August 2019 StudentZone Quiz Solution

Question:

The circuit in Figure 30 produces a positive DC output voltage. How can it be reconfigured to make a negative output voltage? Construct the voltage inverter and repeat the experiment/simulation.

Answer:

The voltage doubler circuit can be easily configured to produce a negative output DC voltage by reverting the diodes D1 and D2 as shown in the circuit below:

 

Let’s now examine how the circuit works in this configuration:

  1. During the first negative half-cycle, D2 will be forward biased C2 will charge to a voltage nearly equal to the peak voltage (vpeak) of the AC input, with its upper end being negative with respect to ground.
  2. During the following positive half cycle, D2 will be reverse biased and will not conduct current. Since C1 is not yet charged, this will forward bias D1 and allow the voltage at the right end of C1 to be applied to the top of C2. C1 will charge as C2 discharges.
  3. On the next negative half-cycle, C2 charges again to -vpeak, through D2. If there is no load to discharge C2, its output will remain at -vpeak.
  1. On the second positive half-cycle, C1is still charged to -vpeak on its right end, while the voltage at the left end of D2 is again -2vpeak.

A simulation example for the negative voltage doubler is presented in the figure below, by applying at the input a sine wave having 2V amplitude and 1KHz frequency.