Ramp Generator Circuit-using NE555

We know that if a capacitor is charged from a voltage supply through a resistor, an exponential waveform is created while charging of a capaci­tor from a continuing current supply produces a ramp. this is the concept behind the circuit. The circuit of a ramp generator using timer 555 is shown in figure. Here the resistor of previ­ous circuits is replaced by a PNP transistor that produces a continuing charging current.

Charging current produced by PNP constant current source is

i_C = Vcc-V_E / R_E

where V_E = R₂ / (R1 + R2) * V_CC + V_BE

When a trigger starts the monostable multivibrator timer 555 as shown in figure, the PNP current supply forces a continuing charging into the capacitor C. The voltage across the capacitor is, therefore, a ramp as illustrated in the figure. The slope of the ramp is given as Slope, s = I/C