## Ramp Generator Circuit-using 555 Timer IC

We know that if a capacitor is charged from a voltage source through a resistor, an exponential waveform is produced while charging of a capacitor from a constant current source produces a ramp. This is the idea 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 constant charging current.

Ramp Generator Circuit

Charging current produced by PNP constant current source is

**i**_{C }**= Vcc-V**_{E }**/ R**_{E}

_{ }

where **V**_{E }**= R**_{2 }**/ (R1 + R2) * V**_{CC }**+ V**_{BE}

When a trigger starts the **monostable multivibrator timer 555** as shown in figure, the PNP current source forces a constant 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*