【正文】 當(dāng)輸出為高電平時(shí)， C1通過(guò) R1充電，直到 C1上存儲(chǔ)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(guò) 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 。 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 。 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。 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?，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(tīng)其自然，則在很短的時(shí)間內(nèi)， |FA|就會(huì)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(lái)限制振幅。 雖然還可以總結(jié)出其他可用來(lái)確定頻率的原則，但可以證明，它們同上述原則是一致的 。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。, and is precisely 0176。 inputtooutput phase shift, there is zero overall phase inversion as seen at the inverting opamp 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. Fig. 29 shows an alternative method of amplitude control, which results in slightly less distortion. Here, DY provides a feedback signal via potentiometer R5. That diode reduces the circuit 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 oscillation is just sustained. Under those conditions, the output signal has an amplitude of about 500 mV pp. Further R5 adjustment enables the output signal to be varied between 170 mV and 300mV RMS. Note that twinT circuits make good fixedfrequency oscillators t but are not suitable for variablefrequency operation due to the difficulties of varying three or four work ponents simultaneously. Fig. 29 Dioderegulated 1kHz twinT oscillator. Fig. 210 Relaxation square wave oscillator. Squarewave generator An opamp can be used to generate squarewaves by using the relaxation oscillator configuration of Fig. 210. The circuit uses dual power supplies, and the opamp output switches alternately between positive and negative saturation levels. When the output is high, C1 charges via R1 until the stored voltage bees more positive than the value set by R2R3 at the noninverting input. The output then regeneratively switches negative, which causes C1 to start discharging via R1 until C1 voltage falls to the negative value set by output then regeneratively switches positive again, and the whole sequence repeats ad infinitum. A symmetrical square wave is developed at the output, and a nonlinear triangular waveform is developed across C1。當(dāng)然要注意， Xf’=Xi這種說(shuō)法意味著 X’f,和Xi的瞬時(shí)值在所有時(shí)刻都完全相等。 若干實(shí)際的考慮參看圖 12可以看出，如果 |FA|在振蕩頗率處正好為 1，那么將反饋信號(hào)接到輸入 端，再除去外部信號(hào)源將不會(huì)造成任何影響 。 圖 21 要求輸入輸出間的相移為 0，全部增 益為 1的穩(wěn)定的正弦波振蕩器框圖 圖 22表示了文氏橋式正弦波振蕩器的實(shí)際構(gòu)成方法。 圖 23 用熱敏電阻穩(wěn)幅的 1kHz文氏橋式振蕩器 熱敏電阻穩(wěn)幅的另一種方法如圖 24所示．在這種情況下，低電流的燈被用作具有正溫度系數(shù)的熱敏電阻，放在增益限定網(wǎng)絡(luò)的低電位（接地）端。圖 27為一個(gè)具有三個(gè) 10倍開(kāi)關(guān)檔的，頻率范圍在 15H15kHz內(nèi)可調(diào)的文氏振蕩器。要使電路起振，首先調(diào)節(jié) R5使之對(duì)地阻值最大，再調(diào)節(jié) R4使振蕩僅能維持 。 圖 211 500Hz5kHz方波振蕩器 圖 212 改進(jìn)型 500Hz5kHz方波振蕩器 圖 2。 這里 D1通過(guò) R5電位器提供一反饋信號(hào)。 