wc<<R Microelectronic Circuits SJTU Yang Hua 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 Microelectronic Circuits SJTU Yang Hua The Wien-Bridge Oscillator A Wien-bridge oscillator with a limiter used for amplitude control. Microelectronic Circuits SJTU Yang Hua 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 Microelectronic Circuits SJTU Yang Hua The Phase-Shift Oscillator A practical phase-shift oscillator with a limiter for amplitude stabilization. Microelectronic Circuits SJTU Yang Hua 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 Microelectronic Circuits C1C2 L( ) C1 C2 SJTU Yang Hua 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 Microelectronic Circuits C1C2 L( ) C1 C2 SJTU Yang Hua Crystal Oscillators A piezoelectric crystal. (a) Circuit symbol. (b) Equivalent circuit. Microelectronic Circuits SJTU Yang Hua Crystal Oscillators Crystal reactance versus frequency (neglecting the small resistance r, ). A series resonance at s 1 LCs A parallel resonance at p 1 Microelectronic Circuits L( Cs C p Cs C p ) SJTU Yang Hua Homework: June 12th, 2008 12.13;12.14 Microelectronic Circuits SJTU Yang Hua Bistable Circuit -three basic factors The output signal only has two states: positive saturation(L+) and negative saturation(L-). The circuit can remain in either state indefinitely and move to the other state only when appropriate triggered.(threshold voltage) The direction of one stage moving to the other stage. A positive feedback loop capable of bistable operation. Microelectronic Circuits SJTU Yang Hua Bistable Circuit The bistable circuit (positive feedback loop) The negative input terminal of the op amp connected to an input signal vI. R1 v vo vo R1 R2 Microelectronic Circuits SJTU Yang Hua Bistable Circuit The transfer characteristic of the circuit in (a) for increasing vI. Positive saturation L+ and negative saturation L- VTH L Microelectronic Circuits SJTU Yang Hua Bistable Circuit The transfer characteristic for decreasing vI. VTL L Microelectronic Circuits SJTU Yang Hua Bistable Circuit The complete transfer characteristics. Microelectronic Circuits SJTU Yang Hua A Bistable Circuit with Noninverting Transfer Characteristics R2 R1 v v I vo R1 R2 R1 R2 Microelectronic Circuits SJTU Yang Hua A Bistable Circuit with Noninverting Transfer Characteristics The transfer characteristic is noninverting. VTH L( R1 R2) VTL L( R1 R2) R1 R2 Microelectronic Circuits SJTU Yang Hua Application of Bistable Circuit as a Comparator Comparator is an analog-circuit building block used in a variety applications. To detect the level of an input signal relative to a preset threshold value. To design A/D converter. Include single threshold value and two threshold values. Hysteresis comparator can reject the interference. Microelectronic Circuits SJTU Yang Hua Application of Bistable Circuit as a Comparator Block diagram representation and transfer characteristic for a comparator having a reference, or threshold, voltage VR. Comparator characteristic with hysteresis. Microelectronic Circuits SJTU Yang Hua Application of Bistable Circuit as a Comparator Illustrating the use of hysteresis in the comparator characteristics as a means of rejecting interference. Microelectronic Circuits SJTU Yang Hua Making the Output Level More Precise For this circuit L+ = VZ1 + VD and L– = –(VZ2 + VD), where VD is the forward diode drop. Microelectronic Circuits SJTU Yang Hua Making the Output Level More Precise For this circuit L+ = VZ + VD1 + VD2 and L– = –(VZ + VD3 + VD4). Microelectronic Circuits SJTU Yang Hua Generation of Square Waveforms Connecting a bistable multivibrator with inverting transfer characteristics in a feedback loop with an RC circuit results in a square-wave generator. Microelectronic Circuits SJTU Yang Hua Generation of Square Waveforms The circuit obtained when the bistable multivibrator is implemented with the positive feedback loop circuit. Microelectronic Circuits SJTU Yang Hua Waveforms at various nodes of the circuit in (b). This circuit is called an astable multivibrator. Time period T = T1+T2 v L ( L L )e t / v L ( L L )e t / T1 RC ln 1 ( L L ) 1 T2 RC ln 1 (L L ) 1 1 T 2RC ln 1 Microelectronic Circuits SJTU Yang Hua Generation of Triangle Waveforms Microelectronic Circuits SJTU Yang Hua Generation of Triangle Waveforms VTH VTL L T1 CR VTH VTL L T2 CR V VTL T1 RC TH L VTH VTL T2 RC L Microelectronic Circuits SJTU Yang Hua Homework: June 19th,2008 12.28; 12.32; 12.33 Microelectronic Circuits SJTU Yang Hua