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OSCILLATORS & MULTIVIBRATORS

OSCILLATOR

· An oscillator is a circuit that is designed to produce an output waveform without any external AC source.

· The only input to an oscillator is a DC supply.

· The key to its operation is positive feedback

· The feedback voltage (vf) is in phase with the input voltage (vin).

· The amplifier produces a 180° phase shift.

· The feedback network must also produce a 180° phase shift.

· Positive feedback is known as regenerative feedback.

· An oscillator needs a brief trigger signal to get it started. Usually, by turning on the circuit will create the trigger signal.

· Two key things for an oscillator to work:

· positive feedback or regenerative feedback

· a trigger signal

But a third thing must be present to keep oscillations going.

· Barkhausen criterion

· The amplifier produces a voltage gain (Av)

· The feedback network produces a loss or attenuation (αv)

· To sustain oscillation;

· (Av)(αv) = 1

· If (Av)(αv) < 1, the oscillation die out in a few cycles. Called damping.

· If (Av)(αv) > 1, the oscillator will go into saturation and cutoff clipping.

· Because of power loss in the circuit components, the (Av)(αv) must be slightly greater than 1.

TYPES OF OSCILLATORS

1. Phase – Shift

a. Uses three RC circuits in its feedback circuit

b. Phase shift calculated by:

i. Θ = tan -1(-Xc/R)

c. Because there are three RC networks, and each produces 60° phase – shift you would expect the 180° total shift. Not true. Each network loads the previous network which causes the phase shift to not be equal.

d. Phase shift oscillators not used because:

i. They are unstable in frequency

ii. And in amplitude.

2. Wien – Bridge

a. It is a low frequency oscillator that achieves regenerative feedback by having 0° phase shift in the positive feedback path.

b. R1C1 is a low-pass filter, R2C2 is a high-pass filter, and both have the same cutoff frequency (R1C1 = R2C2).

c. Combined these filters create a band-pass filter and a band-pass filter has no phase shift.

d. The diodes D1 and D2 limit the feedback voltage to resistors in the negative feedback circuit.

e. Trimmer resistors can be added in series with R1 and R2 to fine tune the oscillator.

f. Because of propagation delay caused by the components, the Wien-Bridge is used below 1MHZ.

3. Colpitts Oscillator

a. Uses a discrete LC oscillator circuit, called a tank circuit, to produce the regenerative feedback.

b. fr = 1/2π

c. The feedback circuit produces a 180° phase shift by:

i. The amplifier output voltage is developed across C1

ii. The feedback voltage is developed across C2.

iii. The voltage across C2 is 180° out of phase with the voltage across C1.

d. Attenuation factor is: (αv) = XC2/XC1 = C1/C2

e. Circuit gain: (Av) = Vout/Vf ≈ C2/C1

f. Because some efficiency is lost when connected to resistive loads the Colpitts is often transformer-coupled to the load.

4. Hartley Oscillator

a. Is similar to the Colpitts except it uses two inductors and one capacitor.

b. Attenuation factor is: (αv) = XL2/XL1 = L2/L1

5. Clapp Oscillator

a. A Clapp oscillator is a Colpitts with an extra capacitor in series with the inductor.

b. The function of C3 is to reduce the effects of the junction capacitance of the transistor on the operating frequency.

c. C3 is always much smaller that C1 or C2 and becomes dominant is any frequency calculation.

d. fr = 1/2π

6. Armstrong Oscillator

a. The Armstrong oscillator uses a transformer to get the 180° phase shift needed for oscillation.

b. Key to the transformer is the winding direction. Notice the polarity dots on the transformer.

c. C1 and the primary of the transformer (inductance) determine the frequency.

IDENTIFYING FEATURES OF LC OSCILLATORS
Oscillator	Circuit Recognition Feature

Colpitts	Tapped capacitors
Hartley	Tapped inductors or a tapped transformer with a single parallel capacitor.
Clapp	Looks like a Colpitts with an added capacitor in series with the coil.
Armstrong	A transformer (not tapped) in parallel with a single capacitor.

7. Crystal Controlled Oscillators

There are applications where extremely stable operating frequencies are required. The transistor oscillators fall short, especially when changing components.

a. Transistors have different gain characteristics between them.

b. Capacitors and inductors have different characteristics and values

c. Resistors can be sensitive to temperature variations.

To overcome these problems, crystal-controlled oscillators are used. They use quartz crystals to control the frequency.

i. Crystal oscillators make use of the piezoelectric effect.

ii. Physical dimensions determine the frequency of vibration.

iii. Types of crystals

1. Rochelle salt

2. Tourmaline

3. Quartz falls between the above two.

a. Naturally grown

b. Silicon Dioxide (SiO2)

d. In the equivalent circuit of the crystal:

· CC = capacitance of the crystal itself

· CM = mounting capacitance

· L = inductance of the crystal

· R = resistance of the crystal

· fs the crystal acts as a series resonant circuit

· fp the crystal acts as a parallel resonant circuit

e. So the crystal can be used to replace either the series or parallel resonant LC circuit.

f. Crystal oscillators are used at frequencies below 10MHZ. But by using harmonics, stable outputs above 10MHZ can be achieved.

g. A Colpitts oscillator can be modified into a crystal controlled oscillator (CCO).

8. Pierce crystal oscillator

OSCILLATOR TROUBLESHOOTING

Oscillator troubleshooting can be very challenging. Most likely the problems will be with the active device or the capacitors and or inductors.

MULTIVIBRATORS

1. Astable – Discrete

A relatively simple flip-flop circuit made up of two transistors, two capacitors and four resistors along with a power source. LEDs may be used to indicate the flip-flop nature of the circuit.

The astable multivibrator is an oscillator that produces a square wave output.

1. The two transistors Q1 and Q2 operate as switches. When Q1 is on, Q2 is off and when Q2 is on Q1 is off.

2. R1 and R4 are current limiting resistors, to keep the current through the transistor to a safe level. R1 and R4 could be replaced by an LED and current limiting resistor, a relay, a motor or another circuit to turn on or off.

3. R2 and R3 along with C1 and C2 provide the time constant for the oscillation frequency.

4. The output waveform signal is generated from point A or D to ground.

5. If R2 = R3, and C1 =C2, the frequency of oscillation is f = 1/(1.4RC), where R = R2 = R3, and C = C1 = C2.

6. The pulse width can be varied by:

a. Q1 cutoff time, t = .7R2C2

b. Q2 cutoff time, t = .7R3C1

7. Circuit operation analysis, Q1 is off and Q2 is on.

A	B	C	D	Description
VCC	0V	>0.7V	0V	Q1 off, Q2 on. C2 will begin to charge via R2 until node B reaches 0.7V
0V	>0.7V	0V	VCC	When node B charges to 0.7V, Q1 turns on. Node A drops from VCC to 0V which pulls node C below 0.7V and turns of C2. C1 will begin to charge via R3 until node C reaches 0.7V.
VCC	0V	>0.7V	0V	When node C charges to 0.7V, Q2 turns on. Node D drops from VCC to 0V, which pulls node B below 0.7V and turns off Q1. C2 begins to charge via R2 until node B reaches 0.7V.

OSCILLATORS Page 1

VCC

R1R2R3

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