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9.3 Astable Multivibrator

Return. Next. 9.3 Astable Multivibrator. 9.3.1 Introduction. Astable multivibrator is a kind of self-oscillation circuit . When power is applied It can generate rectangle pulse which has a certain frequency and amplitude.

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9.3 Astable Multivibrator

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  1. Return Next 9.3 Astable Multivibrator 9.3.1 Introduction • Astable multivibrator is a kind of self-oscillation circuit . When power is applied It can generate rectangle pulse which has a certain frequency and amplitude. • Because of the abundant high-order resonance wave ponderance, it named astable multivibrator. • It consists of switch device, feedback network, and delay device. 分量

  2. Return Back Next 9.3 Astable Multivibrator 9.3.2 CMOS Astable Multivibrator Switch device（gate,voltage comparator,BJT etc.） generate high or low level ；feedback network returns output voltage to switch device and makes it change its state.delay device change oscillation frequency by charge and discharge of RC circuit.

  3. VO VDD VI 0.5VDD Return Back Next 9.3 Astable Multivibrator VON= VOFF=Vth=VDD/2 First metastable: t=0 power on，circuit prime state VO1=VOH， VI=VO2=VOL When capacitance is charged， makeVI =Vth The circuit comes into second metastable. Second metastable: Capacitance C discharge， VI decline，when VI =Vth，the circuit turn to first metastable

  4. Return Back Next 9.3 Astable Multivibrator • In RC serial circuit， f(t)= f()+[f(0) - f()] e-t/ Calculation of oscillation period • if Vth = VDD/2 then T=RCln3 2.2RC

  5. Return Back Next CMOS astable multivibrator’s frequency stability is not very good, the oscillation frequency is mainly decided by the time of gate’s input voltage rising to transition level. 9.3 Astable Multivibrator • Transition level is not stable enough，it is affected easily by power and temperature. • The circuit work is easily interfered，lead to the circuit state transition ahead or lag. • On the circuit state transition, charge and discharge is slower. If any slight change of transition level will affect oscillation period badly.

  6. Return Back Next 9.3 Astable Multivibrator CMOS Astable multivibrator witha compensatory resistance generally RS=10R 补偿性的

  7. Return Back Next 9.3 Astable Multivibrator 9.3.3 Quartz crystal oscillator Quartz crystal has stable frequency characteristic and high quality factor，its frequency selectivity is very good.

  8. Return Back 9.3 Astable Multivibrator If the input signal frequency is equal to fs，the quartz crystal’s equivalent impedance is minimum. So the oscillator’ frequency is also fs，has nothing to do with the external resistance and capacitance .

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