Chapter Two: Radio-Frequency Circuits

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

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

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

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

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

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

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:

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

Common-Base Amplifier One solution to the Miller Effect is to use a common-base amplifier configuration as shown at the right

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

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

Class B RF Amplifier A simple Class B amplifier is shown at the right It uses transformer coupling Both transistors are biased near cutoff

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

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

Frequency Multipliers Sometimes it is useful to use harmonic operation to generate a frequency higher than is conveniently generated by using a frequency multiplier

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

LC Oscillators Practical RF circuits whose frequency is controlled by a resonant LC circuit are: –Hartley Oscillator –Colpitts Oscillator –Clapp Oscillator

Hartley Oscillator Common configurations for a Hartley Oscillator

Colpitts Oscillator Common configurations for a Colpitts Oscillator

Clapp Oscillator Common configuration for a Clapp Oscillator

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

Crystal-Controlled Oscillators Crystal-controlled oscillators are more stable than LC oscillators Crystal oscillators utilize the piezoelectric effect to generate a frequency-variable signal

Mixers Mixers are nonlinear circuits that combine two signals to produce the sum and difference of of the two input frequencies

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

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

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

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

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