Radio 2002, 5
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.
Figure 1. Square wave generator circuit diagram
C1 - 470 pF, VT1 - BC547, VT2 - BC557
The circuit works this way: when 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 first stage (resistor R1), and in the second stage the emitter circuit of transistor VT2 has a resistor R5.simple_square_wave_generator_circuit_e.html
The generator operates steady at a supply voltage in range from 1.5 to 12 V, while the consumption current is in range from 0.15 mA to some mA. The amplitude of the output pulses on the "Out 1" is a little more than half of the supply voltage, and at the "Out 2" it is about 10 times smaller. If you want, you can add another stage of the voltage divider (1/100), by inserting the resistor of 24 Ohms between the lower terminal of the resistor R4 and the common wire.
With the values of components, shown in fig. 1, and with the supply voltage of 2.5 V the consumption current of the circuit is 0.2 mA and the frequency is 1000 Hz, the duty-cycle is 50% (meander), the amplitude of the signal at the "Out 1" is 1V.
Of course, in such a simple generator the parameters of the signal is significantly depends on the power supply voltage. Therefore, the generator should be adjusted at the same voltage with which it will be used. In case of the absence of the generation, adjust the value of resistor R1 and, possibly, R5 (replace it by resistors with another value). Duty cycle is set by selecting the value of the resistor R2.
One possible application of this generator is a flashing light. Then the load (LED or incandescent lamp) is connected in series with the resistor R5, and use the capacitor C1 with capacity of 1 μF to get the frequency of oscillation about 0.5...1 Hz. To obtain the necessary brightness of the light, the value of resistors R3, R5 can be reduced, and the resistor R4 can be eliminate as unnecessary.
V. Polyakov, Moscow
Download LTSpice model of the circuit: simple_square_wave_generator_circuit_e.asc. It uses KT315G and KT361G transistors. Download PSpice models of this transistors and insert them into the file "LTspiceIV\lib\cmp\standard.bjt" of LTSpice.