The ramp generator in Fig 1a and the triangle-wave generator in Fig 1b charge capacitor C₁ linearly. The key to the circuits' linearity is an op-amp adder that sums a reference voltage with the voltage on C₁.

For Fig 1a's ramp generator, the adder comprises op-amp IC₁, R₄, R₅, and associated components. The adder sums capacitor C₁'s voltage, V_CAP, via R₄ with a constant voltage, V_SET, via R₅. (Size R₂ through R₄ to suit your application.) The voltage at the output of the adder is thus

and the voltage across resistor R₁ is

Because the adder holds the voltage across R₁ constant, a constant current flows through R₁, charging C₁ linearly. Fig 1b operates in a similar manner, except that comparator IC₁ switches the polarity of V_SET, ultimately producing a triangle-wave output. (DI #1631)