Simple square wave generator

Posted on Feb 5, 2014

Typically square wave generators are based on symmetrical multivibrator with bipolar transistors of the same structure and with two frequency determining network. However, you can construct more simple oscillator with two transistors of different structure (see figure 1) with only one frequency determining network. The circuit works this way: when

Simple square wave generator
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the supply voltage is applied (capacitor C1 is not charged) the transistor VT1 begins to conduct the current flowing through the bias resistor R1. Collector current of this transistor is the base current for the transistor VT2 and this collector current opens the transistor VT2. The voltage at the collector load of the transistor VT2 increases through a network of C1R2, and this further opens the transistor VT1, and as a result there is an avalanche process of opening the two transistors - it`s forming the front of the rectangular pulse. The duration of the pulse is determined by charging the capacitor C1 through resistor R2. As the capacitor C1 is charging, the base current of the transistor VT1 is reduced, and there comes a time when there is starting an avalanche process of closing both transistors. On the load is formed a falling pulse edge. The duration between pulses is determined by the duration of the discharge of the capacitor C1 by the current flowing through resistors R1 and R2. Then the process repeats. The work of the generator can be explained differently. Two-stage amplifier circuit has a positive feedback (network R2C1) and in the same time the amplifier is put in the linear mode of the transistor VT1 by applying bias on its base through resistor R1. Therefore, the relaxation oscillations are generated. To stabilize the operation of the generator each cascade has a negative feedback - it is weak in the...

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