# Simple Square Wave Generator Circuits

Posted on Feb 6, 2014

Using a field effect transistor at the input of a schmitt trigger, means a low value capacitor can be employed. The trigger by Q1 and O2 has a hysteresis of approximately 3V. This is controlled by the 3V zenen Mth C1 uncharged O1 is off and O2 is forward biased. The voltage at the source of Q1 is approximately +4V. O2 conducts, thus turning on TR

3. The output is therefore at +1OV. C1 then charges via R1 and the gate voltage of Q1 goes positive. When the gate voltage is sufficiently positive Q1 conducts, turning off Q2. The positive feedback from the emitter of (12 to the source of (11 ensures a rapid switch off. Q3 also switches off and the output goes to -5V. Capacitor C1 now discharge towards -5V, but when the voltage across C1 falls by approximately 3V, Ol ceases to conduct, turning on 0. 2. The collector load of Q3 is connected to a negative supply giving a 50% duty cycle. (The circuit still oscillates if R7 is connected to GV but the duty cycle will change, the output remaining at OV for a longer period than at +10V). With the components as shown the frequency of the output is approximately 0. 025Hz. The multiple amplifiers in the LM3900N device are very suitable for use ln waveform generators at frequencies of up to about 10 kHz. Voltage controlled oscillators (the frequency of which is dependent on an input voltage) can also be designed using the device. A simple square wave generator is shown. The capacitor Cn alternately charges and discharges between voltage limits which are set by R2, R 3 and R4. The circuit is basically of the Schmitt trigger type, the voltages at which triggering occurs being approximately Vt/3 and 2V+/ 3. The time constant T of this circuit is equal to 0. 7 R;C2. Where T is in seconds, R1 in ohms and C2 in farads. For example when R1...

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