Pulse generators (multivibrators, self-oscillators). Encyclopedia of radio electronics and electrical engineering

Encyclopedia of radio electronics and electrical engineering / Beginner radio amateur

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A variant of the simplest generator (multivibrator) is shown in fig. 1a.

Rice. 1. Pulse generator on two inverters

The circuit has two dynamic states. In the first of them, when the output D1.1 state is log. "1" (output D1.2 log. "0"), capacitor C1 is charging. In the process of charging, the voltage at the input of the inverter D1.1 increases, and when the value Upor=0,5Upit is reached, there is an abrupt transition to the second dynamic state, in which the outputs D1.1 log. "0", D1.2 - "1". In this state, the capacitance is recharged (discharged) by the reverse current. When the voltage at C1 Unop is reached, the circuit returns to the first dynamic state. The voltage diagram explains the operation. Resistor R2 is limiting, and its resistance should not be less than 1 kOhm, and so that it does not affect the calculated frequency, we select the value of resistor R1 much more than R2 (R2<0,01R1). A limiting resistor (R2) is sometimes installed in series with the capacitor. When using a non-polar capacitor C1, the pulse duration (ti) and pause (to) will be almost the same: ti=to=0,7R1C1. Full period T=1,4R1C1. Resistor R1 and capacitor C1 can be in the range of 20 k0m ... 10 MΩ; 300 pF...100 uF.

When using two inverters of the K1LN561 chip in the circuit (Fig. 2b) (they have only one protective diode at the input), the capacitor will be recharged from the Upit + Unop level. As a result, the symmetry of the pulses is broken ti=1,1R1C1, to=0,5R1C1, period T=1,6R1C1.

Since the switching threshold of the logic elements does not correspond exactly to half the supply voltage, in order to obtain the symmetry of the pulses, a circuit of R2 and VD1 can be added to the traditional oscillator circuit, fig. 1c. Resistor R2 allows tuning to get a meander (ti=to) at the output of the generator.

Fig 2. Pulse generator with separate setting of pulse duration and pause between them

The scheme in fig. 2 makes it possible to separately adjust the duration and pause between pulses: ti=0,8C1R1, to=0,8C1R2. With the values ​​of the elements indicated in the diagram, the pulse duration is about 0,1 s, the repetition period is 1 s.

Rice. 3. Pulse generator on three inverters

The frequency is more stable for generators made on three inverters (Fig. 3). The process of recharging C1 in the direction of decreasing the voltage on the left plate starts from the voltage Up+Unop, as a result of which it takes more time ti=1,1C1R2. The full period of oscillations will be T=1,8C1R2.

Rice. 4. Pulse generator with separate adjustment a) pulse duration and pause between them b) pulse duty cycle

On fig. 4 shows diagrams of similar generators, which allow you to separately adjust the duration and pause between pulses or, at a constant frequency, adjust the duty cycle of the pulses. A multivibrator based on a Schmidt trigger is shown in fig. 5.

Rice. 5. Overlapping pulse generator

If it is required to obtain symmetrical pulses without adjustment at the output of the above generator circuits, then after the circuit it is necessary to put a trigger or use the circuit on three inverters, fig. 6.

Rice. 6. Generator with symmetrical output pulses

Element D1.1 is used to create a second negative feedback circuit that surrounds inverter D1.2 (resistor R5 forms the main feedback circuit for the signal) IC element D1.1 operates in low-gain mode with closed feedback, similar to an operational amplifier operating in linear part of the characteristic As a result of this, the inverted threshold voltage of the inverter D1 1 can be summed with the negative feedback voltage and applied to the input of the element D1.2. If the ratio R2/R1 is equal to the ratio R3/R5, full compensation of errors due to changes in the threshold voltages of the elements D1.1 and D1.2 can be obtained. It is assumed that all elements of the circuit are located in the same package and their threshold voltages are actually equal. The pulse frequency of such a circuit is determined from the ratio F=1/R5C1 (it will be approximately twice as high as compared to the circuit shown in Fig. 1).

Rice. 7. Symmetrical multivibrators

a) on an RS flip-flop with two capacitors

d) on two RS flip-flops

A symmetrical multivibrator can be made on the basis of an RS flip-flop, Fig. 7. A variant of the circuit in Fig. 7v allows resistors R1 and R2 to be chosen with lower resistance, because the diodes separate the charge circuit from the trigger outputs. The second advantage of this scheme is that it allows you to easily and independently control the period and duty cycle of the generated pulses within certain limits. The duty cycle can be adjusted linearly if R1 and R2 are combined into one potentiometer, and the period can be adjusted if the common end of R1 and R2 is connected to a power source through a potentiometer. In order to reduce the number of discrete elements, a multivibrator circuit based on two RS flip-flops is proposed, fig. 7y.

Rice. 13. Circuits that provide increased frequency stability when the ambient temperature changes over a wide range

With a stabilized power supply, the change in the duration of the pulses of multivibrators and the frequency in generators on RC circuits is usually no better than 1% per 15 ° C (in the case of using thermostable capacitors). Greater frequency stability can be obtained using quartz stabilization. On fig. 12 and 13 show typical schemes for constructing such generators. For a small frequency adjustment, sometimes a 10 ... 100 pF capacitor is installed in series with a quartz resonator. The frequency of pulses and their stability in this case at the generator is set by the parameters of the quartz resonator.

Publication: irls.narod.ru

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