CHAPTER 14: Nonsinusoidal Oscillators pps

• The term astable covers a group of oscillator circuits, many based on hysteresis in one form or another.. • The old term “multivibrator” is also used to name these circuits.. It goes

CHAPTER 14

Nonsinusoidal

Oscillators

Describe and Analyze:

• Operation of the 555 IC

• Inverter oscillators

• Schmitt oscillators

• Wave-shaping

• Sawtooth oscillators

• Troubleshooting

• There are other ways to make an oscillator besides phase-shifters and resonators

• The term astable covers a group of oscillator

circuits, many based on hysteresis in one form or

another It also covers chips designed for the

purpose, such as the 555

• The old term “multivibrator” is also used to name

these circuits It goes back to vacuum tube days

when they actually used electromechanical vibrators

in circuits

Square-Wave Oscillators

Square wave from a “free-running” 555 circuit

The “Internals” of a 555

Frequency set by R A , R B , and C.

Functions of the 555

• The 555 is still popular after all these years because

it is easy to use It performs two functions:

– Square-wave oscillator (astable)

– One-shot (monostable)

• Strictly speaking, a square-wave has a 50% duty

cycle But unless the duty cycle is low, astables are called square-wave oscillators even if it’s not 50%

• A one-shot produces a fixed-width output pulse

every time it is “triggered” by a rising or falling edge

at its input

555 Oscillator

fOSC = 1.44 / [(R A + 2R B)  C]

555 One-Shot

t = 1.1RC

Inverter Oscillator

fOSC depends on the number of inverters (must be odd)

A Calculation

• For the circuit of the previous slide, find the frequency range if each inverter has a delay of 10 ns  1 ns

Period T = delay  2  # of inverters,

so TLONG = 11 ns  2  3 = 66 ns

and TSHORT = 9 ns  2  3 = 54 ns

So fLO = 1 / 66 ns  15.2 MHz

and fHI = 1 / 54 ns  18.5 MHz

<insert figure 14-15 here>

Commonly used for microprocessor clock

Hysteresis Oscillator

Schmitt trigger circuit on an op-amp

Example Calculation

• For the circuit of the previous slide:

• Let R1 = R2 = R3 = 10 k  Let C1 = 01 μF

• Find the frequency of oscillation

• [Hint: it takes about 1.1 time constants to get 67% voltage on capacitor.]

• The 2:1 divider formed by R2 & R3 keeps the (+) input at

Vout / 2 C1 has to charge up to Vout / 2 to flip the

compara-tor But it starts from –Vout / 2, which is equivalent to charging

from 0 to 2V / 3 with V applied So, 1.1R1C1 = 110 μs, but it

takes two “flips” for one cycle So f = 1 / 220 μs  4.5 kHz

Square to Triangle

Integrating a square wave makes a triangle wave

Triangle to Sine

With enough diodes, the signal is very close to a sine

Sawtooth Oscillator

Also called a “ramp generator”, it can be used to generate the horizontal sweep in a CRT circuit

A Relaxation Oscillator

Shockley diode converts integrator into a “relaxation” oscillator, so called because the diode periodically

relieves the capacitor’s “tension” (voltage)

Sample Calculation

• For the circuit of the previous slide, let the input

resistor R i = 100 k, the feedback capacitor C =

0.1 F, and let Vin = –1 Volt Calculate the frequency

if the Shockley diode “fires” at 10 Volts

• Iin = 1V / 100 k = 10 A, and charging a capacitor with

a constant current means the voltage ramps up

linearly at a rate of V / t = I / C So t = (C / I) V

• The period T = (0.1 F / 10 A)  10 Volts = 0.1 sec

• So f = 1 / T = 10 Hertz

• As always, check all DC voltages

• Typically, these oscillators either work or they do

not; they do not tend to drift

• Frequencies are not precise (except for crystal

stabilized) so oscilloscope measurements are OK

• Though not often used, if an aluminum electrolytic is the timing capacitor, it is a suspect

• If a potentiometer is used to adjust an RC time

constant, check if it has been “tweaked”

• Look for physical damage to components