The square wave generator is defined as an oscillator that gives the output without any input, any input in the sense we should give input within zero seconds which means it must be an impulse input. This generator is used in digital signal processing and electronic applications.
The new range of low-noise, highspeed, and low-distortion BiMOS opamps from Texas Instruments, type TLC070 to TLC075, is intended for use in instrumentation, audio, and automotive applications. This oscillator is an ideal example of its application: a stable, highly accurate squarewave at frequencies up to 60 kHz can be produced with an output current of ±30 mA.
The TLC073, a dual opamp with a shutdown function, is used here. IC1a is configured as a standard squarewave generator, IC1b as a driver. The frequency of oscillation depends on Cx and Rx and is calculated (for frequencies up to 20 kHz).
The table shows preferred values that give various frequencies. Note that the frequency variation is largely determined by the capacitor since Rx must always be significantly larger than feedback resistor R3. The effect of the supply voltage, at –130 dB, is negligibly small, and the temperature coefficient of frequency is very low: only 1.5 %. At frequencies above 20 kHz, the oscillator remains stable but increasingly non-linear.
The mark-space ratio of the signal can be adjusted in the range 10% to 90% by changing the ratio of resistors R1 to R2. If the two resistors are equal, the output is symmetrical. The output of the driver swings between +0.3 V (low) and 1 V below the supply voltage (high).
The oscillator is switched on and off via the shutdown input of IC1a. The output of the opamp goes to high impedance and the current consumption drops to 35 nA.
The oscillator can, of course, be built using the common or garden TL071 (Ub=7 V, Uout=1.2/6.2 V, Iout=1.75 mA, fmax=50 kHz). As can be seen, the output drive capability is rather lower.