Astable Multivibrators ©Paul Godin Created February 2007 Oscillator Basics

Definitions ◊Astable ◊No stable state ◊Produces alternate high/low states ◊Astable Multivibrators are also known as: ◊Clocks ◊Oscillators astable 1.2

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 astable 1.3

Period and Duty Cycle ◊Duty cycle describes the ratio of the time in the high state versus the overall period of the pulse. T tHtH tLtL Review astable 1.4

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 astable 1.5

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. ! astable 1.6

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 astable 1.7

Schmitt-Triggered Oscillators astable 1.8

Oscillator Circuits ◊Describe the output for the following device: astable 1.9

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 astable 1.10

Controlling the Simple Oscillator ◊The output frequency of the oscillator can be adjusted by adding an RC to the circuit: astable 1.11

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 astable 1.12

Simple Oscillator Output Vc: Charge/Discharge Cycle Discharge Time Charge Time Oscillator Animation astable 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. astable 1.14

Calculating Charge/Discharge Instantaneous Voltages astable 1.15

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. astable 1.16

Charge/Discharge Formula (for time) Where: v = change in charge of the capacitor (between VT+ and VT-) E = applied voltage to the capacitor ln = natural log t = time for the charge or discharge astable 1.17

Determining “E” Vcc Gnd Vt+ Vt- E Vcc Gnd Vt+ Vt- E Capacitor is at V T -, about to start a charge cycle: Capacitor is at V T +, about to start a discharge cycle E = the value between the current capacitor charge (V T -) and the applied voltage Vcc. E = the value between the current capacitor charge (V T +) and the applied voltage (Ground). astable 1.18

Application of the Formula ◊Determine the frequency and duty cycle for the following circuit, given: Vt+= 1.6V Vt- = 0.9V R= 1k C= 1F Assume Logic High = 5V and Logic Low = 0V EXAMPLE astable 1.19

Solution Solve for time High (also the rise time for the capacitor voltage): v = Vt+ - Vt- = 1.6V - 0.9V = 0.7V E = Vcc - Vt- = 5V - 0.9V = 4.1V Vcc Gnd Vt+ Vt- E EXAMPLE astable 1.20

Solution Solve for time Low (also the fall time for the capacitor voltage): v = Vt+ - Vt- = 1.6V - 0.9V = 0.7V E = Vt+ - 0 = 1.6V - 0V = 1.6V Vcc Gnd Vt+ Vt- E EXAMPLE astable 1.21

Solution Solve for frequency: T = t H +t L = 187.2s s = 762.6s f = 1/T= 1/762.6s = 1.311kHz Solve for duty cycle: DC= t H /(t H +t L )= 187.2s / 762.6s =24.5% EXAMPLE astable 1.22 Note: this formula will be given for tests/exams

Practice Problem astable 1.23 ◊Determine the frequency and duty cycle for the following circuit, given: Vt+= 1.4V Vt- = 1.1V R= 10k C= 1F Assume Logic High = 5V and Logic Low = 0V

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. astable 1.24

Crystal Oscillators astable 1.25

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 astable 1.26

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. astable 1.27

Crystal Oscillator Circuits There are many different configurations for crystal oscillators. Following are some examples of basic circuits: astable 1.28

Crystal Oscillator Circuits There are many other ways to create a stable oscillation with crystals. astable 1.29

Crystal Oscillator Circuits ◊The following circuit will be used in lab: astable 1.30

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). astable 1.31

Operation of the Simple Oscillator 0 1- Logic 0 read by input of inverter. 1 2-Output becomes logic The capacitor discharges to VT-. 4-Output becomes logic Capacitor voltage increases to VT+. The gate senses a logic 1 input. 1 Animated astable 1.32

©Paul R. Godin prgodin gmail.com END astable 1.33