【文章內(nèi)容簡(jiǎn)介】 reater than unity when the output is close to zero, causing the circuit to oscillate。 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。 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。 inverted phase shift with a smallsignal f0. Because the inverting opamp also causes a 180186。 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。 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。 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