op amp oscillators

7/28/2019 OP Amp Oscillators

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Sine-wave os cillator

In Fig. 1, an op-'amp can b e made tooscillate by feeding a portion of theoutput back to the input via a frequen-cy-selective network, and controlling the overall voltage gain.

For optimum sine-wave genera-tion, the frequency-selective network must feed back an overall phase shiftof zero degrees, while the gain net-

work prov ides unity amplification atthe desired oscillation frequency. Thefrequency network often has a nega-tive gain, which must be compen-sated for by additional amplificationin the gain network, so that the totalgain is unity. If the overall gain is lessthan unity, the circuit will not oscil-late; if the overall gain is greater thanunity, the output waveform will bedistorted.

As Fig. 2 shows, a Wien-bridgenetwork is a practical way of imple-menting a sine-wave oscillator. Thefrequency-selective Wien-bridge isconstructed from the R1-C1 and. R2-C2 networks. Normally, the Wienbridge is symmetrical, so that C1= C2 = C and R1= R2 = R. Whenthat condition is met, the phase rela-tionship between the output and inputsignals varies from — 90° to + 90°,and is precisely 0° at a center frequen-cy, f o, which can be calculated using this formula:

f =1/(2iCR)

The Wien network is connected‘ltween the op-amp's output and thenon-inverting input, so that the circuitgives zero overall phase shift at f

o,

where the voltage gain is 0.33; there-fore, the op-amp must be given a volt-age gain of 3 via feedback network R3-R4, which gives an overall gain of unity. That satisfies the basic require-

ments for sine-wave oscillation. Inpractice, however, the ratio of R3 toR4 must be carefully adjusted to givethe overall voltage gain of precisely unity, which is necessary for a low-distortion sine wave.

Op-amps are sensitive to tem-perature variations, supply-voltagefluctuations, and other conditions thatcause the op-amp's output voltage to vary. Those voltage fluctuationsacross components R3-R4 will also



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The desired feedback thermistor re-sistance value is triple that of R3, sothe feed-back gain is x 3. When thefeedback gain is multiplied by the fre-quency network's gain of 0.33, theoverall gain becomes unity. If the os-cillator output amplitude starts torise, R T heats up and reduces its resis-tance, thereby automatically reducing

the gain of the circuit, which sta-bilizes the amplitude of the outputsignal.

An alternative method of ther-mistor stabilization is shown in Fig.4.In that case, a low-current lamp isused as a Positive Temperature Co-efficient (PTC) thermistor, and isplaced in the lower part of the gain-determining feedback network. If the

output amplitude increases, the lampheats up thereby increasing its resis-tance, reducing the feedback gain,and providing automatic amplitudestabilization. That circuit also showshow the Wien network can be modi-fied by using a twin-ganged potenti-ometer to make a variable-frequency

oscillator over the range 150 Hz to 1.5kHz. The sine-wave output amplitudecan be made variable using R5.

A slightly annoying feature of ther-mistor-stabilized circuits is that, in variable-frequency applications, theoutput amplitude of the sine wavetends to "jitter" or "bounce" as thefrequency control potentiometer isswept up and down its range.

Diode stabilization

The jitter problem of variable-fre-quency circuits can be minimized by using the circuits of Figs. 5 or 6, which rely on the onset of diode orZener conduction for automatic gain

control. In essence, R3 is for a circuitgain slightly greater than unity whenthe output is close to zero, causing thecircuit to oscillate; as each half-cyclenears the desired peak value, one of the diodes starts to conduct, whichreduces the circuit gain, automat-ically stabilizing the peak amplitudeof the output signal. That "limiting"technique typically results in the gen-eration of 1% to 2% distortion on thesine-wave output. The maximum

peak-to-peak output of earoughly double the breakage of its diode regulator

In Fig. 5, the diodes sduct at 500 mV, so the ciroutput of about 1-volt peIn Fig. 6, the Zener diodeare connected back-to-bahave values as high as 5giving a p-p (peak-to-pea

about 12 volts. Each circby adjusting R3 for the maue (minimum distortion) ciliation can be maintainedfrequency band.

The frequency rangbridge oscillators can bechanging the Cl and C2



creasing Cl and C2 by a decade re-duces the output frequency by adecade. Figure 7 shows the circuit of avariable-frequency Wien oscillatorthat covers the range 15 Hz to 15 kHz

in three switched-decade ranges. Thecircuit uses Zener-diode amplituderegulation, and its output is adjustableby both switched and fully-variableattenuators. Notice that the maximum

useful operating frequency is re-stricted by the slew-rate limitations of

the op-amp. The limit is about 25 kHz

using a LM741 op-amp, or about 70kHz using a CA3140.

