Question

A.C. Fundamentals - Comparison of series and parallel resonant circuit

Answer 1

Series and parallel resonant circuits are two common types of circuits used in electronics and electrical engineering for various applications. They both involve the concept of resonance, which occurs when the circuit's reactances balance out, resulting in maximum current flow. However, they have different characteristics and are used for different purposes. Let's compare series and parallel resonant circuits:

Series Resonant Circuit:

Configuration:

In a series resonant circuit, the components (usually a resistor, an inductor, and a capacitor) are connected in series.

Resonant Frequency:

The resonant frequency of a series resonant circuit is determined by the values of the inductor (L) and capacitor (C) according to the formula:

res
=
1
2



f
res
    ​

=
2π
LC
    ​

1
    ​

.

Impedance:

At resonance, the reactive components (inductive reactance and capacitive reactance) cancel each other out, resulting in minimum impedance. The impedance is mainly resistive and equal to the resistance (R) of the circuit.
Below resonance, the circuit is inductive, and the impedance is mainly inductive.
Above resonance, the circuit is capacitive, and the impedance is mainly capacitive.

Current:

At resonance, the current is maximum because the impedance is minimum. This can be advantageous for applications where a high current is desired.

Applications:

Series resonant circuits are often used in applications where a specific frequency needs to be filtered or selected, such as in radio tuning circuits.

Parallel Resonant Circuit:

Configuration:

In a parallel resonant circuit, the components are connected in parallel.

Resonant Frequency:

The resonant frequency of a parallel resonant circuit is again determined by the values of the inductor and capacitor, but this time using the formula:

res
=
1
2



f
res
    ​

=
2π
LC
    ​

1
    ​

.

Admittance:

At resonance, the reactive components cancel each other out, resulting in maximum admittance (reciprocal of impedance) and minimum current impedance.
Below resonance, the circuit is capacitive, and the admittance is mainly capacitive.
Above resonance, the circuit is inductive, and the admittance is mainly inductive.

Voltage:

At resonance, the voltage across the circuit is maximum due to the minimum current impedance.

Applications:

Parallel resonant circuits are often used in applications where a specific frequency needs to be rejected or attenuated, such as in notch filters or noise reduction circuits.

In summary, series resonant circuits are known for their high current at resonance, making them suitable for applications requiring high current flow at a specific frequency. Parallel resonant circuits, on the other hand, have maximum voltage across them at resonance and are used for applications where a specific frequency needs to be attenuated. The choice between series and parallel resonant circuits depends on the specific requirements of the application.