2. INTRODUCTION
Oscillators are circuits that produce specific, periodic waveforms such as square, triangular saw
tooth, and sinusoidal. They generally use some form of active device, lamp, or crystal
surrounded by passive devices such as resistors, capacitors, and inductors, to generate the
output.
Sinusoidal oscillators consist of amplifiers with external components used to generate
oscillation, or crystals that internally generate the oscillation. The focus here is on sine wave
oscillators, created using operational amplifiers op amps.
Sine wave oscillators are used as references or test waveforms by many circuits. A pure sine
wave has only a single or fundamental frequency—ideally no harmonics are present. Thus, a
sine wave may be the input to a device or circuit, with the output harmonics measured to
determine the amount of distortion.
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3. CIRCUIT DIAGRAM FOR SINE WAVE GENERATOR
Twin-T oscillators
A twin-T network between the output and input of an inverting op-amp, as shown in Fig .The
twin-T network comprises R1-R2-R3-R4 and CI-C2-C3. In a "balanced" circuit, those
components are in the ratios R1 = R2 = 2(R3 + R4), and Cl = C2 = C3/2. When the network is
perfectly balanced, it acts as a notch filter that gives zero output at a center frequency (f0), a
finite output at all other frequencies, and the phase of the output is 180° inverted. When the
network is slightly unbalanced by adjusting R4, the network will give a minimal output at fo.
By critically adjusting R4 to slightly unbalance the network, the twin-T gives a 180° inverted
phase shift with a small-signal fo. Because the inverting op-amp also causes a 180° input-to-
output phase shift, there is zero overall phase inversion as seen at the inverting op-amp input,
and the circuit oscillates at a center frequency of 1 kHz. In practice, R4 is adjusted so that
oscillation is barely sustained, and under that condition the sine wave has less than 1%
distortion.
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4. COMPONENTS USED:-
SR NO COMPONENT RATING
1. Capacitor .01uf,.01um,.02uf
C1,C2,C3
2. Op amp Ic1 LM741
3. Power supply 9V
4. Resistors 15K,15K,6.8K,1K,27K,10K
R1&R2&R3&R4,R5
WORKING OF SINE WAVE GENERATOR
op-amp can be made to oscillate by feeding a portion of the output back to the input via a
frequency-selective network, and controlling the overall voltage gain.
For optimum sine-wave generation, the frequency-selective network must feed back an overall
phase shift of zero degrees, while the gain network provides unity amplification at the desired
oscillation frequency. The frequency network often has a negative gain, which must be
compensated for by additional amplification in the gain network, so that the total gain is unity.
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5. If the overall gain is less than unity, the circuit will not oscillate; if the overall gain is greater
than unity, the output waveform will be distorted.
Op-amp oscillators are circuits that are unstable—not the type that are sometimes
unintentionally designed or created in the lab—but ones that are intentionally designed to
remain in an unstable or oscillatory state. Oscillators are useful for generating uniform signals
that are used as a reference in such applications as audio, function generators, digital systems,
and communication systems.
Two general classes of oscillators exist: sinusoidal and relaxation. Sinusoidal oscillators consist
of amplifiers with RC or LC circuits that have adjustable oscillation frequencies, or crystals that
have a fixed oscillation frequency. Relaxation oscillators generate triangular, sawtooth, square,
pulse, or exponential waveforms, and they are not discussed here.
Op-amp sine-wave oscillators operate without an externally-applied input signal. Instead, some
combination of positive and negative feedback is used to drive the op amp into an unstable
state, causing the output to cycle back and forth between the supply rails at a continuous rate.
The frequency and amplitude of oscillation are set by the arrangement of passive and active
components around a central op amp.
Op-amp oscillators are restricted to the lower end of the frequency spectrum because op amps
do not have the required bandwidth to achieve low phase shift at high frequencies.
Voltage-feedback op amps are limited to a low kHz range because their dominant, open-loop
pole may be as low as 10 Hz. The new current-feedback op amps have a much wider
bandwidth, but they are very hard to use in oscillator circuits because they are sensitive to
feedback capacitance. Crystal oscillators are used in high-frequency applications up to the
hundreds of MHz range.
APPLICATIONS:-
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