Ramp Generator and its Low Pass Filtered Output

This is circuit is an example of a current source charging a capacitor. The current source is shown explicitly and consists of a zener diode biasing what is essentially a non-inverting (op-amp) amplifier. The load is a capacitor, which gets charged and displays a linear increase of voltage with time. Diode D2 acts as a clamp and prevents the capacitor going above the diodes forward voltage. Switch S1 allows the capacitor to be discharged with a TC of R3*C1. U2 forms a buffer before feeding a low pass filter. Due to the DC bias of U1 being 5V, the output DC bias is also 5V. The votlage on the capacitor swings between 5V and 5V + diode voltage.