Voltage Multipliers

Voltage multipliers use a clamping action to increase peak rectified voltages without the need to increase the input transformer's voltage rating. Multiplication factors of two, three, and four are commonly used. Voltage multipliers are used in high-voltage, low-current applications. They can be used to generate bias voltages ranging from a few volts for electronic appliances to millions of volts for purposes such as high-energy physics experiments and lightning safety testing. We will shortly introduce the concept of limiters and clampers so that understanding voltage multipliers afterwards is more straightforward.

Diode Limiters:

A diode limiter creates a threshold voltage at the output. In the circuits shown next, when the diode becomes forward biased, the anode cannot exceed 0.7 V (assuming silicon) because the cathode is at ground potential. So the output is limited to 0.7 V when the input exceeds this value. When the input goes back below 0.7 V, the diode reverses-biases and appears as an open circuit. The same principle, but for limiting the negative part, is shown on the right.

Diode Limiter Configurations:



Diode Limiter with Additional DC Level Configuration:


In order to adjust the level to which the voltage signal is limited, a bias voltage can be added in series with the diode as shown next. If the bias voltage is varied up or down, the limiting level changes correspondingly. The battery can be connected in opposite polarity to the diode, and output voltages of specified positive or negative levels can be achieved. Other configurations such as forward- and reverse-biased diodes in parallel achieve a symmetric signal at the output.

Diode Clampers:  

Clampers add a dc level to an ac signal, so they are also known as dc restorers. The circuit shown next represents a diode clamper that inserts a positive dc level. When the input initially goes negative, the diode is forward-biased, allowing the capacitor to charge to near the peak of the input (Vp(in) - 0.7 V). Just above the negative peak, the diode is reverse-biased because the cathode is held near Vp(in) - 0.7 V by the charge on the capacitor. The capacitor can only discharge through the high resistance of RL so from the peak of one negative half-cycle to the next, the capacitor discharges very little. The amount discharged depends on the value of RL. For a good clamping action, the RC time constant should be at least ten times the period of the input frequency. In this case, the capacitor acts essentially as a battery in series with the input signal. Clampers are often used in television receivers as dc restorers.


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