Twin-T oscillatorsAnother way of designing a sine-

wave oscillator is to wire a twin-Tnetwork between the output and input

of an inverting op-amp, as shown inFig. 8. The twin-T network comprises

R1-R2-R3-R4 and CI-C2-C3. In a"balanced" circuit, those compo-nents are in the ratios

R1 = R2 = 2(R3 + R4), andCl = C2 = C3/2. When the network isperfectly balanced, it acts as a notchfilter that gives zero output at a center

frequency (f 0 ), a finite output at allother frequencies, and the phase of the output is 180° inverted. When the

network is slightly unbalanced by ad-

justing R4, the network will give aminimal output at f o.

By critically adjusting R4 toslightly unbalance the network, thetwin-T gives a 180° inverted phaseshift with a small-signal f o. Becausethe inverting op-amp also causes a

180° input-to-output phase shift, thereis zero overall phase inversion as seenat the inverting op-amp input, and the

circuit oscillates at a center frequency

of 1 kHz. In practice, R4 is adjustedso that oscillation is barely sustained,

and under that condition the sine wavehas less than 1% distortion.

Figure 9 shows an alternative meth-od of amplitude control, which resultsin slightly less distortion. Here, Dlprovides a feedback signal via poten-tiometer R5. That diode reduces thecircuit gain when its forward voltage

exceeds 500 mV. To set up the circuit,first set R5 for maximum resistance to

ground, then adjust R4 so that oscilla-

tion is just sustained. Under thoseconditions, the output signal has anamplitude of about 500 mV p-p. Fur-ther R5 adjustment enables the output

signal to be varied between 170 mVand 300-mV RMS.

Note that twin-T circuits make

good fixed-frequency oscillators, butare not suitable for variable-frequency

operation due to the difficulties of

varying three or four network compo-nents simultaneously.

Square-wave generatorAn op-amp can be used to generate

square-waves by using the relaxationoscillator configuration of Fig. 10.The circuit uses dual power supplies,and the op-amp output switches alter-

nately between positive and negative



saturation levels. When the output ishigh, Cl charges via R1 until thestored voltage becomes more positivethan the value set by R2-R3 at thenon-inverting input. The output thenregeneratively switches negative, which causes Cl to start discharging via R1 until Cl voltage falls to thenegative value set by R2-R3. The out-put then regeneratively switchespositive again, and the whole se-quence repeats ad infinitum.

A symmetrical square wave is de- veloped at the output, and a non-lin-ear triangular waveform is developedacross Cl; those waveforms swing symmetrically on both sides of ground. Notice that the operating fre-quency can be varied by altering ei-ther the R1 or Cl values, or by altering the R2-R3 ratios, which makes thatcircuit quite versatile.

Figure 11 shows how to design apractical 500 Hz to 5-kHz square- wave generator, with frequency varia-tions obtained by altering the attenua-tion ratio of R2-R3-R4. Figure 12shows how to improve Fig. 11 by using R2 to preset the range of frequency control R4, and by using R6 as anoutput amplitude control.

Figure 13 shows how to design a

general purpose square-wave gener-ator that covers the 2 Hz to 20-kHzrange in four switched-decaderanges. Potentiometers R1 to R4 areused to vary the frequency within eachrange: 2 Hz-20 Hz, 20 Hz-200 Hz,200 Hz-2 kHz, and 2 kHz-20 kHz,respectively.

Variable duty-cycleIn Fig. 10, Cl alternately charges

and discharges via R1, and the circuitgenerates a symmetrical square-waveoutput. That circuit can be modified

to give a variable duty-cycle output by providing C1 with alternate chargeand discharge paths.

In Fig. 14, the duty cycle of theoutput waveform is fully variablefrom 11:1 to 1:11 v ia R2, and the fre-quency is variable from 650 Hz to 6.5kHz via R4. The circuit action is suchthat Cl alternately charges throughR1-D1 and the bottom of R2, and dis-charges through R1-D2 and the top of

R2. Notice that any variation has negligible effect on the opefrequency of the circuit.

In Fig. 15, the duty cycle is mined by CI-DI-RI (mark), aC1-D2-R2 (space). The pulquency is variable between 3003 kHz via R4.

Resistance activation

Notice from the descriptionoscillator in Fig. 10 that the ochanges state at each half cyclethe Cl voltage reaches the thr value set by the R2-R3 voltageer. Obviously, if Cl is unable tothat value, the circuit will notlate. Figure 16 shows a resistanc vated oscillator that will oscilla when R4, which is in paralleCl, has a value greater than Rratio of R2:R3 must be 1:1. Tthat R4 is a potentiometer is oillustration. Most resistance-ac

oscillators use either thermistLDR's, which simulate the poometer action.