圖 25 用二極管穩(wěn)幅的文氏橋式 振蕩器 圖 26 用齊納二極管穩(wěn)幅的文氏橋式振蕩器 文氏橋式振蕩器的頻率范圍也可以通過(guò)改變 C1和 C2的值來(lái)調(diào)節(jié)。該電阻的阻值需為 R3的 3倍，以使得反饋增益為 3,將反饋增益和頻率網(wǎng)絡(luò)的 0. 33增益相乘，總增益為 1。如果整個(gè)增益小于 1，則電路不能振蕩。上述原則與反饋公式 Af=A/(1+FA)是一致的。反饋網(wǎng)絡(luò)的輸出為 Xf =FX0=AFXi,混合電路（現(xiàn)在就是一個(gè)反相器）的輸出為 Xf’＝ Xf =AFXi 由圖 1，環(huán)路增益為 環(huán)路增益 =Xf’/Xi=Xf/Xi=FA 圖 11 尚未連成閉環(huán)的增益為 A的放大器和反饋網(wǎng)絡(luò) F 假定恰好將信號(hào) Xf’ ，調(diào)整到完全等于外加的輸入信號(hào) Xi。 increasing C1 and C2 by a decade reduces the output frequency by a decade. Fig. 27 shows the circuit of a variablefrequency Wien oscillator that covers the range 15 Hz to 15 kHz in three switcheddecade ranges. The circuit uses Zenerdiode amplitude regulation, and its output is adjustable by both switched and fullyvariable attenuators. Notice that the maximum useful operating frequency is restricted by the slewrate limitations of the opamp. The limit is about 25 kHz using a LM741 opamp, or about 70 kHz using a CA3140. 2. 4 TvuinT oscillators Another way of designing a sinewave oscillator is to wire a twinT work between the output and input of an inverting opamp, as shown in Fig, 28. The twinT work prises R1R2R3R4 and C1C2C3. In a balanced circuit, those ponents are in the ratios R1=R2=2(R3＋ R4), and C1=C2=C3/2. When the work 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 work is slightly unbalanced by adjusting R4, the work will give a minimal output at f0. Fig. 28 1kHz twinT oscillator. By critically adjusting R4 to slightly unbalance the work, the twinT gives a 180186。s output and the noninverting input, so that the I circuit gives zero overall phase shift at f0, where the voltage gain is 。s to the noninverting input The output remains in that state until the input falls to 80 mV。 只要電路能振蕩，其頻率就由上述原則來(lái)確定。 所以，在實(shí)際振蕩器的調(diào)試中，總是要調(diào)整 |FA|多少比 1大一些 (比方說(shuō)大 5%),以保證在晶體管和電路參數(shù)發(fā)生偶然變化時(shí) ， |FA|不致下降到 1以下。如果利用增益穩(wěn)定網(wǎng)絡(luò)來(lái)代替被動(dòng)（無(wú)源）的 R3和 R4增益限定網(wǎng)絡(luò)，即將反饋網(wǎng)絡(luò)變?yōu)榫哂凶詣?dòng)增益控制功能的網(wǎng)絡(luò)，則可增加放大器的穩(wěn)定性。這種限制技術(shù)常引起輸出正弦波有 1%2%的失真。 通過(guò)慢慢改變 R4使網(wǎng)絡(luò)趨向不平衡，網(wǎng)絡(luò)將產(chǎn)生 180?相移和 f0小信號(hào)輸出 。這些波形在原點(diǎn)兩側(cè)均勻分布，工作頻率可以通過(guò)改變 R1或 C1或改變 R2R3比率來(lái)調(diào)整 。電路使用雙電源供電。如圖 28所示 。 二極管穩(wěn)幅 頻率調(diào)節(jié)時(shí)，電路的輸出抖動(dòng)問(wèn) 題可以被最大限度地減小。間變化，在中心頻率 f0處恰好是零。隨著振幅的增大，有源器件的非線性變得更加明顯 。因?yàn)樾盘?hào)在通過(guò)電抗網(wǎng)絡(luò)時(shí)引入的相移總是頻率的函數(shù)，所以我們有如下重要原則 ： 正弦振蕩器的工作頻率是這樣一個(gè)頻率，在該頻率下，信號(hào)從輸入端開(kāi)始，經(jīng)過(guò)放大器和反饋網(wǎng)絡(luò)后，又回到輸 入 端時(shí)，引入的總相移正好是零（當(dāng)然，或者是 2π 的整數(shù)倍）。 resistor R3 enables the fullscale frequency to be set to precisely 1 kHz, The amplitude of the triangular waveform is fully variable via R5 and the square wave via R8. The output generates symmetric waveforms, since C1 alternately charges and discharges at equal cur