Multivibrator with a short back-to-back rectangular pulses

 

(57) Abstract:

The invention can be used in the field of pulsed electronic equipment when creating switching, counting devices, and also for processing of binary information in digital technology. The multivibrator includes two electronic key (K) (VT1, VT2), performed on the transistor (T) of different type conductivity, the positive feedback circuit containing ramasadaya capacitors (K) (C1, C2) and two series-connected amplifier (Y) on T (VT3, VT4). Resistor voltage dividers form the collector load U. To connect the manifold T each At a common point, uniting collectors TC. Base T circuit PIC form a common output device with respect to the middle point of the power source. In the multivibrator formation of the trailing edge is the same as the front, under the influence of positive feedback. Technical result: the scheme generates rectangular pulses that are symmetric with respect to the time axis. 4 Il.

The invention relates to techniques of automatic control and can be used when developing an electronic pulse circuit to generate switching, counting devices, the device shall thevibrator [1, 2] - shapers recurring almost rectangular voltage pulses. In Fig. 1 shows the most common (classical) multivibrator circuit with the collector-base connections. Multivibrator with two electronic keys transistors VT1 and VT2, the output voltage of each of which is fed to the input of another, the result is a positive feedback between the electronic key and two-stage system acquires the ability to excitation. Is the periodic transfer of a circuit from one state to another, accompanied by a rapid increase in current of one transistor, and switching the transistor into saturation and the simultaneous decline of the current of the other transistor and the transition into the state of cutoff. The multivibrator is in one of the two quasi-stable States. The duration of finding it in this state is determined breathtaki chains RB1C1and RB2C2< / BR>
If, for example, at time t0opens the transistor VT1, and the transistor VT2 is closed, and the process of charge of the capacitor C2chain +E open the emitter junction of transistor VT1, C2, RK2, -E to the voltage UC2E with a constant time is of ancestor, creates a voltage drop UK2= i2RK2. Thus, the voltage at the locked transistor is reduced to a value-E is not a leap, and decreases exponentially with a time constant of2= RK2C2. In the process of operation of the multivibrator is formed by a pulse with a steep leading edge and stretched back, the duration of which considerably exceeds the length of the front, as shown in the plots of Fig. 2.

The capacitor at t > t0that was in the previous half-period charged to a voltage UC1-E, begins to reload, and in that moment, when the voltage across it passes through zero, the voltage at the base of VT2 is also zero, and it opens, closing in the feedback circuit of the transistor VT1. As and when closing, the voltage on the collector is not immediately reaches the value E, and decreases with a time constant of1= RK1C1< / BR>
The delay of the trailing edge is because unlike the front, arising under the influence of positive feedback, it is simply the charge of the capacitor with a constant temporizing = Rtoc.

Poor pulse shape associated with the sloping rear front, is the main weeks the Directors. The most famous are:

1. The multivibrators with shut-off diodes [3];

2. The multivibrators with clamping diodes [3].

In the first case, the locking transistor timing capacitor is disconnected from the collector resistance in the closure of the shut-off of the diodes, and the voltage at the collector of the jump is reduced to-e. the Length of the front and rear fronts here almost equal, the maximum duty cycle is less than the basic scheme [3].

In the second case, the reduction of the trailing edge is achieved by the fact that the supply voltage by using an additional source is fixed at a low level. The smaller the additional voltage source, the less the duration of the trailing edge. However, to the same extent decreases the amplitude of the pulse of multivibrator [3].

In circuits with shut-off and clamping diodes the formation of pulses does not differ from the processes in the main circuit, so as a prototype, select the multivibrator circuit with the collector-base connections.

The objective of the invention is to develop multivibrator that generates a pulse with a short back-to-back, not exceeding in duration the front.

This task deactiviated with short back-to-back powered by two voltage sources E, connected in series and having a grounded common point around which removed the output voltage. He established two electronic keys ab and cd transistors of the opposite conductivity VT1 and VT2 with their collector resistors RK1and RK2included in parallel circuit with positive feedback MN. This circuit includes two series-connected amplifier transistors of the opposite conductivity VT3 and VT4, the emitters of which are connected and grounded, a base connected between a common point About' United collectors VT1 and VT2, forming the output of the circuit relative to the earth. The General point About' are connected via capacitors C1and C2with collectors of transistors VT3 and VT4. The base of the main transistor VT1 and VT2 are connected with the collector resistors of the amplifiers so that the base of the transistor of the main circuit (VT1 or VT2) connected to the transistor circuit positive feedback MN, which has the opposite conductivity type. For simplicity, we will choose the elements of the flowchart shown in Fig. 3 from the condition

RK1= RK2; R3= R'3+R3= R'4+ R4= R4; C1= C2.

