RF Design: Varactor Diode Applications in the RF Design Arena

Varactor Diode (VDR) is an electrical diode composed of two vertically disposed aluminum plates having their respective ends attached to a grounded piece of electric cable. Electrical power is supplied to the terminals of the Varactor Diode by passing an electrical current through the lead plates. The current flow through the wire increases the resistance value of the wire and the voltage induced in the device due to the difference in the applied potential. Consequently, the current flowing through the wire changes its resistance value, which results into higher voltage across the device. To understand the working of a Varactor Diode it is important to know that a typical VDR consists of an insulated plate, a thin-film transistor, a variable resistive bridge, a variable capacitor and a controlling switch.

In electronics, a varactor diode, reverse bias condition diaphragm, variable resistive bridge, variable cap diaphragm, tuning fork or capacitor control switch and a variable resistive plate are some kinds of conventional diaphragms arranged in series. Varactor Diode can be used as an active or passive device. In order to control the amount of electric current flowing through the wire, a series combination of a variable resistive plate and a capacitor will control the amount of electric current flowing through the wire. To use varactor diode as an active device, an initial forward bias voltage is set up between the gate of the capacitor and the terminal of the varactor diode.

The range of frequencies of operation for most types of the Varactor Diode can be adjusted with the help of the variable resistor bridge network. However, the current limit cannot be changed unless and until the total amount of current flowing through the wire is equal to or less than the maximum forward bias voltage. In order to achieve this, the capacitor is set up with a variable resistor value between the gate and the terminal. As a result, the current limit is controlled by the resistance level variation between the gate and the terminal. As far as the characteristics of the varactor diode are concerned, they are very similar to the Wheatstone bridge.

However, the RF circuit designers have included some interesting features into the new designs. For instance, instead of using a single capacitive diodes, there are now two or more such devices. By doing so, the RF interference is reduced considerably. Further, it reduces the power dissipation for the same voltage rating for that of non-bias varactor diodes.

The biggest advantage of the new designs is that the RF interference is eliminated without any sacrifice of the forward bias capability of the circuit. They have also found another advantage in that the current drain current for a non-bias varactor diode system is less when compared to a bias operated system. Therefore, the RF circuit designers have introduced a lot of benefits in their new designs. In fact, the RF interference is almost completely eliminated for any type of voltage swing.

The capacitors in the past often tried to overcome the RF cancellation by increasing the RF excursion as the circuit was biased into the area where the RF energy was detected. This process results in an increased capacitance in the area and thus results in a reduction in the output signal strength for the tuning source. However, the designers of the new designs have found a way out by using a new generation of non-bias capacitors. These varactor tuners do not create a RF imbalance and thus eliminates the RF cancellation issue. They have also made it possible for the tuner to be set to a wide range of frequencies by using a variable RF bandwidth selection.