Varactor Diode in Voltage Controlled Oscillators

1. Varactor Diodes

Varactor diodes are extraordinary sorts of diodes. When a forward predisposition is connected, a expansive current is produced, narrowing the consumption locale of the PN intersection, expanding the capacitance, and shaping a dissemination capacitance impact. However, in reverse bias, a transition capacitance effect occurs. Due to leakage current under forward bias, varactor diodes are typically used with reverse bias in applications.

A varactor diode can be seen as a PN intersection. When a turn around voltage V (since varactor diodes are utilized in switch) is connected to the PN intersection, electrons within the N-type semiconductor are drawn towards the positive electrode, and gaps within the P-type semiconductor are drawn towards the negative cathode, shaping a exhaustion layer with no free electrons or gaps. The width of the exhaustion layer (d) changes with the switch voltage V. When V increments, the exhaustion layer width d extends, diminishing the diode capacitance C (C=kS/d); on the other hand, when V diminishes, the exhaustion layer width d contracts, expanding the capacitance C. By changing the turn around voltage V, the intersection capacitance C of the varactor diode can be balanced to attain control.

2. Utilizing Varactor Diodes in Voltage-Controlled Oscillators: A Circuit Chart Examination

Underneath may be a ordinary voltage-controlled oscillator circuit appearing the common utilize of varactor diodes.

Voltage Controlled Oscillator

Ordinarily, the cathode is grounded or runs at a common DC potential. The other end can then be biased. The biasing circuit needs to isolate the RF signals from the tuning circuit to prevent performance degradation.

In this circuit, two diodes are used. One is directly grounded, and the other is grounded through an inductor. Using two diodes helps better balance changes caused by RF oscillations. This circuit overcomes the problem of RF modulation tuning voltage. As the RF voltage increases, the capacitance of one diode increases while the other diode's capacitance decreases. The back-to-back configuration halves each diode's capacitance since they are in series. Additionally, note that the series resistance doubles, which will affect the Q factor.

If needed, a single capacitor can replace D1. If not done, the control voltage will ground through the inductor, causing the circuit to malfunction.

The control voltage needs to be applied to the junction of the two diodes. Resistors or inductors can be used for this purpose since the diodes work in reverse bias with high DC resistance.

Inductors can be very effective in some cases as they provide a low resistance path for biasing. However, they may introduce stray inductance, especially in oscillators, which could lead to unwanted oscillations. Resistors can also be used. The resistor must be high enough to isolate the biasing circuit from the tuning circuit without lowering the Q factor, yet low enough to control the bias on the diodes and counteract the RF passing through the diodes. 10 kΩ is a good starting point.

When designing varactor diode circuits, ensure the diodes do not become forward-biased. Sometimes, especially when using low-level reverse bias, the RF signal in the circuit's RF section may overcome the bias during certain periods, driving the diodes forward.