Tuned Circuits

1
Tuned Circuits RCL Networks
Infrared Radiometer
2
Interest

• Magnetic Resonance Imaging (MRI) Looks at the
  body based on principles of magnetism and radio
  wave transmission.

3
Outline

• Resonance
• Tank Circuit
• Series RCL Circuit
• Parallel RCL Circuit
• Amplifiers
• Filters

4
Tuned Circuits

• Series Tuned Circuits
• . Theory
• . Ideal Series resonant circuit contains no
  resistance. It contains only inductance and
  capacitance that are in series with each other
  and with the source voltage.
• . Operation
• . At Resonance ( XL XC ) therefore, XL XC
  0. The resultant reactance is equal to 0.
  Impedance ( Z ) is minimum.
• . Since Z is minimum, current is maximum for a
  given voltage. Maximum current flow causes
  maximum voltage drops across individual
  reactances.

5
Formulas

• XL 2 II f L.
• XC 1 / 2 II f C.
• Z XL XC R
• At resonance XL XC so 2 II f L 1 / 2 II f C,
  therefore Fr .159/ LC

6
Questions

• Q. What is the formula for XL?
• A. XL 2 II f L.
• Q. What is the formula for XC?
• A. XC 1 / 2 II f C.
• Q. What is the resonant frequency in a typical
  tuned circuit?
• A. XL XC, Fr .159/Square root LC.

7
Tuned Circuit Operation

• . Operation (Continued)
• . When Frequency is lt Resonance
• XC ? gt current is lower gt voltage drops across
  reactances are lower.
• . When Frequency is gt Resonance
• XL ? gt current is lower gt voltage drops across
  reactances are lower.

8
Series Tuned Circuit Schematic
9
Tuned Circuit Operation

• . Series Tuned Circuit Analysis

XL
o
R Z
0o
XC
XL
RESONANCE
XL
XL - XC
o
Capacitive
o
Inductive
Resistive
CURRENT
IMPEDANCE
XC - XL
XC
XC
ABOVE RESONANCE
BELOW RESONANCE
100
200
300
500
600
700
Fr
Z R
10
Parallel Tuned Circuits

• 1. Theory
• . Called a tank circuit because it can store
  energy.
• . It has the ability to take the energy fed to it
  from a power source and store this energy
  alternately in the inductor and capacitor.
• . The resulting output is a continuous ac sine
  wave.
• . Operation
• . Voltage is the same across the inductor and
  capacitor. (parallel)
• . Current through the components varies inversely
  with their reactances.
• . Total current through the circuit is the
  vectoral sum of the two individual component
  currents.
• . IL and IC are 180o out of phase.
• . At resonance, IL and IC cancel each other out
  gt no current from source.

11
Parallel Tuned Circuits Continued

• . Application
• . At resonance, the circuit has a maximum
  impedance which results in minimum current drawn
  from the source.

12
Parallel Tuned Circuit Schematic
13
Parallel Tuned Circuits
IL

• . Circuit analysis

o
IC
IC
RESONANCE
IC
I
IC - IL
o
o
Capacitive
Resistive
Inductive
CURRENT
IMPEDANCE
IL - IC
IL
IL
Z
ABOVE RESONANCE
BELOW RESONANCE
100
200
300
500
600
700
Fr
14
Parallel Tuned Circuits

• . Applications
• . Tuned Amplifier

VCC
RB

IMAX
L1
C1
0
VIN
0
RL
CC
T
15
Questions

• Q. What are some examples of a parallel tuned
  amplifiers?
• A. Antenna tuners, air signal tracker, ham radio,
  transponders (ID aircraft etc).
• Q. What crystal can replace the RLC circuit to
  make it last longer?
• A. Piezoelectric Crystal.

16
Pulsed Amplifier

• . Pulsed Amplifier 3 main sections
• 1. Gain Amp 2. Input Gate Signal 3. Tank Circuit

VCC
R1

C1
0
VIN
OUTPUT SIGNAL
L1
C2
17
Pulsed Amplifier
T0
T1
T2
T3
INPUT GATE
OUTPUT SIGNAL
18
Tuned Amplifier

• c. Tuned Amplifier 3 main sections
• 1. Gain Amp 2. Positive Feedback Circuit 3.
  Frequency Determining Device

VCC
R1

C1
0
VIN
OUTPUT SIGNAL
L1
Cy1
19
Overdriven Amplifier
SATURATION
CUTOFF
C1
C2
Q1
R2
R1
OUTPUT
INPUT
VCC
-VEE
SATURATION
CUTOFF
20
Overdriven Amplifier

• . The input signal drives the transistor into and
  out of saturation and cutoff.
• . When the transistor is in saturation and / or
  cutoff, that portion of the input waveform is
  clipped and the output is distorted.

21
Conclusions

• Q. What is the difference between series and
  parallel resonant circuits?
• A. The characteristics for current impedance,
  and capacitance inductance are reversed.
• Q. What are some examples of circuits that use
  resonant circuits?
• A. MRI machines, infrared radiometer, transistor
  radios others.