Triangular Wave Generator:

In the relaxation oscillator discussed in the previous lecture, capacitor voltage V_C has a near triangular wave shape but the sides of the triangles are exponentials rather than straight line. To linear size the triangles, it is required that C be charged with a constant current rather that the exponential current through R. The improved circuit is shown in fig. 1.

Figure 21.1

In this circuit an OPAMP integrator is used to supply a constant current to C so that the output is linear. Because of inversion through the integrator, this voltage is fedback to the non-inverting terminal of the comparator rather than to the inverting terminal. The inverter behaves as a non-inverting schmitt trigger. The voltage v_R is used to shift the dc level of the triangular wave and voltage v_s is used to change the slopes of the triangular wave form is shown in fig. 2.

Fig. 2

To find the maximum value of the triangular waveform assume that the square wave voltage v_Ois at its negative value = -V_sat. With a negative input, the output v (t) of the integrator is an increasing ramp. The voltage at the non-inverting comparator input v₁ is given by

When v₁ rises to V_R, the comparator changes state from - V_sat to +V_sat and v(t) starts decreasing linearly similarly, when v₁ falls below v_R the comparator output changes from +v_sat to -v_sat. Hence the minimum value of triangular v_min occurs for v₁ = v_R. Hence the peak value V_max of the triangular waveform occurs for v₁ = V_R.

Therefore,

The peak to peak swing is given by

The average output voltage is given by .
If V_R = 0, the waveform extends between -V_sat (R₂ / R₁ ) and +V_sat (R₂ /R₁).
The sweep times T₁ and T₂ for V_s = 0 can be calculated as follows:
The capacitor charging current is given by

where, v_c = -v_out is the capacitor voltage.

For v_out = -V_sat, . Therefore,
                

When the output voltage of first OPAMP is +V_sat, then, the voltage v₁ is given by

GOTO >> 1 || 2 || 3 || Home