Transcript Astable Multivibrators

Astable Multivibrators
Oscillator Basics
Technician Series
©Paul Godin
Created February 2007
Modified March 2015
Definitions
◊ Astable
◊ No stable state
◊ Produces alternate high/low states
◊ Astable Multivibrators are also known as:
◊ Clocks
◊ Oscillators
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Uses of Astables
◊ Provide edges for edge-triggered devices
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flip-flops
counters
shift registers
Digital to Analog / Analog to Digital converters
microprocessors
communications, etc…
◊ Can provide sound for certain applications
◊ practical audible sound in the 100Hz to 5kHz range
◊ exercise caution when applying square waves
to speakers
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Period and Duty Cycle
◊
Review
Duty cycle describes the ratio of the time in the high state
versus the overall period of the pulse.
T
tH
tL
tH
D.C. 
 100%
tH  tL
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Does Duty Cycle Matter?
Review
◊ To an edge-triggered device, does the duty
cycle affect its operation?
◊ If a 10% D.C. clock is applied to the following
circuit, what is the output D.C.?
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Square waves and speakers
◊ Cautions:
!
◊ The average power for a square wave is higher than
for a sine wave with the same peak voltage. Speaker
coil damage may result.
◊ A speaker is an electro-mechanical device. It is
physically unable to produce the instantaneous
motion of a square wave. Damage to the cone and
physical structure may result.
◊ Speakers have a low impedance and likely represents
a greater load than the driving circuit is capable of
handling. Damage to the driving circuit may result.
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Speaker Interfaces
◊
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Use cheap speakers!
Keep the output voltages low.
Use an output device that can handle the load.
Filter the output square waves
◊ Use an RC circuit in series.
◊ Use an audio transformer.
Discussion in class
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Schmitt-Triggered Oscillators
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Oscillator Circuits
◊ Describe the output for the following device:
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Oscillator Parameters
◊ In
◊
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the previous oscillator circuit:
What determines the output frequency?
What is the waveform of the output?
What determines the duty cycle?
◊ How can we slow the process down?
In-Class Discussion
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Controlling the Simple Oscillator
◊ The output frequency of the oscillator can be
adjusted by adding an RC to the circuit:
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Schmitt Oscillator
◊ The separation between Vt+ and Vt- can be used
to create an oscillating circuit.
◊ An RC network is used to control the oscillation
rate by controlling the charge and discharge time
of the capacitor voltage.
◊ Easy oscillator to build. Used where precise or
accurate frequency isn’t necessary.
◊ displays
◊ visual effects
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Simple Oscillator Output
Vc: Charge/Discharge Cycle
Charge Time
Discharge Time
Oscillator Animation
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Schmitt Trigger Oscillator Control
Schmitt Triggered Oscillators may be controlled
by the use of RC circuits.
To achieve a specific frequency, the values of R
and C may be calculated.
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The Simple Schmitt Oscillator
◊ Advantages:
◊ Easy to build
◊ Fair range of frequency
◊ Small footprint
◊ Disadvantages:
◊ Unstable, as the frequency will vary with temperature
variations.
◊ Difficult to predict values due to the range of Vt+ and
Vt- between different gates, even within the same IC
package.
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Crystal Oscillators
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Crystal Oscillators
◊ Crystal Oscillators are commonly used in
conjunction with microprocessors,
communications circuits and other frequencysensitive devices because of their:
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reliability
stability
accuracy
ease of use
Symbol
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Crystal Oscillators
◊
A crystal oscillator is constructed from a piece of quartz
crystal that is cut and shaped to the appropriate size.
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A property called piezoelectricity happens with quartz
crystals.
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If pressure is applied, it creates voltage
If voltage is applied, it physically vibrates
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When a voltage is applied to the crystal, it vibrates at a
very specific frequency.
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Crystal oscillators commonly require small capacitors
to aid with the back-and-forth voltage, and require a
source of current.
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Crystal Oscillator Circuits
There are many different configurations for crystal
oscillators. Following are some examples of basic circuits:
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Crystal Oscillator Circuits
There are many other ways to create a
stable oscillation with crystals.
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Crystal Oscillator Circuits
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Crystal Oscillators
◊
Crystal Oscillators are often packaged in an ovalshaped metallic “can”.
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Those with 2 leads require external circuitry; those
with 4 leads typically already possess the internal
circuitry required to produce the oscillation (voltage
and ground needs to be applied).
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Operation of the Simple Oscillator
1- Logic 0 read by
input of inverter.
4-Output becomes
logic 0.
5- The capacitor
discharges to VT-.
1
0
0
1
2-Output becomes
logic 1.
3- Capacitor voltage
increases to VT+. The
gate senses a logic 1
input.
Animated
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END
©Paul R. Godin
prgodin°@ gmail.com
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