Where does the series-pass transistor get switched in order to produce Pulse Width Modulation.

Where does the series-pass transistor get switched in order to produce Pulse Width Modulation (PWM) in any series switching regulator circuit ?

a. Between cut-off & saturation at high frequency
b. Between active & cut-off at high frequency
c. Between cut-off & saturation at low frequency
d. Between active & cut-off at low frequency

Correct Answer : a. Between cut-off & saturation at high frequency

Explanation :

Efficiency of a regulator can be increased by using series-pass transistor as a switch. The configuration arrangement of series switching regulator is in such a way that the series-pass transistor is allocated between cut-off and saturation region. This is so because it leads to the generation of square wave output representing pulse width modulation. Low pass LC filter filters filters the modulated square wave output so as to produce average DC output voltage. Since the output voltage is proportional to pulse width & frequency, the efficiency can be seen improved upto approximately 95%.
These regulators find their applications in many kinds of circuits like push-pull,feed-forward and so on.