The Parallel Resonance Circuit
In many ways a parallel resonance circuit is exactly the same as the series resonance
circuit we looked at in the previous tutorial. Both are 3-element networks that contain two reactive components making
them a second-order circuit, both are influenced by variations in the supply frequency and both have a frequency point
where their two reactive components cancel each other out influencing the characteristics of the circuit. Both circuits
have a resonant frequency point.
The difference this time however, is that a parallel resonance circuit is influenced by the currents
flowing through each parallel branch within the parallel LC tank circuit. A tank circuit
is a parallel combination of L and C that is used in filter networks
to either select or reject AC frequencies. Consider the parallel RLC circuit below.
Parallel RLC Circuit
Let us define what we already know about parallel RLC circuits.
A parallel circuit containing a resistance, R, an inductance,
L and a capacitance, C will produce a parallel resonance
(also called anti-resonance) circuit when the resultant current through
the parallel combination is in phase with the supply voltage.
At resonance there will be a large circulating current between the
inductor and the capacitor due to the energy of
the oscillations.
A parallel resonant circuit stores the circuit energy in the magnetic field of the inductor
and the electric field of the capacitor. This energy is constantly being transferred back and forth between the inductor
and the capacitor which results in zero current and energy being drawn from the supply. This is because the corresponding
instantaneous values of IL and IC will
always be equal and opposite and therefore the current drawn from the supply is the vector addition of these two
currents and the current flowing in IR.
In the solution of AC parallel resonance circuits we know that the supply voltage is common for
all branches, so this can be taken as our reference vector. Each parallel branch must be treated separately as with
series circuits so that the total supply current taken by the parallel circuit is the vector addition of the individual
branch currents. Then there are two methods available to us in the analysis of parallel resonance circuits. We can
calculate the current in each branch and then add together or calculate the admittance of each branch to find the
total current.
We know from the previous series resonance tutorial that resonance takes place when
VL = -VC and this situation occurs when
the two reactances are equal, XL = XC. The admittance of a
parallel circuit is given as:
Resonance occurs when XL = XC and the imaginary parts of Y become zero. Then:
Notice that at resonance the parallel circuit produces the same equation as for the series resonance circuit. Therefore, it makes no difference if the inductor or capacitor are connected in parallel or series. Also at resonance the parallel LC tank circuit acts like an open circuit with the circuit current being determined by the resistor, R only. So the total impedance of a parallel resonance circuit at resonance becomes just the value of the resistance in the circuit and Z = R as shown.
At resonance, the impedance of the parallel circuit is at its maximum value and equal to the resistance of the circuit and we can change the circuit's frequency response by changing the value of this resistance. Changing the value of R affects the amount of current that flows through the circuit at resonance, if both L and C remain constant. Then the impedance of the circuit at resonance Z = RMAX is called the "dynamic impedance" of the circuit.
Impedance in a Parallel Resonance Circuit
Note that if the parallel circuits impedance is at its maximum at resonance then consequently, the
circuits admittance must be at its minimum and one of the characteristics of a parallel resonance circuit is that
admittance is very low limiting the circuits current. Unlike the series resonance circuit, the resistor in a parallel
resonance circuit has a damping effect on the circuits bandwidth making the circuit less selective.
Also, since the circuit current is constant for any value of impedance, Z,
the voltage across a parallel resonance circuit will have the same shape as the total impedance and for a parallel
circuit the voltage waveform is generally taken from across the capacitor.
We now know that at the resonant frequency, ƒr the admittance
of the circuit is at its minimum and is equal to the conductance, G given by
1/R because in a parallel resonance circuit the imaginary part of admittance, i.e. the
susceptance, B is zero because BL = BC
as shown.
Susceptance at Resonance
From above, the inductive susceptance, BL is inversely proportional to the frequency as represented by the hyperbolic curve. The capacitive susceptance, BC is directly proportional to the frequency and is therefore represented by a straight line. The final curve shows the plot of total susceptance of the parallel resonance circuit versus the frequency and is the difference between the two susceptance's.
Then we can see that at the resonant frequency point were it crosses the horizontal axis the total
circuit susceptance is zero. Below the resonant frequency point, the inductive susceptance dominates the circuit producing
a "lagging" power factor, whereas above the resonant frequency point the capacitive susceptance dominates producing a
"leading" power factor. So at resonant frequency, the circuits current must be "in-phase" with the applied voltage as
there effectively there is only the resistance in the circuit so the power factor becomes one or unity,
( θ = 0o ).
Current in a Parallel Resonance Circuit
As the total susceptance is zero at the resonant frequency, the admittance is at its minimum and is
equal to the conductance, G. Therefore at resonance the current flowing through the circuit must
also be at its minimum as the inductive and capacitive branch currents are equal
( IL = IC ) and are 180o
out of phase.
