【正文】 plitude 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。, and is precisely 0176。 (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。 inverted phase shift with a smallsignal f0. Because the inverting opamp also causes a 180186。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。s output is at a positive saturation value of 8 volts. Under that condition the R^R? divider feeds a positive reference voltage about 80 mV to the noninverting input* Consequently, the output remains in that state until the input voltage rises to a value equal to 80 mV, The opamp39。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 。由于放大器無法辨別加給它的輸入信號的來源，于是就會出現(xiàn)如下情況 ， 如果除去外加信號源，而將 2端同 1端接在一起，則放大器將如以前一樣，繼續(xù)提供一個(gè)同樣的輸出信號 X0。 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 。 雖然還可以總結(jié)出其他可用來確定頻率的原則，但可以證明，它們同上述原則是一致的 。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。因?yàn)槿绻?FA=1,則 Af → ∞ ,這可以解釋為，即使沒有外加信號電壓，也仍然有輸出電壓 。于是，似乎 |FA|大于 1時(shí)，振蕩器的振幅會無限制地增大?，F(xiàn)假設(shè)即使最初能滿足這個(gè)條件，由于電路元件特性，特別是晶體管特性受老化、溫度和電壓等影響發(fā)生變化（漂移）， 于是很顯然，如果整個(gè)振蕩器聽其自然，則在很短的時(shí)間內(nèi)， |FA|就會變得不是小于 1,就是大于 1．在前一種情況下，只是振蕩停止而已，而在后一種情況下，我們就又需要用非線性來限制振幅。 圖 12 三極點(diǎn)傳遞函數(shù)在 S平面上的根軌跡 2. 運(yùn)放振蕩器 正弦振蕩器 圖 21是一個(gè)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(gè)電壓增益的運(yùn)放振蕩器 。如果增益大于 1，則輸出波形將失真 。當(dāng)上述條件滿足時(shí)，輸出和輸入間的相位關(guān)系在 90176。 實(shí)際中 ， R3和 R4之比必須仔細(xì)調(diào)整以使總增益為 1，這是產(chǎn)生 低失真正弦波所必須的。 圖 23為 1kH：固定頻率振蕩器。當(dāng)振蕩器輸出幅度上升時(shí)， RT被加熱 ， 阻值降低，自動減小電路增益，從而穩(wěn)定輸出信號的幅度 。輸出正弦波的振幅可以利用 R5來改變 。 實(shí)質(zhì)上， 當(dāng)輸出接近零時(shí)， R3可使電路增益稍大于 1，從而使電路振蕩。 在圖 25中，二極管在電壓為 500mV時(shí)就開始導(dǎo)通，故輸出峰 峰值大約為 1V。增加 C1C2十個(gè)數(shù)量級，可以減小輸出頻率十個(gè)數(shù)量級（即 10倍）。 利用 CA3140時(shí)大約為 70kHz。 在平衡電路中， R1=R2=2(R3＋ R4),C1=C2=C3/2,當(dāng)網(wǎng)絡(luò)平衡完美時(shí)，為陷波濾波器在中心頻率 f0處輸出為 0，而在其他頻率處有確定輸出，輸出相移 180?可變 。電路將振蕩在中心頻率為 1kHz處。當(dāng)二極管正向電壓超過 500mV時(shí)，將減小增益。但不適宜可以變化頻率的網(wǎng)絡(luò)，因?yàn)檎{(diào)節(jié) 3或 4個(gè)網(wǎng)絡(luò)中的元件，使之同步是很困難的 。當(dāng)輸出為高電平時(shí)， C1通過 R1充電，直到 C1上存儲的電壓比在同相輸入端由 R2R3分壓建立的正值更正時(shí)，輸出再次轉(zhuǎn)為負(fù)向飽和電壓，使 C1又通過 R1放電，直到C1上的電壓降到由 R2R3分壓建立的負(fù)值更負(fù)時(shí)，輸出再次轉(zhuǎn)換為正向飽和電壓，如此循環(huán)下去 。該發(fā)生器的頻率調(diào)整可通過改變衰耗器 R2R3R4來實(shí)現(xiàn)