Parallel Resonance Circuit (Anti-Resonance) – Working Principle, Impedance and Characteristics

Introduction

A parallel resonance circuit consists of a resistance (R), an inductance (L), and a capacitance (C) connected in parallel. When the resultant current flowing through this parallel network is in phase with the supply voltage, the circuit is said to be in parallel resonance. This condition is also known as anti-resonance.

At resonance, a large amount of circulating current flows between the inductor and the capacitor due to the continuous exchange of energy. The inductor stores energy in its magnetic field, while the capacitor stores energy in its electric field. This constant transfer of energy between L and C results in almost zero current drawn from the supply.

The current drawn from the supply is the vector sum of the current through the resistor (I_R) and the instantaneous currents through the inductor (I_L) and capacitor (I_C). At resonance, I_L and I_C are equal in magnitude and opposite in phase, so they cancel each other out.

Condition for Resonance

In a parallel RLC circuit, resonance occurs when the inductive reactance equals the capacitive reactance:

X_L = X_C

At this point, the imaginary part of the total admittance becomes zero, and the circuit behaves as a purely resistive circuit. Interestingly, the parallel circuit follows the same resonance condition as a series resonance circuit — the difference lies only in the connection of components.

Impedance at Parallel Resonance

At resonance, a parallel LC tank circuit behaves like an open circuit. The only component that limits current is the resistor R. Therefore, the total impedance of the circuit becomes:

Z = R

This means that the impedance of a parallel resonant circuit is maximum at resonance. As impedance is very high, the total circuit current is minimum. Since the impedance is purely resistive at this point, the total current drawn from the supply is in phase with the supply voltage (V_S).

By changing the value of the resistance R (while keeping L and C constant), the frequency response and the circulating current of the circuit can be controlled. The impedance of the circuit at resonance is called the dynamic impedance, and it is represented as:

Z = R_max

Admittance in a Parallel Resonance Circuit

Since impedance is maximum at resonance, it logically follows that the admittance is minimum. One important characteristic of a parallel resonant circuit is its extremely low admittance, which greatly restricts the current drawn from the source.

Compared to a series resonance circuit, the resistor in a parallel resonance circuit dampens the bandwidth, making the circuit less selective in terms of frequency response.

Voltage in a Parallel Resonance Circuit

The voltage across a parallel resonance circuit generally follows the same pattern as the total impedance. Since the circuit current remains almost constant regardless of impedance, the voltage waveform of the parallel circuit is typically measured across the capacitor.

Conclusion

A parallel resonance (anti-resonance) circuit exhibits maximum impedance and minimum current at the resonant frequency. The circulating energy between the inductor and capacitor makes the circuit electrically efficient at resonance. Due to its unique characteristics, the parallel resonance circuit is widely used in filters, oscillators, and tuning circuits in communication systems.