Voltage Doubler

Definition: A multiplier circuit that generates a dc output voltage having amplitude twice the maximum amplitude of the ac input supply voltage is known as Voltage Doubler. The circuit shows its necessity in all such applications where a high level of voltage is required when the input source is of low amplitude.

Here, the name itself is indicating that it generates a voltage having peak double as that of input at the output.

Voltage doubler circuits are mainly characterized as:

Let us first understand half-wave voltage doubler.

Half Wave Voltage Doubler

The figure below shows the circuit of half-wave voltage doubler:

The circuit is made of two diodes D₁ and D₂, two capacitors C₁ and C₂ and an ac supply voltage. This input ac voltage gets doubled by the circuit and a double amplitude dc signal is achieved at the output.

Let us now understand how this happens:

When positive half of the input signal is applied, diode D₁ gets forward biased. Due to this, it behaves as a short circuit and current flows through the diode. This current charges the capacitor C₁ up to the peak value of the applied input signal.

At the same time, the positive half of the input signal reverse biases the diode D₂. Due to this current does not flow through that portion of the circuit. Hence, C₂ will not get charged.

Due to the absence of a return path for the current, the charge present in capacitor C₁ will not get discharged. Hence, C₁ stores the charge because of its energy storing property.

When negative half of the input signal is applied, diode D₂ comes in forward biased although D₁ falls in reversed biased.

Due to reverse the biased condition of D₁, capacitor C₁ will not get charged. However, the capacitor C₁ will now discharge. While C₂ charges due to forward biased condition of diode D₂.

The capacitor C₂ present in the circuit charges will gets charged but with double the peak of applied input voltage i.e., 2V_m. This is so because the discharging capacitor voltage of C₁ and applied input voltage gets added.

Hence producing a voltage that is twice the peak of the applied input voltage i.e., 2V_m. Now, the charge present on the capacitor gets discharged through the load. Hence the dc output with the peak of 2V_m is achieved.

The thing that has to be kept in mind in case of half-wave voltage doubler is that the output does not increase rapidly. But with each applied input cycle it rises slowly.

Another factor is that simply one-half cycle charges the capacitor C₂. Thus, the discharging voltage consists of ripple (unwanted fluctuations) frequency that is equivalent to supply frequency of applied input signal.

The figure below shows the input and output waveform of half-wave voltage doubler:

Let us now understand full wave voltage doubler.

Full Wave Voltage Doubler

The figure below shows the circuit of full wave voltage doubler:

The circuit has an ac source, two capacitors C₁ and C₂ and two diodes D₁ and D₂.

Let us have a look at the operation of the circuit:

When positive half of supply is provided, the diode D₁ gets forward biased. This forward voltage charges the capacitor C₁ up to the maximum value of applied input voltage V_m.

At this particular cycle, D₂ gets reverse biased, causing no any current to flow through it. Hence, capacitor C₂ will not be charged.

When the negative half cycle of the signal is applied, the diode D₂ will be forward biased. However, D₁ will now come to reverse biased state. Due to forward biasing applied at D₂, the current flowing through the circuit will charge capacitor C₂ up to V_m. However, capacitor C₁ will not be charged this time due to reverse biasing of D₁.

As the two capacitors form a series connection. Thus, the output achieved will be V_m + V_m i.e., 2V_m. However, in the presence of a load, this output will be a little bit less than a no-load condition (2V_m).

The figure below shows the input and output waveform of full wave voltage doubler:

Advantages of Voltage Doubler

• It eliminates the use of a high voltage transformer. As it changes a low voltage to high at a low rate.
• Voltage multiplication can be greatly increased by cascading such circuits.

Disadvantages of Voltage Doubler

• The output present has undesired fluctuations called ripples.

Applications of Voltage Doubler

Voltage doublers are used widely in cathode ray tubes, in x-ray and radar systems along with LCD backlight, in laser systems and in oscilloscopes etc.