When the npn base has positive voltage, the transistor is switched on and the upper current mirror pushes its current down to ground. When the npn base voltage drops to 0V, the npn turns off, so the current mirror can charge the capacitor to 6V in 10ms with a linear voltage ramp. Then, when the npn base voltage raises again the capacitor voltage drops to 0V.
Try to switch the collector of the npn on the 680k resistor and compare the linear current charging with the classic exponential RC envelope.
