Controlled single vibrator. Encyclopedia of radio electronics and electrical engineering

Encyclopedia of radio electronics and electrical engineering / Radio amateur designer

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Controlled generators in general and single vibrators in particular are most often performed by radio amateurs on standard microcircuits of the AG and GG groups. Meanwhile, non-standard implementations of such generators, in addition to optimizing the design, sometimes predetermine the appearance of a number of new interesting effects and properties of a particular device as a whole. However, there are very few publications on this topic in Radio and other popular literature.

The author of this article shares his experience in mastering controlled single vibrators built according to a non-trivial scheme.

Described in [1] (scheme - in Fig. 8, a) single vibrator on the trigger has a fairly wide range of capabilities, but it also has some disadvantages. Firstly, the charging of the capacitor C1 occurs through the output resistance of the trigger. On fig. 1a shows a fragment of the circuit of this single vibrator with time-setting circuits, the output resistance Rout is conditionally shown outside the trigger. Changing Rout affects the duration of the generated pulse. Secondly, the time for restoring the voltage on the capacitor to a predetermined level is long (relative to the duration of the generated pulse). Thirdly, there is no functionality for electronic control of the duration of the output pulse, which narrows the scope of the node.

Consider the charging and discharging circuits of the capacitor C1 in a single vibrator. At the stage of formation of the time interval tо, the capacitor is charged from 0 (more precisely, from the residual voltage) to the threshold voltage Uthr through the circuit: positive output of the power source - Rout-R1-C1-common wire.

At the recovery stage, the capacitor is discharged from Upor to 0, first through the VD1 diode and the output resistance Rout, and at the end, when the VD1 diode closes, through the resistor R1.

The diode closes almost completely when the voltage across it drops below 0,5...0,6 V, and the capacitor finishes discharging with the same time constant as when the time interval was formed. Thus, with more stringent requirements for the residual voltage on the capacitor, the recovery time increases, limiting the allowable pulse repetition rate for a given recovery error.

Of course, the recovery time can be significantly reduced to bring the capacitor to its original state by using an additional discharge transistor, but this will complicate and increase the cost of the design. It turns out that it is possible to reduce the recovery time of a single vibrator and expand its functionality without complicating it in a fairly simple way.

In a single vibrator according to the scheme in Fig. 1, b there are the same number of parts, but the right terminal of the resistor R1 is connected to the positive power wire. Here, the output impedance of the trigger does not affect the charging time of the capacitor C1.

Capacitor C1 is charged from the voltage Ud on the VD1 diode to Uth along the circuit: the positive power wire-resistor R1-capacitor C1-common wire, and is discharged - from Uth to Ud through the diode VD1-output resistance Rout.

Thus, in a single vibrator according to the scheme in Fig. 1b, firstly, there is no effect of the output resistance of the trigger on the generated time interval, and, secondly, the second part of the recovery stage (capacitor discharge through the resistor), which increases the total recovery time, is excluded. Indeed, after the completion of the formation of a given period of time by a single vibrator, the diode remains an open current flowing through the resistor R1. The resistance of the diode remains low, which provides a quick recovery of the initial voltage across the capacitor. True, this somewhat increases the power consumption of the single vibrator in standby mode.

On fig. 2 shows the voltage diagrams at the input R of the trigger at the recovery stage for a single vibrator according to the circuit of fig. 1a (curve 1) and Fig. 1b (curve 2). In both cases, the discharge of the capacitor to the closing voltage of the diode UD (for a silicon diode - about 0,5 ... 0,6 V) practically ends by the time t1. For the second case, recovery almost ends here, so the recovery time is close to t1-t0.

In the first case, the capacitor should be discharged almost to zero, but due to the fact that after the moment t1 the diode is closed, the discharge is delayed and even after the time R1C1 the voltage across the capacitor will be equal to 0,6 / e ~ 0,2V (e is the base of the natural logarithm). Therefore, the recovery time here is much longer.

Single vibrator according to the scheme of fig. 1b has another significant advantage - the output of the resistor R1 can be energized not from the positive power wire, but, for example, from a source with adjustable voltage, which makes it possible to control the pulse duration electronically by changing the voltage at the output of the resistor. The scheme of a controlled single vibrator is shown in fig. 3, and control characteristics - in fig. 4, curve 1.

Note that if the values ​​of the time constant of the RC-circuit of single vibrators are equal according to Fig. 1a and 3 and Ucontrol = Upit the duration t0 of the output pulse of the second is slightly less than the first. The reason for this is that the capacitor C1 of the second single vibrator is charged not from zero, but from some initial voltage Ud, so the capacitor will be charged up to Upor in less time.

The interval of control voltage values ​​must satisfy the condition: Upr < Ucontrol < Upit (1), which corresponds to curve 1 in fig. 4.

In cases where such an interval turns out to be inconvenient, it can be extended to 0 < Ucontrol < Upit (2) by introducing another resistor - R2 - of approximately the same rating, as shown in Fig. 5. The control characteristic for this case is shown in fig. 4, curve 2. If the single vibrator is controlled by an operational amplifier, by choosing R1=3R2, the control interval can be extended to -Upit < Ucontrol < +Upit (3) - this option is illustrated by curve 3 in fig. 4.

If it is necessary to make a ready-made single vibrator, made according to the scheme of Fig. 1,a, it is enough to introduce an additional resistor into it, like R1 - in fig. 5. To preserve the pulse duration at Ucontrol = Upit, it is necessary that the resistance of R1 and R2 connected in parallel (according to Fig. 5) be equal to the resistance R1 in the initial node - this is condition (4).

It should be noted that in single vibrators according to Fig. 1, b, 3 and 5 resistors serve to set a certain current that charges the capacitor C1. This current can be provided in the absence of resistors by an external source of control current, collected, for example, on pnp transistors. Such a solution makes it possible to implement an inversely proportional dependence of the duration of the generated pulse on the control current.

Ratings of resistors of single vibrators according to the scheme in fig. 3 and 5 it is permissible to vary over a wide range - from 10 kOhm or more, capacitors - from 100 pF or more. To provide the possibility of increasing the capacitance of the capacitor, it is necessary to connect another resistor in series with the diode, which limits the discharge current of the capacitor. The pulse duration at Ucontrol = Upit, keeping in mind condition (4), must be estimated according to the relationships described in [1].

The considered controlled single vibrator requires 1/2 of the microcircuit package for implementation, and the one described, for example, in [2] (in Fig. 2) requires 3/4 of the package. In general, the RS-trigger for a single vibrator can be implemented on various logic elements and nodes of digital technology [3]. The connection of two single vibrators into a ring makes it possible to implement a pulse generator controlled by two inputs with a wide overlap in frequency and duty cycle.

Literature

1. Alekseev S. Shapers and generators based on microcircuits of the CMOS structure. - Radio, 1985, No. 8, p. 31 - 35.
2. Ignatenko A. Voltage controlled generator. - Radio, 1994, No. 6, p. 22.
3. Samoilenko A. Options for constructing an RS flip-flop. - Radio, 1998, No. 9, p. 53 - 56.

Author: A.Samoilenko, Klin, Moscow Region

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