Transcript 9.3 Astable Multivibrator

9.3 Astable Multivibrator
9.3.1 Introduction
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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.
分量
It consists of switch device, feedback network,
and delay device.
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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.
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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， make VI = Vth
The circuit comes into
second metastable.
VO
VDD
Second metastable:
Capacitance C discharge， VI decline，when
0.5VDD
VI VI = Vth，the circuit turn to first metastable
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9.3 Astable Multivibrator
Calculation of oscillation period
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In RC serial circuit， f(t)= f()+[f(0) - f()] e-t/
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if Vth = VDD/ 2 then T=RCln3 2.2RC
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9.3 Astable Multivibrator
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.
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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.
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9.3 Astable Multivibrator
CMOS Astable multivibrator with a
compensatory resistance
generally RS=10R
补偿性的
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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.
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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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