Square Wave Generator Using Op amp:

The Square Wave Generator Using Op amp means the astable multivibrator circuit using op-amp, which generates the square wave of required frequency. The Fig. 2.83 shows the square wave generator using op amp.

It looks like a comparator with hysteresis (schmitt trigger), except that the input voltage is replaced by a capacitor. The circuit has a time dependent elements such as resistance and capacitor to set the frequency of oscillation.

As shown in the Fig. 2.83 the comparator and positive feedback resistors R₁ and R₂ form an inverting schmitt trigger.

When V_o  is at +V_sat, the feedback voltage is called the upper threshold voltage V_UT and is given as

When V_o is at -V_sat, the feedback voltage is called the lower-threshold voltage V_LT and is given as

When power is turn ON, V_o automatically swings either to +V_sat or to -V_sat since these are the only stable states allowed by the schmitt trigger. Assume it swings to +V_sat. With V_o = +V_sat we have – V_p = V_UT and capacitor starts charging towards +V_sat through the feedback path provided by the resistor R_f to the inverting (-) input. This is illustrated in Fig. 2.84 (a). As long as the capacitor voltage V_C is less than V_UT, the output voltage remains at +V_sat.

As soon as V_C charges to a value slightly greater than V_UT, the (-) input goes positive with respect to the (+) input. This switches the output voltage from +V_sat to -V_sat and we have V_p = V_LT , which is negative with respect to ground. As V_o switches to -V_sat, capacitor starts discharging via R_f, as shown in the Fig. 2.84 (b).

The  current   I – discharges capacitor to 0 V and recharges capacitor to V_LT. When V_C becomes slightly more negative than the feedback voltage V_LT, output voltage V_o switches back to +V_sat. As a result, the condition in Fig. 2.84(a) is reestablished except that capacitor now has a initial charge equal to V_LT. The capacitor will discharge from V_LT to 0V and then recharge to V_UT, and the process is repeating. Once the initial cycle is completed, the waveform become periodic, as shown in the Fig. 2.84(c).

Frequency of Oscillation:

The frequency of oscillation of Square Wave Generator Using Op amp is determined by the time it takes the capacitor to charge from V_UT to V_LT and vice versa. The voltage across the capacitor as a function of time is given as

where

• V_C(t) is the instantaneous voltage across the capacitor.
• V_initial is the initial voltage
• V_max is the voltage toward which the capacitor is charging.

Let us consider the charging of capacitor from V_LT to V_UT, where V_LT is the initial voltage, V_UT is the instantaneous voltage and +V_sat is the maximum voltage. At t = T₁, voltage across capacitor reaches V_UT and therefore equation (3) becomes

The time taken by capacitor to charge from V_UT to V_LT is same as time required for charging capacitor from V_LT to V_UT. Therefore, total time required for one oscillation is given as

The frequency of oscillation can be determined as f_o = 1/T, where T represents the time required for one oscillation.

Substituting the value of T we get,