Then the processes occurring below the middle line O is the connection of the supply voltage in the point O' in the first moment due to the equality RK1= RK2is equal to 0. At the moment when t0arise charging current of the capacitor C1, powered by chain E, R3C1About' and capacitor C2, powered by chain O', C2, R4, -E. the Charging currents flowing through the resistance R3and R4create on the bases of the transistors VT1 and VT2 voltage relative to the emitter, open transistors and enter into the active mode. Due to the incomplete symmetry, technological factors and random processes, the collector currents of transistors VT1 and VT2 are different from each other and the voltage drop across the resistors RK1and RK2not the same, which leads to the deviation of the voltage at point O' 0. And here comes into action positive feedback: if the voltage at point' was higher than 0, the voltage at the bases of transistors VT3 and VT4 is also greater than 0. When this transistor VT3 slightly open and its collector current will increase the voltage on the base of the transistor VT1 with respect to the emitter (which is necessary for this mode is active). At the same time, the current of the transistor VT4 will decrease. The current under the action of positive feedback VT1 is in saturation and transistor VT2 is closed. In this case the action of the feedback circuit is stopped, and the whole system only voltage at point' was below 0, opens the transistor VT4, and the positive feedback leads to saturation of the transistor VT2 and the emergence of the cutoff current of the transistor VT1. The process of opening one transistor and simultaneously closing the other there is an avalanche, because both transistors are in the active region and between acts positive feedback. Output almost instantly sets the voltage slightly differs from the voltage source E.

Open state of the transistor VT1 (VT2) is maintained by the voltage drop on the part of the collector load VT3 (VT4), and current transistors VT3 and VT4, in turn, is provided by the voltage at the collector of the main transistor VT1 or VT2. The closed state of the other transistor provides a zero voltage on its base when closed, the transistor VT3 (or VT4). Change mode transistors, resulting in the change of state is due to the fact that the capacitors C1and C2charged at the first moment of operation of the circuit until the voltage Usoand Usoand saved these voltage in the avalanche process of transformation will continue to change its state.

After opening of the charging transistor VT1 t is another way E, RK2C1, outdoor transistor VT3, 0. But when you change the direction of the charge in the direction of the current in the resistor R3will not change and the transistor VT1 is still open.

At the same time, the negative voltage on the capacitor C2at closing of the transistor VT2 at time t0and therefore VT4, is formed with a negative voltage source and their sum is less than-that is, the Capacitor C2will be recharged to change its charge to the opposite.

But in the moment when the voltage across it goes to zero, and the voltage on the base will be slightly less than the source voltage E, the transistor VT2 will open and cause the locking of the transistor VT1 multivibrator moved to another quasi-stable state.

Now the voltage at the point O' negative and not significantly different from the negative voltage source to the collector value of the saturation voltage of transistor VT2. Then, after locking the transistors VT1 and VT3 voltage at point M will jump to a value close to E, and the voltage on the capacitor C1maintains its charge will be higher voltage source + E. Begins charging capacitor C1and after passing CLASS="ptx2">

Sources of information

1. Kaganov, I. L. Industrial electronics. -M.: Higher school, 1983, 558 S.

2. Isakov, Y. A. Industrial electronics. -Kiev: high school, 1975, 328 S.

3. Stepanenko, I. P. fundamentals of theory of transistors and transistor circuits. - M. : Energy, 1977, 672 S.

Multivibrator with a short back-to-back rectangular pulses formed by two electronic keys and a positive feedback circuit containing ramasadaya capacitors, characterized in that the power source has an output medium point, transistors, electronic keys have a different conductivity type, the positive feedback circuit includes two series-connected amplifier with transistors of different conductivity type, the emitters of which are connected to the middle point of the power source, and the collector circuit is enabled resistor voltage dividers, forming a collector load amps, part of which is between the poles of the power source and the midpoint of the divider is connected to the input of the transistor so that the conductivity type of the transistor of the key opposite to the conductivity type of the transistor amplifier, ramasadaya capacitor connecting the collector of each transistor politely connection form a common output device with respect to the middle point of the power source

 

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