Figure 17 is a precision "ligh vated" oscillator (or alarm), ana LDR as the resistance activatiment. The circuit can be convea "dark-activated" oscillattransposing the position of LDR1. Figure 18 uses a NTC therRT, as the resistance-activatinment, and is a precision oveperature oscillator/alarm. Thecan be converted to an unde

perature oscillator by transposand Rl.

The LDR or RT can have antance in the range from 2000 o2 megohms at the required level, and R1 must have the samue as the activating elementdesired trigger level. RI sets thger level; the Cl value can be to change the oscillation frequ

Triangle/square generationFigure 19 shows a function

ator that simultaneously prod



linear triangular wave and a squarewave using two op-amps. IntegratorICI is driven from the output of IC2,where IC2 is wired as a voltage com-

parator that's driven from the outputof ICI via voltage divider R2-R3. The

square-wave output of IC2 switchesalternately between positive andnegative saturation levels.

Suppose, initially, that the outputof IC1 is positive, and that the outputof IC2 has just switched to positivesaturation. The inverting input of IC1is at virtual ground, so a current IR1equals + VsAT /R1. Because RI andCl are in series, IRSand Iclare equal.

Yet, in order to maintain a constantcurrent through a capacitor, the volt-age across that capacitor must changelinearly at a constant rate. A linearvoltage ramp therefore appears across

Cl, causing the output of ICI to startto swing down linearly at a rate of 1/

Cl volts per second. That output is fed

via the R2-R3 divider to the non-in-verting input of IC2.

Consequently, the output of ICIswings linearly to a negative valueuntil the R2-R3 junction voltage fallsto zero volts (ground), at which pointIC2 enters a regenerative switchingphase where its output abruptly goesto the negative saturation level. Thatreverses the inputs of IC1 and IC2, so

IC1 output starts to rise linearly untilit reaches a positive value that causes

the R2-R3 junction voltage to reach

the zero-volt reference value, whichinitiates another switching action.

The peak-to-peak amplitude of thelinear triangular-waveform is con-trolled by the R2-R3 ratio. The fre-quency can be altered by changingeither the ratios of R2-R3, the values

of R1 or Cl, or by feeding RI from theoutput of IC2 through a voltage divid-er rather than directly from op-amp1C2 output.

In Fig. 20, the current input to Cl(obtained from R3-R4) can be variedover a 10:1 range via RI, enabling thefrequency to be varied from 100 Hz to

1 kHz; resistor R3 enables the full-

scale frequency to be set to precisely 1kHz. The amplitude of the triangular

waveform is fully variable via R5, andthe square wave via R8. The outputgenerates symmetric waveforms,since Cl alternately charges and dis-charges at equal current values deter-

mined by R3-R4.Figure 21 shows how to modify Fig.

20 to make a variable symmetry

ramp/rectangular generator, wherethe slope of the ramp and duty cycle isvariable via R4. Cl alternately

charges through R3-D1 and the upperhalf of R4, and discharges throughR3-D2 and the lower half of R4.

Switching circuitsFigure 22 shows the connections

for making a manually triggered bi-stable circuit. Notice that the invert-ing terminal of the op-amp is tied toground via RI, and the non-invertingterminal is tied directly to the output.

Switches SI and S2 are normallyopen. If switch Si is briefly closed,the op-amp inverting terminal is mo-mentarily pulled high, and the outputis driven to negative saturation; con-sequently, when S1 is released again,

the inverting terminal returns to zero

volts, but the output and the non-in-verting terminal remains in negativesaturation. The output remains in that

state until S2 is briefly closed; thatswitches the output to a stablepositive saturation state until SI isclosed again.

Figure 23 shows how Fig. 22 can bemodified for operation from a single-ended power supply.

Finally, Fig. 24 shows how to con-nect an op-amp as a Schmitt trigger,which can be used to convert a sine

wave into a square wave. Suppose,-initially, that the op-amp's output is ata positive saturation value of 8 volts.Under that condition the R1-R2 divid-

er feeds a positive reference voltageabout 80 mV to the non-inverting in-put. Consequently, the output re-mains in that state until the input

voltage rises to a value equal to 80mV. The op-amp's output will thenswitch regeneratively to a negativesaturation level of — 8 volts, therebyfeeding a reference voltage of — 80mV's to the non-inverting input. Theoutput remains in that state until theinput falls to — 80 mV; at that point,the output regeneratively switchesback to the positive saturation level.The switching levels can be altered bychanging the R1 value. R-E

op amp oscillators

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