Electronically switchable LC resonant circuit arrangement - has supplementary capacitor connected in parallel to existing LC circuit by diode and transistor switch

摘要

The resonant circuit (C3, L1) incorporates a supplementary capacitor (C1) connected to earth via a diode (D1) and a high frequency coupling capacitor (C2). When a voltage is applied to the base of an NPN transistor (T1) across a resistor (R4), the voltage drop from the potential divider (R1, R2) forward biases the diode, connecting the supplementary capacitor to earth via the diode and coupling capacitor and via the transistor. When the voltage is removed from the base of the transistor, the diode ceases to conduct, effectively removing the supplementary capacitor from the resonant circuit. An additional high-value resistor (R3) ensures that a positive voltage is present on the transistor collector at all times. ADVANTAGE - Low impedance add-on circuit only marginally affects Q-factor of resonant circuit, while circuit switching is effected by application of simple positive voltage.