We remember that the total current flowing in a parallel RLC circuit is equal to the vector sum of the
individual branch currents and for a given frequency is calculated as:
At resonance, currents IL and IL are
equal and cancelling giving a net reactive current equal to zero. Then at resonance the above equation becomes.
Since the current flowing through a parallel resonance circuit is the product of voltage divided by impedance, at resonance the impedance, Z is at its maximum value, ( =R ). Therefore, the circuit current at this frequency will be at its minimum value of V/R and the graph of current against frequency for a parallel resonance circuit is given as.
Parallel Circuit Current at Resonance
The frequency response curve of a parallel resonance circuit shows that the magnitude of the current is a function of frequency and plotting this onto a graph shows us that the response starts at its maximum value, reaches its minimum value at the resonance frequency when IMIN = IR and then increases again to maximum as ƒ becomes infinite. The result of this is that the magnitude of the current flowing through the inductor, L and the capacitor, C tank circuit can become many times larger than the supply current, even at resonance but as they are equal and at opposition ( 180o out-of-phase ) they effectively cancel each other out.
As a parallel resonance circuit only functions on resonant frequency, this type of circuit is also
known as an Rejector Circuit because at resonance, the impedance of the circuit is at its maximum thereby
suppressing or rejecting the current whose frequency is equal to its resonant frequency. The effect of resonance in a
parallel circuit is also called "current resonance".
The calculations and graphs used above for defining a parallel resonance circuit are similar to
those we used for a series circuit. However, the characteristics and graphs drawn for a parallel circuit are exactly
opposite to that of series circuits with the parallel circuits maximum and minimum impedance, current and magnification
being reversed. Which is why a parallel resonance circuit is also called an Anti-resonance circuit.
Bandwidth & Selectivity of a Parallel Resonance Circuit
The bandwidth of a parallel resonance circuit is defined in exactly the same way as for the series
resonance circuit. The upper and lower cut-off frequencies given as: ƒupper
and ƒlower respectively denote the half-power frequencies where the power
dissipated in the circuit is half of the full power dissipated at the resonant frequency
0.5( I2 R ) which gives us the same -3dB points at a current value
that is equal to 70.7% of its maximum resonant value, ( 0.707 x I )2 R.
As with the series circuit, if the resonant frequency remains constant, an increase in the
quality factor, Q will cause a decrease in the bandwidth and likewise, a decrease in the quality factor will
cause an increase in the bandwidth as defined by: BW = ƒr /Q
or BW = ƒ2 - ƒ2.
Also changing the ratio between the inductor, L and the capacitor, C,
or the value of the resistance, R the bandwidth and therefore the frequency response of the
circuit will be changed for a fixed resonant frequency. This technique is used extensively in tuning circuits for
radio and television transmitters and receivers.
The selectivity or Q-factor for a parallel resonance circuit is generally defined as the
ratio of the circulating branch currents to the supply current and is given as:
Note that the Q-factor of a parallel resonance circuit is the inverse of the expression for the Q-factor of the series circuit. Also in series resonance circuits the Q-factor gives the voltage magnification of the circuit, whereas in a parallel circuit it gives the current magnification.
Bandwidth of a Parallel Resonance Circuit
Example No1
A parallel resonance network consisting of a resistor of 60Ω, a capacitor of 120uF and an
inductor of 200mH is connected across a sinusoidal supply voltage which has a constant output of 100 volts at all
frequencies. Calculate, the resonant frequency, the quality factor and the bandwidth of the circuit, the circuit
current at resonance and current magnification.
Resonant Frequency, ƒr
Inductive Reactance at Resonance, XL
Quality factor, Q
Bandwidth, BW
The upper and lower -3dB frequency points, ƒH and ƒL
Circuit Current at Resonance, IT
At resonance the dynamic impedance of the circuit is equal to R
Current Magnification, Imag
Note that the current at resonance (the resistive current) is only 1.67 amps, while
the current flowing around the LC tank circuit is larger at 2.45 amps. We can check
this value by calculating the current flowing through the inductor (or capacitor) at resonance.
Parallel Resonance Tutorial Summary
We have seen that Parallel Resonance circuits are similar to series resonance
circuits. Resonance occurs in a parallel RLC circuit when the total circuit current is "in-phase" with the supply
voltage as the two reactive components cancel each other out. At resonance the admittance of the circuit is at its
minimum and is equal to the conductance of the circuit. Also at resonance the current drawn from the supply
is also at its minimum and is determined by the value of the parallel resistance.
The equation used to calculate the resonant frequency point is the same for the previous
series circuit. However, while the use of either pure or impure components in the series RLC circuit does not
affect the calculation of the resonance frequency, but in a parallel RLC circuit it does.
In this tutorial about parallel resonance, we have assumed that the components are purely inductive
and purely capacitive with negligible resistance. However in reality the coil will contain some resistance. Then the
equation for calculating the parallel resonant frequency of a circuit is therefore modified to account for the additional
resistance.
Resonant Frequency using Impure Components
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