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Communication
Electronics
Principles & Applications
Third Edition
Louis E. Frenzel
Chapter 6
Radio Transmitters
©2001
Glencoe/McGraw-Hill
TRANSMITTER COMPONENTS
• Carrier signal source, usually a crystal
oscillator or a frequency synthesizer
• Multiple RF amplifiers to increase
power level
• High power final amplifier
• Impedance matching circuits to ensure
maximum power transfer
• Modulator
AM TRANSMITTER WITH
HIGH-LEVEL MODULATION
Carrier
oscillator
Buffer
Driver
Final power
amplifier
Modulation
Driver amplifier
Audio
amplifier
Speech
processing
TYPICAL SSB TRANSMITTER
Antenna
Linear driver amplifier
Carrier
oscillator
Balanced
modulator
Linear power
amplifier
Sideband
filter
Buffer
Mixer
Tuned
circuit
LO
Microphone
Audio
amplifier
Speech
processing
TYPICAL FM TRANSMITTER
USING INDIRECT FM
Driver
Carrier
oscillator
Final power
amplifier
Phase
modulator
Buffer
Frequency multipliers
Microphone
Audio
amplifier
Speech
processing
CLASSES OF AMPLIFIERS
• Class A
– Conducts continuously
– Linear, lowest distortion
– Least efficient (<50%)
• Class B
– Conducts for 180º of input
– More efficient than class A
– Push pull circuit reduces distortion
CLASSES OF AMPLIFIERS
(continued)
• Class C
– Conducts for less than 180º
– Highly efficient
– Generates distortion and harmonics
• Class D and E
– Switching amplifiers
– Introduce distortion and harmonics
– Most efficient (>90%)
– Use BJT and enhancement mode
MOSFETS
Class A Operation
Class B Operation
1 KW PUSH-PULL RF
POWER AMPLIFIER
2.7 nH
RF
in
20 nH
16:1
+50 V
MRF 154
3 nF
0.47 F
10 H
3 nF
20 
RF
out
0.1 F
300 pF
20 nH
410
pF
20 
2.7 nH
Note:
3 nF
MRF 154
gate bias circuits not shown
9:1
Toroid
input
and output
transformers
provide for
Gate
bias
circuits
maintain
Class
B
operation
Negative
feedback
provides
stability
andgain
wideband
Power
MOSFETs
provide
apush-pull
power
of 11 toperformance.
14
dB.
impedance
matching,
operation,
and
good efficiency and minimum distortion.
broad-band performance (10 to 90 MHz).
Bias
Bias
TUBES VERSUS TRANSISTORS
IN POWER AMPLIFIERS
• Individual RF transistors can achieve power
levels up to about 500 watts.
• Using a push pull circuit and/or paralleling
transistors produces power levels over 1 kW.
• For power levels over 1 kW, the outputs of
several transistor amplifiers can be combined.
• For power levels over about 2 kW, tube
amplifiers are easier to implement and cheaper.
• Crossover Distortion
• If we look at the diagram for the input and output
waveform, we
• can see some distortion at the cross over where one
transistor
• stops conducting and the other one starts to conduct.
• 􀂄 A small base-emitter voltage is needed to turn on a
• transistor
• 􀂄 QN actually only conducts when vin > 0.7 V
• 􀂄 QP actually only conducts when vin < -0.7 V
• 􀂄 When 0.7 > vin > -0.7, nothing conducts and the output is
• zero.
• 􀂄 i.e. the input-output relationship is not at all linear.
Class C Amplifier
IMPEDANCE MATCHING
NETWORKS
• Impedance matching networks are used
between amplifier stages and between
the final amplifier and the antenna load
to ensure maximum power transfer.
• Impedance matching networks consist
or inductors and capacitors combined in
, T or L configurations.
TRANSFORMERS AND BALUNS
• Transformers are widely used
between amplifier stages for
impedance matching.
• A balun is a special type of
transformer that is also used for
impedance matching between stages.
SPEECH PROCESSING CIRCUITS
• Special circuits are used to modify and
enhance the voice signal from the
microphone.
• Speech processing includes filtering, levelclipping or compressing to minimize
bandwidth.
• In modern wireless equipment, speech
processing is performed by a digital signal
processor (DSP).