1. 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. 9.3 Astable Multivibrator 9.3.1 Introduction 分量 ReturnNext

2. 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. NextBackReturn

3. NextBackReturn 9.3 Astable Multivibrator VOVO VIVI V DD 0.5V DD V ON = V OFF = V th = V DD /2 First metastable: t=0 power on ， circuit prime state V O1 = V OH ， V I = V O2 = V OL When capacitance is charged ， make V I = V th The circuit comes into second metastable. Second metastable: Capacitance C discharge ， V I decline ， when V I = V th ， the circuit turn to first metastable

4. 9.3 Astable Multivibrator Calculation of oscillation period In RC serial circuit ， f(t)= f(  )+[f(0) - f(  )] e -t/  if V th = V DD / 2 then T=RCln3  2.2RC NextBackReturn

5. 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. 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. 9.3 Astable Multivibrator NextBackReturn

6. 9.3 Astable Multivibrator CMOS Astable multivibrator with a compensatory resistance generally R S =10R NextBackReturn 补偿性的

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

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