【正文】 ternal generator will result in a cessation of oscillations. But now suppose that |FA| is greater than unity. Then, for example, a 1V signal appearing initially at the input terminals will, after a trip around the loop and back to the input terminals, appear there with an amplitude larger than 1V. This larger voltage will then reappear as a still larger voltage, and so on, It seems j then, that if |FA| is larger than unity, the amplitude of the oscillations will continue to increase without limit, But of course, such an increase in the amplitude can continue only as long as it is not limited by the onset of nonlinearity of operation in the active devices associated with the amplifier. Such a nonlinearity bees more marked as the amplitude of oscillation increases. This onset of nonlinearity to limit the amplitude of oscillation is an essential feature of the operation of all practical oscillators, as the following considerations will show: The condition |FA|=1 does not give a range of acceptable values of |FA| , but rather a single and precise value. Now suppose that initially it were even possible to satisfy this condition. Then, because circuit ponents and, more importantly, transistors change characteristics (drift) with age, temperature, voltage, etc., it is clear that if the entire oscillator is left to itself, in a very short time |FA| will bee either less or larger than unity. In the former case the oscillation simply stops, and in the latter case we are back to the point of requiring nonlinearity to limit the amplitude. An oscillator in which the loop gain is exactly unity is an abstraction pletely unrealizable in practice. It is accordingly necessary, in the adjustment of a practical oscillator, always to arrange to have |FA| somewhat larger (say 5 percent) than unity in order to ensure that, with incidental variations in transistor and circuit parameters , |FA| shall not fall below unity. While the first two principles stated above must be satisfied on purely theoretical grounds, we may add a third general principle dictated by practical considerations, .: Fig. 12 Root locus of the threepole transfer functions in the s plane. The poles without feedback (FA0 = 0) are s1, s2, and s3, whereas the poles after feedback is added are s1f, s2f, and s3f. In every practical oscillator the loop gain is slightly larger than unity, and the amplitude of the oscillations is limited by the onset of nonlinearity. 2 Opamp Oscillators Opamps can be used to generate sine wave, triangularwave, and square wave signals. We’ll start by discussing the theory behind designing opamp oscillators. Then we’ll examine methods to stabilize oscillator circuits using thermistors, diodes, and small incandescent lamps. Finally, our discussion will round off with designing bistable opamp switching circuits. Sinewave oscillator In , an opamp can be made to oscillate by feeding a portion of the output back to the input via a frequencyselective work and controlling the overall voltage gain. For optimum sinewave generation, the frequencyselective work must feed back an overall phase shift of zero degrees while the gain work provides unity amplification at the desired oscillation frequency. The frequency work often has a negative gain, which must be pensated for by additional amplification in the gain work, so that the total gain is unity. 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. Fig 21 Stable sinewave oscillation requires a zero phase shift between the input and output and an orerall gain of 1. As Fig. 22 shows, a Wienbridge work is a practical way of implementing a sinewave oscillator. The frequencyselective Wienbridge is coostructed from the R1C1 and R2C2 works. Normally, the Wien bridge is symmetrical, so that C1=C2=C and R1 =R2=R. When that condition is met, the phase relationship between the output and input signals varies from90176。, and is precisely 0176。s output and the noninverting input, so that the I circuit gives zero overall phase shift at f0, where the voltage gain is 。 (NTC) thermistor Rt which, together with R3 forms a gaindetermining feedback work. The thermistor is heated by the mean power output of the opamp The desired feedback thermistor resistance value is triple that of R3, so the feedback gain is X3. When the feedback gain is multiplied by the frequency work39。 as each halfcycle nears the desired peak value, one of the diodes starts to conduct, which reduces the circuit gain, automatically stabilizing the peak amplitude of the output signal. That limiting” technique typically results in the generation of 1% to 2% distortion on the sinewave output. The maximum peaktopeak output of each circuit is roughly double the breakdown voltage of its diode regulator element. In Fig 25, the diodes start to conduct at 500 mV, so the circuit gives an output of about 1volt peaktopeak. In Fig, 26, the Zener diodes D1 and D2 are connected backtoback, and may have values as high as 5 to 6 volts, giving a pp (peaktopeak) output of about 12 volts. Each circuit is set up by adjusting R3 for the maximum value (minimum distortion) at which oscillation can be maintained across the frequency band. The frequency range of Weinbridge oscillators can be altered by changing the C1 and C2 values。 inverted phase shift with a smallsignal f0. Because the inverting opamp also causes a 180186。 those waveforms swing symmetrically on both sides of ground. Notice that the operating frequency can be varied by altering either the R1 or C1 values, or by altering the R2R3 ratios, which makes that circuit quite versatile. Fig. 211 shows how to design a practical 500 Hz to 5kHz squarewave generator, with frequency variations obtained by alter