【正文】 tain a constant current through a capacitor, the voltage across that capacitor must change linearly at a constant rate, A linear voltage ramp therefore appears across C1, causing the output of IC1 to start to swing down linearly at a rate of 1/C1 volts per second. That output is fed via the R2R3 divider to the noninverting input of IC2. Consequently, the output of IC1 swings linearly to a negative value until the R2 –R3 junction voltage falls to zero volts (ground), at which point IC2 enters a regenerative switching phase where its output abruptly goes to the negative saturation level. That reverses the in puts of IC1 and IC2 so IC1 output starts to rise linearly until it reaches a positive value that causes the R2R3 junction voltage to reach the zerovolt reference value, which initiates another switching action. The peaktopeak amplitude of the linear triangularwaveform is controlled by the R2 –R3 ratio. The frequency can be altered by changing either the ratios of R2R3 the values of R1 or C1 or by feeding R1 from the output of IC2 through a voltage divider rather than directly from opamp IC2 output. In Fig. 210, the current input to C1 (obtained from R3R4) can be varied, over a 10: 1 range via R1, enabling the frequency to be varied from 100 Hz to 1kHz。 圖 11表示了放大器、反饋網(wǎng)絡(luò)和輸入混合電路尚未連成閉環(huán)的情況。 對(duì)正弦波而言，條件 Xf’ =Xi等同于 Xi和 Xf’，的幅度、相位和頻率都完全一樣的條件。 環(huán)路增益為 1,即 FA=1這個(gè)條件叫做巴克豪森判據(jù) 。其實(shí)，只是在不受放大器中有源器件的非線性的限制時(shí)，振幅的增大才能繼續(xù)下去。 為了獲得最佳正弦波，當(dāng)網(wǎng)絡(luò)增益在振蕩頻率處提供單位增益時(shí)，頻率選擇網(wǎng)絡(luò)整個(gè)相位移為零。到＋ 90176。輸出振幅被具有負(fù)溫度 系數(shù) (NTC)的熱敏電阻 RT和 R3所組成的增益限定反饋網(wǎng)絡(luò)穩(wěn)定。 圖 24 用燈泡穩(wěn)幅的文氏橋式振蕩器 熱敏電阻穩(wěn)幅電路的缺點(diǎn)是，應(yīng) 用于頻率可變情況下，當(dāng)調(diào)節(jié)控制頻率電位器時(shí)，輸出幅度將抖動(dòng)不穩(wěn)。 在圖 26中齊納二極管采用雙向聯(lián)接，導(dǎo)通電壓可達(dá) 56V，所以輸出峰 峰值大約為 12V。 圖 27 15Hz— 15kHz三檔量程十進(jìn)位文氏橋 式振蕩器 T振蕩器 另一種設(shè)計(jì)正弦波振蕩器的方法是在一反相運(yùn)放的輸入輸出之間接入一個(gè)雙 T網(wǎng)絡(luò)。實(shí)際上調(diào)節(jié) R4在臨界狀態(tài)時(shí)，正弦波具有小于 1％ 的失真。 2. 5方波發(fā)生器 運(yùn)放組成由圖 210所示的張弛振蕩形式時(shí)，就能產(chǎn)生方波。 圖 212示出了如何改進(jìn)圖 211。 圖 211示出了如何設(shè)計(jì)一個(gè)實(shí)用的 500Hz5kHz的方波發(fā)生器。 雙 T網(wǎng)絡(luò)能構(gòu)成一個(gè)很好的固定頻率的振蕩器。 因?yàn)榉聪噙\(yùn)放在輸入輸出之間有 180?相移， 所以在運(yùn)放輸人端總相移為 0?。注意，有效工作頻率受運(yùn)放的壓擺率限制，上限頻率，當(dāng)用 LM741時(shí)大約為 25kHz。每個(gè)電路的最大峰 峰值 輸出約為穩(wěn)壓二級(jí)管擊穿電壓的兩倍。這個(gè)電路還示出了用雙聯(lián)可變電位器來改變文氏橋式網(wǎng)絡(luò)，從而構(gòu)成一個(gè)頻率可以在 150Hz1. 5kHz之間變化的振蕩器。圖 23至圖 27列出了幾種實(shí)際使用的，具有自動(dòng)穩(wěn)幅功能的文氏橋式振蕩器 。通常，文氏橋是平衡的，所以 R1 =R2=R,C1=C2=C。上述兩條原則是在純理論的基礎(chǔ)上必須要滿足的，同時(shí)，我們根據(jù)實(shí)際的考慮，再添上第三條一般原則，即： 在每個(gè)實(shí)際的振蕩器中，環(huán)路增益 都略大于 1，并且振蕩幅度由非線性特性來限制?，F(xiàn)在假定 |FA|大于 1，那么，最初出現(xiàn)在輸 入 端的信號(hào)，例如是 1V，在繞回路一周又回到輸入端時(shí)，其幅值將大于 1V,然后 這個(gè)較大的電壓又會(huì)以更大的電壓再出現(xiàn)于輸入端，如此循環(huán)往復(fù)。顯然還必須滿足另一個(gè)條件，即 Xi和 Xf’的幅度必須相等。 巴克豪森判據(jù) :在以下關(guān)于振蕩器的討論中我們假定 ，整個(gè)電路工作在線性 狀 態(tài)，并且放大器或反饋網(wǎng)絡(luò)或它們兩者是含有電抗元件的 。 at that point, (the input voltage falls to a value equal to 80 mV) the output regenerative switches back to the positive saturation level. The switching levels can be altered by changing the R1 value Fig 224 Schmitt trigger prevents output oscillations caused by triggering off a slow sinewave. 