【正文】 ature Coefficient39。 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。 as each halfcycle nears the desired peak value, one of the diodes starts to conduct, which reduces the circuit gain, automatically stabilizing the peak amplitude of the output signal. That limiting” technique typically results in the generation of 1% to 2% distortion on the sinewave output. The maximum peaktopeak output of each circuit is roughly double the breakdown voltage of its diode regulator element. In Fig 25, the diodes start to conduct at 500 mV, so the circuit gives an output of about 1volt peaktopeak. In Fig, 26, the Zener diodes D1 and D2 are connected backtoback, and may have values as high as 5 to 6 volts, giving a pp (peaktopeak) output of about 12 volts. Each circuit is set up by adjusting R3 for the maximum value (minimum distortion) at which oscillation can be maintained across the frequency band. The frequency range of Weinbridge oscillators can be altered by changing the C1 and C2 values。 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 current 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。當(dāng)信號(hào) Xi直接加到放大器的輸入端時(shí)，放大器提供一個(gè)輸出信號(hào) X0。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）。當(dāng)然，這個(gè)條件意味著不僅要求 |AF|=1，而且要求 AF的相位是零。隨著振幅的增大，有源器件的非線性變得更加明顯 。因?yàn)轭l率選擇網(wǎng)絡(luò)通常有負(fù)增益 ， 為了保持全部增益為 1， 必須用增益網(wǎng)絡(luò)中的附加增益來(lái)補(bǔ)償。間變化，在中心頻率 f0處恰好是零。熱敏電阻由運(yùn)放輸出的平均功率來(lái)加熱。 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。 在整個(gè)頻帶中，每個(gè)電路均通過(guò) R3調(diào)節(jié)到最大電壓輸出。如圖 28所示 。 圖 28 1kHz 雙 T振蕩器 圖 29 二極管穩(wěn)幅的 1kHz 雙 T振蕩器 圖 29示出了可以使失真更小的幅度控制方法 。電路使用雙電源供電。用 R2微調(diào)頻率，同 R4轉(zhuǎn)換頻段，用 R6來(lái)控制輸出幅度。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 。 在這些條件下，輸出信號(hào)將有峰值約為 500mV的振幅，進(jìn)一步改變 R5可使輸出信號(hào)的有效值在 170300mV間變化。 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 。利用齊納二極管穩(wěn)幅，電路的輸出幅度用開(kāi)關(guān)和可變衰耗器來(lái)調(diào)節(jié)。這種限制技術(shù)常引起輸出正弦波有 1%2%的失真。如果輸出電壓幅度上升，則燈被加熱而 電阻增加，反饋增益減小，從而實(shí)現(xiàn)自動(dòng)振幅穩(wěn)定。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。選頻文氏橋由 R1C1，和 R2C2網(wǎng)絡(luò)構(gòu)成 。 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 |FA|小于 1，那么除去外部信號(hào)源將會(huì)導(dǎo)致停振。 只要電路能振蕩，其頻率就由上述原則來(lái)確定。條件 Xf’ =Xi等價(jià)于 AF=1,即環(huán)路增益必須等于 1。s to the noninverting input The output remains in that state until the input falls to 80 mV。 those waveforms swing symmetrically on both sides of ground. Notice that the operating frequency can be varied by altering either the R1 or C1 values, or by altering the R2R3 ratios, which makes that circuit quite versatile. Fig. 211 shows how to design a practical 500 Hz to 5kHz squarewave generator, with frequency variations obtained by altering the attenuation ratio of R2R3R4. Fig11212 shows how to improve Fig. 211 by using R2 to preset the range of frequency control R4, and by using R6 as an output amplitude control. Fig. 211 500Hz5kHz squarewave oscillator. Fig. 212 Improved 500Hz 5k Hz squarewave oscillator. Fig. 213 shows how to design a general purpose squarewave generator that covers the 2Hz to 20kHz range in four switcheddecade ranges. Potentiometers R1 to R4 are used to vary the frequency within each range。s output and the noninverting input, so that the I circuit gives zero overall phase shift at f0, where the voltage gain is 。to ＋ 90176。 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, t