CHAPTER 13: Sine Wave Oscillator Circuits. pptx

• Oscillators use feedback to produce periodic AC output with DC power as the only input.. A Bit of Theory• We know that with negative feedback in an op-amp, the equation for closed-lo

CHAPTER 13

Sine Wave Oscillator Circuits

Describe and Analyze:

• Feedback oscillator theory

• RC phase-shifting oscillators

• LC resonant oscillators

• Crystal oscillators

• Troubleshooting

• Oscillators use feedback to produce periodic AC

output with DC power as the only input

• In contrast, function generators produce periodic

outputs by joining pieces of wave-forms together

A Bit of Theory

• We know that with negative feedback in an op-amp, the

equation for closed-loop gain in terms of open-loop gain is:

ACL = AOL / (1 + B  AOL)

where B is the feedback ratio set by external resistors Now, if

there is an additional 180° phase shift in B, we can express it

mathematically as:

ACL = A OL / (1 – B  AOL)

In that case, what happens when B  AOL = 1?

• Mathematically, ACL “goes to infinity” (whatever that means)

Physically, the circuit oscillates: it takes no Vin to get a Vout The trick is to get that 180° shift in B, the feedback network

RC Oscillators

• As a group, RC oscillators use an RC network

inserted into the feedback loop of an amplifier to

produce positive feedback at exactly one frequency

As we just saw, that’s the recipe for oscillation

• One type of RC oscillator is the Wien-Bridge

oscillator Another is simply called the Phase-Shift

oscillator

Wien-Bridge Oscillator

Feedback to the (+) input > feedback to (–) input at fOSC

Starting and Running

• A problem with the Wien-Bridge (and with all

feedback oscillators) is that the feedback necessary

to start oscillating is slightly more than the feedback

to maintain a pure sine wave If the gain is left too

high, the sinewave amplitude will increase until it hits the rails and is clipped

• The cure is to include a means for the circuit to lower its gain a bit once it starts oscillating This is a type of negative feedback based on amplitude

Phase-Shift Oscillator

• The RC phase-shift oscillator is the simplest of its

type A minimum of three RC LPF sections are put in the feedback loop of an inverting amplifier Each RC stage causes an amount of phase shift that changes with frequency At one specific frequency, the phase shifts of the network add up to 180°

• Since the inverting amplifier has a 180° phase shift, the total phase shift is 360°, which means it has

become positive feedback The circuit oscillates

Phase-Shift Oscillator

Oscillations in Amplifiers

The old joke is that, when you’re testing them,

oscillators don’t oscillate but amplifiers do If there’s an accidental feedback path from the output to the input, then it’s a good bet a high-gain amplifier will oscillate Such feedback paths can be:

• Through the power rails

• Through magnetic coupling of signal leads

• Through capacitive coupling of adjacent components

• Through unshielded input cables

• Through putting a microphone too close to a speaker

LC Oscillators

Pulsing an LC “tank circuit” will make it “ring” But the oscillations

back into the tank as fast as it were being dissipated, it would

ring forever That is the basic idea of an LC resonant oscillator

Colpitts Oscillator

A common type of LC oscillator

Hartley Oscillator

Another type of LC oscillator

Crystal Oscillators

• Piezoelectric crystals behave like extremely high-Q

resonant circuits

• A crystal’s frequency depends on its physical

dimensions, which can be tightly controlled by

grinding

• LC resonant oscillators can be “crystal stabilized”

• A crystal can take the place of an LC tank circuit

XTAL Oscillator Examples

Since you’re stuck with it anyway, the Pierce oscillator

(b) utilizes stray capacitance for feedback

• Use a frequency counter to check frequency You

can use an oscilloscope, but your readings will be off

by ±5%

• Use a X10 probe to minimize loading (they work on counters too)

• At very high frequencies, you don’t have to touch the probe to the circuit, just get it close

• There can be high voltage in a transmitter’s tank

circuit even if it uses a 12 Volt DC supply