【正文】 rent values determined by R3R4. Basic function generator for both triangular and square waves. Fig. 220 100 Hz 1 kHz function generator for both triangular and square waves. Fig. 221 shows how to modify to make a variable symmetry ramp/rectangular generator T where the slope of the ramp and duty cycle is variable via R4. C1 alternately charges through R3D1 and the upper half of R4, and discharges through R3D2 the lower half of R4. 400 Hz~1kHz function generator with variable slope and duty cycle. 2. 9 Switching circuits Fig 222 shows the connections for making a manually triggered bitable circuit. Notice that the inverting terminal of the opamp is tied to ground via R1, and the n oninverting terminal is tied directly to the output. Switches S1 and S1 are normally open. If switch S1 is briefly closed the opamp inverting terminal is momentarily pulled high, and the output is driven to negative saturation j consequently , when S1 is released again T the inverting terminal returns to zero volts ,but the output and the noninverting terminal remains in negative saturation. The output remains in that state until S1 is briefly closed。 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。 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。 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 ?，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)。通常，文氏橋是平衡的，所以 R1 =R2=R,C1=C2=C。這個(gè)電路還示出了用雙聯(lián)可變電位器來(lái)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。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。 雙 T網(wǎng)絡(luò)能構(gòu)成一個(gè)很好的固定頻率的振蕩器。 圖 212示出了如何改進(jìn)圖 211。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。 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。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。 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。 環(huán)路增益為 1,即 FA=1這個(gè)條件叫做巴克豪森判據(jù) 。 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。s gain of , the overall gain bees unity. If the oscillator output amplitude starts to rise, RT heats up and reduces its resistance, thereby automatically reducing the gain of the circuit, which stabilizes the amplitude of the output signal. An alternative method of thermistor stabilization is shown in Fig. 24, In that case, a lowcurrent lamp is used as a Positive Temperature Coefficient (PTC) thermistor, and is placed in the lower part of the gaindetermining feedback work. If the output amplitude increases, the lamp heats up thereby increasing its resistance, reducing the feedback gain, and providing automatic amplitude stabilization. That circuit also shows how the Wien work can be modified by using a twinganged potentiometer to make a variablefrequency oscillator over the range 150 Hz kHz. The sinewave output amplitude can be made variable using R5. A slightly annoying feature of thermistorstabilized circuits is that, in variablefrequency applications, the output amplitude of the sine wave tends to jitter or bounce as the frequency control potentiometer is swept up and down its range. Diode stabilization The jitter problem of variablefrequency circuits can be minimized by using the circuits of Figs. 25 or 26 which rely on the onset of diode or Zener conduction for automatic gain control. In essence, R3 is for a circuit gain slightly greater than unity when the output is close to zero, causing the circuit to oscillate。 (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。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(wèn)題 。該條件概括為下述原則： 在振蕩頻