波形發(fā)生器 1. 正弦振蕩器基本原理 許多不同組態(tài)的電路，即使在沒有輸人信號(hào)激勵(lì)的情況下，也能輸出一個(gè)基本上是正弦形的輸出波形。 2Hz20Hz, 20Hz200Hz, 200Hz2kHz, and 2 kHz20 kHz, respectively. Fig. 213 Four decade 2 Hz~20 kHz square wave generator. Variable dutycycle In Fig. 210, C1 alternately charges and discharges via R1, and the circuit generates a symmetrical squarewave output. That circuit can be modified to give a variable dutycycle output by providing d with alternate charge and discharge paths. In Fig. 214, the duty cycle of the output waveform is fully variable from 11:1 to 1: 11 via R2, and the frequency is variable from 650 Hz to kHz via R4, The circuit action is such that C1 alternately charges through R1D1 and the bottom of R2, and discharges through R1 –D2 and the top of R2. Notice that any variation of R2 has negligible effect on the operating frequency of the circuit. In Fig. 215, the duty cycle is determined by C1D1R1 (mark), and by C1D2R2 (space). The pulse frequency is variable between 300 Hz to 3 kHz via R4. Fig. 214 Squarewave generator with variable dutycycle, and frequency. Fig. 215 Variable frequency narrowpulse generator. Resistance activation Notice from the description of the oscillator in Fig. 210 that the output changes state at each half cycle when the C1 voltage reaches the threshold value set by the R2 –R3 voltage divider. Obviously, if C1 is unable to attain that value, the circuit will not oscillate. Fig. 214 shows a resistance activated oscillator that will oscillate only when R4, which is in parallel with C1, has a value greater than R1. The ratio of R2:R3 must be 1:1. The fact that R4 is a potentiometer is only for illustration. Most resistanceactivated oscillators use either thermostats or LDR39。 therefore, the opamp must be given a voltage gain of 3 via feedback work R3R4, which gives an overall gain of unity. That satisfies the basic requirements for sinewave oscillation. In practice, however, the ratio of R3 to R4 must be carefully adjusted to give the overall voltage gain of precisely unity, which is necessary for a lowdistortion sine wave. Opamps are sensitive to temperature variations, supplyvoltage fluctuations, and other conditions that carse the opamp’s output voltage to vary. Those voltage fluctuations across ponents R3R4 will also use the voltage gain to vary. The feedback work can be modified to give automatic gain adjustment (to increase amplifier stability) by replacing the passive R3R4 gaindetermining work with a gainstabilizing circuit. Figs. 23 through 27 show practical versions of Wienbridge oscillators having automatic amplitude stabilization. Fig. 23 Thermistorstabilized 1kHz Weinbridge oscillator. Fig. 24 Lampstabilized Wienbridge oscillator. Fig. 25 Dioderegulated Wienbndge oscillator. Fig. 26 Zenerregulated Wienbridge oscillator. Fig. 27 Three decade 15 Hz~15 kHz Wienbridge oscillator. 2. 2 Thermistor stabilization Fig. 23 shows a 1kHz fixedfrequency oscillator. The output amplitude is stabilized by a Negative Temper