A simple square wave generator. Encyclopedia of radio electronics and electrical engineering

Encyclopedia of radio electronics and electrical engineering / Measuring technology

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To test and adjust various amplifiers, including 3H amplifiers, it is useful to use a rectangular pulse generator. Typically, such generators are made according to the scheme of a symmetrical multivibrator on two bipolar transistors of the same structure and with two frequency-setting circuits. However, it is possible to assemble a simpler generator on two transistors of different structures (see figure) with one frequency-setting circuit.

The generator works like this. When the supply voltage is applied (capacitor C1 is not charged), the transistor VT1 is slightly opened by the current flowing through the bias resistor R1. The collector current of this transistor is the base for VT2 and opens it. The growing voltage on the collector load of the latter through the C1R2 chain opens the transistor VT1 even more, as a result, an avalanche-like process of opening both transistors occurs - the front of a rectangular pulse is formed.

The duration of the top of the pulse is determined by the duration of the charging of the capacitor C1 through the resistor R2. As this capacitor charges, the base current of transistor VT1 decreases and there comes a moment when an avalanche process of closing both transistors occurs. A negative voltage drop is formed on the load - a drop in the pulse. The duration of the pause between pulses is determined by the duration of the discharge of the capacitor C1 by the current flowing through the resistors R1 and R2. Then the process is repeated.

The operation of the generator can be explained differently. The two-stage amplifier is covered by a positive feedback circuit (R2C1 elements) and at the same time brought to the linear mode of the transistor VT1 by applying a bias to its base through the resistor R1. Therefore, relaxation oscillations arise. To stabilize the operation of the generator, each stage is covered by the OOS circuit - in the first stage it is small and is carried out through the resistor R1, and in the second stage, the resistor R2 is connected to the emitter circuit of the transistor VT5.

The generator operates stably at a supply voltage of 1,5 to 12 V, while the current consumption is from 0,15 to milliamps. The amplitude of the output pulses at "Output 1" is slightly higher than half the supply voltage, and at "Output 2" it is about 10 times less. If desired, you can make another step of division (1/100) by adding a resistor with a resistance of 4 m between the lower output of the resistor R240 according to the circuit and the common wire.

With the ratings of the parts indicated in the diagram and at a supply voltage of 2,5 V, the consumed current was 0,2 mA, the pulse frequency was 1000 Hz, the duty cycle was 2 (meander), the pulse amplitude at "Output 1" was 1 V.

Of course, with such a simple generator, the signal parameters noticeably depend on the voltage of the power source. Therefore, the generator should be tuned at the voltage at which it will be used. In the absence of generation, a resistor R1 and, possibly, R5 are selected. The duty cycle of the pulses is set by selecting the resistor R2.

One of the possible applications of the generator is as a flashing light beacon, for example, in a watchdog. Then, in series with resistor R5, an LED or a miniature incandescent lamp is turned on, and a capacitor is used with a capacity of up to fractions of a microfarad so that the generation frequency is 0,5 ... 1 Hz. To obtain the required brightness of the indicator light, you can install resistors R3, R5 of lower resistance, and exclude R4 as unnecessary.

Author: V.Polyakov, Moscow

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