Astable Multivibrators

©Paul GodinCreated February 2007

Oscillator Basics

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◊ 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

◊ Duty cycle describes the ratio of the time in the high state versus the overall period of the pulse.

%100tt

tD.C.

LH

H

T

tH tL

Review

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Does Duty Cycle Matter?

◊ 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.?

Review

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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

◊ 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 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

Discharge Time

Charge Time

Oscillator Animationastable 1.13

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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Calculating Charge/Discharge Instantaneous Voltages

)1()( RC

t

initAppliedinit eEEEv

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RC Calculation Method

◊ Get the Vt+ and the Vt- from the Schmitt-triggered inverter.◊ Note the specifications are for an average device. Your

specific device’s values may be a little different.

◊ Calculate the Rise and Fall times using the charge/discharge formulas for RC circuits.

◊ Add the time low and the time high to get the period.

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Charge/Discharge Formula(for time)

Ev

RCt1

1ln

Where:v = change in charge of the capacitor (between VT+ and VT-)E = applied voltage to the capacitor ln = natural logt = time for the charge or discharge

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Determining “E”

Vcc

Gnd

Vt+

Vt-

E

Vcc

Gnd

Vt+

Vt- E

Capacitor is at VT-, about to start a charge cycle:

Capacitor is at VT+, about to start a discharge cycle

E = the value between the current capacitor charge

(VT-) and the applied voltage Vcc.

E = the value between the current capacitor charge

(VT+) and the applied voltage (Ground).

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Application of the Formula

◊ Determine the frequency and duty cycle for the following circuit, given:

Vt+= 1.6VVt- = 0.9V

R= 1kC= 1F

Assume Logic High = 5V andLogic Low = 0V

Ev

RCt1

1ln

EXAMPLE

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Solution

Solve for time High (also the rise time for the capacitor voltage):

v = Vt+ - Vt- = 1.6V - 0.9V = 0.7VE = Vcc - Vt- = 5V - 0.9V = 4.1V

s

VV

Fk

Ev

RCt

2.187

1.47.01

1ln11

1

1ln

Vcc

Gnd

Vt+

Vt-

E

EXAMPLE

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Solution

Solve for time Low (also the fall time for the capacitor voltage):

v = Vt+ - Vt- = 1.6V - 0.9V = 0.7VE = Vt+ - 0 = 1.6V - 0V = 1.6V

s

VV

Fk

Ev

RCt

4.575

6.17.01

1ln11

1

1ln

Vcc

Gnd

Vt+

Vt- E

EXAMPLE

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Solution

Solve for frequency:

T = tH+tL= 187.2s + 575.4s = 762.6s

f = 1/T= 1/762.6s = 1.311kHz

Solve for duty cycle:

DC= tH/(tH+tL)= 187.2s / 762.6s =24.5%

EXAMPLE

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Note: this formula will be given for tests/exams

Practice Problem

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◊ Determine the frequency and duty cycle for the following circuit, given:

Vt+= 1.4VVt- = 1.1V

R= 10kC= 1F

Assume Logic High = 5V andLogic Low = 0V

Ev

RCt1

1ln

The Simple 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 frequency-sensitive devices because of their:◊ 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.

◊ A property called piezoelectricity happens with quartz crystals.◊ If pressure is applied, it creates voltage◊ If voltage is applied, it physically vibrates

◊ When a voltage is applied to the crystal, it vibrates at a very specific frequency.

◊ 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

◊ The following circuit will be used in lab:

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Crystal Oscillators

◊ Crystal Oscillators are often packaged in an oval-shaped metallic “can”.

◊ 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

0

1- Logic 0 read by input of inverter.

1

2-Output becomes logic 1.

5- The capacitor discharges to

VT-.

4-Output becomes logic 0.

0

3- Capacitor voltage increases to VT+. The gate senses a

logic 1 input.

1

Animated

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©Paul R. Godinprgodin°@ gmail.com

END

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