This circuit is basically a dc-dc converter. The DC output of the chopper will be at a higher level compared to the DC input. The switching is done with the help of a BJT. The output of the chopper again depends on the duty cycle of the pulse. The output of the chopper Va is expressed as Vs/(1-k). The capacitor is provided for smoothing the output, as due to the switching there will be many ripples. An equation is provided to find the average of the output. This can be substituted into the above formula for verification.