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Chapter Two:Radio-Frequency Circuits
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Introduction• There is a need to modulate a signal using an information
signal• This signal is referred to as a baseband signal• The carrier needs to be a higher frequency than the baseband• RF Amplifiers, Oscillators, Mixers, and frequency
synthesizers are used to meet these conditions
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High-Frequency Effects
• At very low frequencies, capacitors and other components behave in very straightforward ways
• A capacitor is considered an open circuit to DC voltages and a short circuit for AC at low frequencies
• As frequencies become higher, component interaction becomes more critical both directly and as “stray” reactances, inductances, and capacitances
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Effect of Frequency on Device Characteristics
• All electronic devices have capacitances and inductances
• As frequency increases, so does inductive reactance
• As frequency increases, capacitive reactance decreases
• At some point, the two reactances will be equal and the circuit will self-resonate
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Lumped & Distributed Constants• At low frequencies, the capacitances and inductances found
between the traces on a printed circuit board are negligible• As frequency increases, the stray capacitances and
inductances are considered as distributed along the length of the pc board
• They are said to be distributed constants
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High-Frequency Construction Techniques
• Circuits are designed to reduce the “stray” capacitances and inductances resulting from the wiring and circuit board
• Traces and wires are kept short and well separated• Conductors and inductors in close proximity are kept at
right angles• Toroidal cores for inductors are used to reduce stray
magnetic fields• Shielding is used • A gimmick is used in some circuits
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Radio-Frequency Amplifiers• RF amplifiers differ from audio amplifiers in that wide
bandwidth may or may not be required• Linearity of the output may or may not be required• Efficiency can be improved through the use of Class C
amplifiers
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Narrowband Amplifiers• Many RF amplifiers are
required to operate only within a narrow range of frequencies
• Filters are used to reduce the bandwidth
• The tuned amplifier is set according to the formula:
11π2
1
CLfo
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Miller Effect
• Inter-electrode capacitance and inductance is a problem in RF circuits
• This problem is especially severe for the collector-base capacitance in a common-emitter amplifier
• The multiplication of the effect of capacitance in this configuration is called the Miller Effect
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Common-Base Amplifier
• One solution to the Miller Effect is to use a common-base amplifier configuration as shown at the right
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Wideband Amplifiers• Baseband parts of RF systems may make use of wideband
amplifiers
• Wideband amplifiers typically use transformer coupling
• Typical wideband amplifiers need negative feedback to compensate for higher low-frequency gain: as frequency increases, negative feedback decreases
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Amplifier Classes• Amplifiers are classified according to the portion of the
input cycle the active device conducts current• This is referred to as the conduction angle and is
expressed in degrees• Single-ended audio amps are operated in Class A where
the device conducts for 360°• Push-pull amps can be a Class B if one of the two
devices is conducting at all times• Most audio power amps operate in Class AB - a
compromise between Class A and Class B
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Class B RF Amplifier
• A simple Class B amplifier is shown at the right
• It uses transformer coupling• Both transistors are biased
near cutoff
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Class C Amplifiers• Class C amplifiers
conduct for less than 180° of the input cycle
• Class C amplifiers can be single-ended or push-pull
• Class C amplifiers are very efficient in RF applications but inherently induce severe distortion
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Neutralization• Transistors or tubes may
introduce sufficient feedback to cause the circuit to oscillate and become unstable
• Neutralization can cancel this type of feedback by feeding back a portion of the output signal to the input in such a way that it has the same amplitude as the unwanted signal but the opposite phase
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Frequency Multipliers• Sometimes it is useful to use harmonic operation to
generate a frequency higher than is conveniently generated by using a frequency multiplier
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Radio-Frequency Oscillators• RF oscillators do not differ in principle than other
oscillators but practical circuits are quite different• Any amplifier can be made to oscillate if a portion of the
output signal is fed back to the input• The Barkhausen criteria establishes the requirements for a
circuit to oscillate
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LC Oscillators• Practical RF circuits whose frequency is
controlled by a resonant LC circuit are:– Hartley Oscillator– Colpitts Oscillator– Clapp Oscillator
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Hartley Oscillator
• Common configurations for a Hartley Oscillator
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Colpitts Oscillator
• Common configurations for a Colpitts Oscillator
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Clapp Oscillator
• Common configuration for a Clapp Oscillator
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Varactor-Tuned Oscillator
• The frequency of an oscillator may be tuned by varying the inductance or capacitance of the circuit
• Varactors are more convenient substitutes than variable capacitors in many circumstances
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Crystal-Controlled Oscillators• Crystal-controlled oscillators are more stable than LC oscillators• Crystal oscillators utilize the piezoelectric effect to generate a
frequency-variable signal
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Mixers
• Mixers are nonlinear circuits that combine two signals to produce the sum and difference of of the two input frequencies
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Types of Mixers
• Square-law mixers: output is derived by the formula:
• Diode Mixers use a diode operated in the forward bias mode• Transistor Mixers use bipolar and FET transistors• Balanced Mixers are mixers where the input frequencies do
not appear at the output
32iiio CvBvAvv
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Frequency Synthesizers• Conventional LC oscillators tend to be unstable because of:
– Vibration– Temperature changes– Voltage changes– Component aging
• Crystal oscillators are more stable but are are limited to a narrow range of operating frequencies
• Frequency Synthesizers overcome these limitations and may end up being more cost effective
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Phase-Locked Loops• The phase-locked loop is the basis of nearly all modern
synthesizer designs
• The loop consists of a:– Phase detector
– Voltage-controlled oscillator (VCO)
– Low-pass filter
– The purpose of the PLL is lock the VCO to the reference signal
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Simple Frequency Synthesizer• In addition to the phase detector, VCO, and filter, a programmable divider is
necessary for frequency synthesis using a PLL as shown below
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Prescaling• Because programmable dividers are unavailable at frequencies
above 100MHz, fixed- and two-modulus prescalers are used• Two-modulus prescalers can be programmed to divide by two
consecutive integers