【正文】 該發(fā)生器的頻率調(diào)整可通過改變衰耗器 R2R3R4來實現(xiàn) 。但不適宜可以變化頻率的網(wǎng)絡(luò)，因為調(diào)節(jié) 3或 4個網(wǎng)絡(luò)中的元件，使之同步是很困難的 。電路將振蕩在中心頻率為 1kHz處。 利用 CA3140時大約為 70kHz。 在圖 25中，二極管在電壓為 500mV時就開始導(dǎo)通，故輸出峰 峰值大約為 1V。輸出正弦波的振幅可以利用 R5來改變 。 圖 23為 1kH：固定頻率振蕩器。當上述條件滿足時，輸出和輸入間的相位關(guān)系在 90176。 圖 12 三極點傳遞函數(shù)在 S平面上的根軌跡 2. 運放振蕩器 正弦振蕩器 圖 21是一個通過選頻網(wǎng)絡(luò)將輸出的一部分，反送到輸入，來控制整個電壓增益的運放振蕩器 。于是，似乎 |FA|大于 1時，振蕩器的振幅會無限制地增大。該條件概括為下述原則： 在振蕩頻率處，如果放大器的轉(zhuǎn)移增益和反饋網(wǎng)絡(luò)的反饋系數(shù)的乘積（環(huán)路增益的幅值）小于 1，則振蕩不能維持下去。 在這些條件下 ， 能保持波形形狀的唯一周期性波形是正弦波 。我們將在下文討論所有這些振蕩器的基本原理，除了確定產(chǎn)生振蕩所需的條件之外，還研究振蕩頻率和振幅的穩(wěn)定問題 。s, which simulate the potentiometer action. Fig. 216 Resistanceactivated relaxation oscillator. Fig. 217 is a precision lightactivated oscillator (or alarm), and uses a LDR as the resistance activating element. The circuit can be converted to a “dark activated oscillator by transposing the position of LDR and R1. Fig, 218 uses a NTC thermistor, RT, as the resistanceactivating element y and is a precision overtemperature oscillator/alarm. The circuit can be converted to an under temperature oscillator by transposing RT and R1. The LDR or RT can have any resistance in the range from 2021 ohms to 2 megohms at the required trigger level, and R1 must have the same value as the activating element at the desired trigger level. R1 sets the trigger level the C1 value can be altered to change the oscillation frequency. Fig 217 Precision lightactivated oscillator Fig. 218 Precision overtemperature oscillator/alarm, Triangle/square generation Fig 219 shows a function generator that simultaneously produces a linear triangular wave and a square wave using two opamps Integrator IC1 is driven from the output of IC2, where IC2 is wired as a voltage parator that39。 (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。WAVEFORM GENERATORS 1 The Basic Priciple of Sinusoidal Oscillators Many different circuit configurations deliver an essentially sinusoidal output waveform even without inputsignal excitation. The basic principles governing all these oscillators are investigated. In addition to determining the conditions required for oscillation to take place, the frequency and amplitude stability are also studied. Fig. 11 shows an amplifier, a feedback work, and an input mixing circuit not yet connected to form a closed loop. The amplifier provides an output signal X0 as a consequence of the signal Xi applied directly to the amplifier input terminal. The output of the feedback work is Xf =FX0=AFXi, and the output of the mixing circuit (which is now simply an inverter) is Xf’＝ Xf =AFXi From Fig. 11 the loop gain is Loop gain=Xf’/Xi=Xf/Xi=FA Suppose it should happen that matters are adjusted in such a way that the signal Xf’ is identically equal to the externally applied input signal Xi. Since the amplifier has no means of distinguishing the source of the input signal applied to it at would appear that, if the external source were removed and if terminal 2 were connected to terminal 1, the amplifier would continue to provide the same output signal Xo as before. Note, of course, that the statement Xf’ =Xi means that the instantaneous values of Xf’ and Xi are exactly equal at all times. The condition Xf’ =Xi is equivalent to –AF=1, or the loop gain, must equal unity. Fig 11 An amplifier with transfer gain A and feedback work F not yet connected to form a closed loop. The Barkhausen Criterion We assume in this discussion of oscillators that the entire circuit operates linearly and that the amplifier or feedback work or both contain reactive elements. Under such circumstances, the only periodic waveform which will preserve, its form is the sinusoid. For a sinusoidal waveform the condition Xi = Xf’ is equivalent to the condition that the amplitude, phase, and frequency of Xi and Xf’ be identical. Since the phase shift introduced in a signal in being transmitted through a reactive work is invariably a function of the frequency, we have the following important principle: The frequency at which a sinusoidal oscillator will operate is the frequency for which the total shift introduced as a signal proceeds from the input terminals, through the amplifier and feedback work, and back again to the input, is precisely zero (or, of course, an integral multiple of 2π). Stated more simply, the frequency of a sinusoidal oscillator is determined by the condition that the loopgain phase shift is zero. Although other principles may be formulated which may serve equally to determine the frequency, these other principles may always be shown to be identical with that stated above. It might be noted parenthetically that it is not inconceivable that the above condition might be satisfied for more than a single frequency. In such a contingency there is the possibility of simultaneous oscillations at several frequencies or an oscillation at a single one of the allowed frequencies. The condition given above determines the frequency, provided that the circuit will oscillate at all. Another condition which must clearly be met is that the magnitude of Xi and Xf’ must be identical. This condition is then embodied in the following principle: Oscillations will not be sustained if, at the oscillator frequency, the magnitude of the product of the transfer gain of the amplifier and the magnitude of the feedback factor of the feedback work (the magnitude of the loop gain) are less than unity. The condition of unity loop gain AF = 1 is called the Barkhausen criterion. This condition implies, of course, both that |AF| =1 and that the phase of A is zero. The above principles are cons