【正文】 om 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。 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。 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 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。 the