CHAPTER 5 - OSCILLATORS

Oscillators

Describe the basic concept of an oscillator

Discuss the basic principles of operation of an

oscillator

Analyze the operation of RC, LC and crystal

oscillators

Describe the operation of the basic relaxation

oscillator circuits

Objectives

OscillatorsIntroduction

• An oscillator is a circuit that produces a periodic

waveform on its output with only the dc supply

voltage as an input.

• A repetitive input signal is not required except to

synchronize oscillations in some applications

• The output voltage can be sinusoidal or

nonsinusoidal, depending on the type of

oscillator.

Oscillators

Oscillators

Two major classifications for oscillators are feedback oscillators and relaxation oscillators.

• The feedback oscillator returns a fraction of the output signal to the input with no net phase shift, resulting in a reinforcement of the output signal.

• The relaxation oscillator makes use of an RC timing circuit to generate a waveform that is generally square wave or other nonsinusoidal waveform.

Oscillators

Feedback Oscillators

Feedback Oscillators

Positive feedback:• is characterized by the condition wherein the portion

of the output voltage of an amplifier is fed to the input

with no net phase shift, resulting in a reinforcement of

the output signal.• This is called “oscillation”.

In phase

Noninverting amplifier

V f V oA v

Feedback circuit

If the feedback circuit returns the signal out of phase, an inverting amplifier produces positive feedback.

Feedback OscillatorsConditions for oscillation:

2. The voltage gain ACL, around the closed feedback loop (loop gain) must equal to 1 (unity).

1. The phase shift around the feedback loop must be 0°.

Start-Up Conditions:• When oscillation starts at t0, the condition ACL > 1 causes the sinusoidal output voltage amplitude to build up to a desired level. • Then ACL decreases to 1 and maintains the desired amplitude.

Feedback Oscillators

Oscillators

Oscillators with RC Feedback Circuits

RC Feedback Circuits

Three types of feedback oscillators that use RC circuits to produce sinusoidal outputs are:

• Wien-bridge oscillator – most widely used type

• Phase-shift oscillator

• Twin-T oscillator

The Wien-Bridge Oscillator

• Wien-Bridge Oscillator is a lead-lag circuit.

• R1 and C1 form the lag portion and R2 and C2 form the lead portion

• At lower frequencies, the lead circuit dominates due to the high reactance of C2.

•As the frequencies increases, XC2 decreases, thus allowing the output voltage to increase.

RC Oscillators – Wien–bridge oscillator

V in V out

The lead-lag circuit of a Wien-bridge oscillator reduces the input signal by 1/3 and yields a response curve as shown.

• The resonant frequency can be determined by the formula below.

• Below fr, the lead circuit dominates and the output leads the input. Above fr, the lag circuit dominates and the output lags the input.

RCfr 2

1

RC Oscillators – Wien-bridge

RC Oscillators – Wien-bridge

It is a low frequency oscillator which ranges from a few kHz to 1 MHz.

Structure of this oscillator is shown below;

Vout

–

+

R4

R3

R2

R1

C1C2 Lead-lag

network

Voltage-divider

The lead-lag circuit is in the positive feedback loop of Wien-bridge oscillator. The voltage divider limits the gain. The lead lag circuit is basically a band-pass with a narrow bandwidth.

RC Oscillators – Wien-bridge

Since there is a loss of about 1/3 of the signal in the positive feedback loop, the voltage-divider ratio must be adjusted such that a positive feedback loop gain of 1 is produced. This requires a closed-loop gain of 3. The ratio of R1 and R2 can be set to achieve this.

RC Oscillators – Wien-bridge

To start the oscillations an initial loop gain greater than 1 must be achieved.

RC Oscillators – Wien-bridge

The back-to-back zener diode

arrangement is one way of achieving

this.

RC Oscillators – Wien-bridge oscillator using back-to-back zener diode

.

R 1

R 2

R 3

D 1 D 2

+

-

V out

Lead-lag1/3

f r

When dc is first applied the zeners

appear as opens. This allows the slight

amount of positive feedback from turn

on noise to pass.

RC Oscillators – Wien-bridge

The lead-lag circuit narrows the

feedback to allow just the desired

frequency of these turn transients to

pass. The higher gain allows

reinforcement until the breakover

voltage for the zeners is reached.

•Automatic gain control is necessary to maintain a gain of exact unity.

•The zener arrangement for gain control is simple but produces distortion because of the nonlinearity of zener diodes.

•A JFET in the negative feedback loop can be used to precisely control the gain.

•After the initial startup and the output signal increases, the JFET is biased such that the negative feedback keeps the gain at precisely 1.

RC Oscillators – Wien-bridge oscillator using a JFET negative feedback loop

RC Oscillators – Wien-bridge

RC Oscillators – Wien-bridge

EXAMPLE

Rf

R3

R1

R2

C1

Q1

C2

–

+

Vout

C3R4

D1

10 k

1.0 k 10 k

680

680

4.7 nF

4.7 nF

1.0 F

What is fr for the Wien bridge shown in figure below?

1

2π1

2π 680 4.7 nF

rf RC

= 48.9 kHz

RC Oscillators – Phase-shift

The attenuation, B of the three-section RC feedback circuit is:

29

1B Where,

fR

RB

3

RC Oscillators – Phase-shift

• Each of the three RC circuits in the feedback loop can provide a maximum phase shift approaching 90o.

• Oscillation occurs at the frequency where the total phase shift through the three RC circuits is 180o.

• The frequency of oscillation is shown in the following equation:

RCfr

62

1

where R1 = R2 = R3 = R and C1 = C2 = C3 = C

RC Oscillators – Phase-shift - EXAMPLE

(a)Determine the value of Rf necessary for the circuit below to operate as an oscillator.

(b) Determine the frequency of oscillation

Answer: Rf = 290 kOhm, fr = 6.5 kHz

RC Oscillators – Twin-T Oscillator

• Two T-type RC filters used in the feedback loop

• One of the twin-T filters has a low-pass response, and the other has a high-pass response

• The combined parallel filters produce a band-stop response with a center frequency equal to frequency of oscillation, fr.

RC Oscillators – Twin-T Oscillator