AN0003
15
A Quick Guide to Tank Circuits
Introduction
The tank circuit, a common building block in electronic systems, is a parallel resonant circuit comprised of an inductor, a
capacitor, and an optional resistor. Since the capacitor and the inductor both store energy, this type of circuit is referred
to as a tank circuit. By taking advantage of its bandpass response characteristic, tank circuits are typically used to limit
bandwidth. This is of particular interest in filtering an IF signal which is to be amplified by a very large gain. By limiting the
bandwidth, one is able to limit the noise power of the unfiltered signal, which could easily saturate the amplifier.
AN0003
A Quick Guide to Tank Circuits
Terminology
The tank circuit has three main specifications: Bandwidth, Quality Factor, and Insertion Loss. These parameters define
the passband, shape, and loss of the tank circuit response.
The half-power bandwidth,
BW,
is defined as the difference between the upper (f
2
) and lower (f
1
) half-power frequencies.
There are two types of quality factor. The first is component
Q.
Component
Q
is defined as the ratio of the magnitude of
the component’s series reactance to its series resistance at a particular frequency. A perfect, lossless component would
have infinite
Q.
Component
Q
will have an effect on the overall circuit
Q.
The second is circuit
Q
which is used to denote the relative selectivity of a circuit. It is defined as
Maximum Instantaneous Energy Stored in the Circuit
-
Q
= 2π
×
-------------------------------------------------------------------------------------------------------------------------------
Energy Dissipated per Cycle
The half-power bandwidth and the circuit Q are related by the equation
f
c
-
Q
= --------
BW
where
f
c
is the center frequency. As can be seen by the above relationship, an increase in Q will result in narrower band-
width and higher selectivity.
The insertion loss,
IL,
of the tank is the measure of how much the applied signal is absorbed by the tank circuit compo-
nents. Ideally, they would not absorb any of the signal, but since the components are not ideal, they will attenuate the sig-
nal by some finite amount. If viewed with a spectrum analyzer, the insertion loss will shift the entire bandpass
characteristic down by some number of decibels.
The source resistance and load resistance are not really tank circuit parameters, however they have a direct relationship
on the tank's performance. The source and load resistance, from a calculation viewpoint, are connected in parallel with
the inductor and resistor. So, if the source or load resistance is a relatively low value, then it will load the circuit and
reduce the circuit
Q.
15
TECHNICAL NOTES
AND ARTICLES
Circuit Topology
As mentioned before, the tank circuit is comprised of a capacitor, an inductor, and optionally a resistor connected in par-
allel as shown in the following figure.
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15-17
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