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In particular, we saw the need to modulate a signal using an information (baseband) signal. The carrier must be much higher in frequency than thc baseband signal. Carrier frequencies can be as low as a few kilohertz but are typically much higher: megahertz or hundreds of megahertz. Microwave communications use carrier frequencies in the gigahertzrange. You are probably already familiar with amplifier and oscillator circuits that operate at audio frequencies. In this chapter, we will explore some of the differences in design and construction that permit these circuits to work at radio frequencies We will also look at some techniques that are used in radio-frequency (RF) circuits but impossible or impractical to implement at lower frequencies. In addition, we will discuss devices such as frequency multipliers and mixers, which allow the frequency of a signal to be changed. 2.2 High-FrequencyE ffects As we extend our study of electronic circuits to higher frequencies, we have to be more careful to include reactive effects, not only those that are included deliberately as circuit elements but also the "stray" reactances in components and even within and between wires and circuit board traces. As we get still higher in frequency, into the UHF range, we find that conventional devices and construction methods become inefficient and innovative approaches to circuit design become important. At microwave frequencies, many circuits seem to bear very little physical resemblance to those used at lower frequencies. The Effect of Frequency on Device Characteristics The series inductive component Ls is mainly due to The leads. The resistive component can be divided into two parts, a small series component R5 due to lead resistance and a large parallel resistance Rp representation dielectric losses. As the frequency increases, so does the inductive reactance. Meanwhile, the capacitive reactance of the component decreases with increasing frequency. eventually, a point will be reached where the two reactances are equal and the capasitor becomes a series-resonant circuit. This point is called the self-resonant frequency. Above this point, the magnitude of the inductive reactance becomes

greater than that of the capacitive reactance, and our so-called capacitor behaves like an inductor. The size of these capacitors depends partly on the physical structure of the transistor and partly on its operating point. As frequency increases, the capacitive reactances will decrease until the performance of the transistor is degraded. The base-to-collector capacitance, for instance, will cause feedback from output to input in an ordinary common-emitter ammplifier circuit. The feedback can lower the gain of the amplifier or cause it to become unstable. As the frequency increases into the gigahertz range, transit-time effects also become important. The transit time is the time it takes a charge carrier to cross a device. In an NPN transistor, it is the time taken for electrons to cross the base; in ; PNP transistor, the holes exhibit transit time. In general, free electrons move more quickly than holes, so NPN transistors are preferred to PNP for high frequency operation. Transit time can be reduced by making devices physically small, but this causes problems with heat dissipation and breakdown voltage Lumped and Distributed Constants At low frequencies, we generally assume that capacitors have capacitance, resistors have resistance, and short sections of good conductors (for example, the traces on a circuit board) have neither. We saw in the previous section that this assumption is really a simplification that becomes less accurate as the frequency increases. For instance, a circuit board trace has a small amount ofinductance in addition to resistance. There will also be capacitance between this trace and every other trace on the board. High-Frequency Construction Technique It is possible to design circuitry to reduce the effect of "stray" capacitance and inductance resulting from the wiring and circuit board traces themselves. In general, keeping wires and traces short reduces inductance, and keeping them well separated reduces capacitance between them. Inductive coupling can be reduced by keeping conductors and inductors that are in close proximity at right angles to each other. The use oftoroidal cores for inductors and transformers also helps to

reduce stray magnetic fields 2.3 Radio-FrequencyAmpliliers Amplifiers for RF signals can be distinguished from their audio countelparts in several important ways. Wide bandwidth may or may not be required. If it is not, gain can be increased and distortion reduced with the use of tuned circuits NarrowbandAmplifiers Often the signals in an RF communication system are restricted to a relatively narrow range offrequencies. In such circumstances it is unnecessary and, in fact, undesirable to use an amplifier with a wide bandwidth. Doing so invites problems with noise and interference. Consequently, many of the amplifiers found in both receivers and transmitters incorporate filters to restrict their bandwidth. In many cases these filters also increase the gain of the amplifier. Wideband Amplifiers Not all amplifiers used in communications have tightly restricted bandwidth. For instance the amplifiers used for the baseband part of the system are usually wideband (also called broadband). You are no doubt already familiar with audio amplifier Those used for baseband video are similar, though their bandwidth is larger (about 4.2MHz for broadcast television signals). Broadband amplifier then, generally incorporate some form of filtering so that the frequency response, while broad, is restricted to the range of interest. Wideband RF amplifiers, like their narrowband counterparts, typically use transformer coupling. This technique was once popular for audio amplifiers as well. but the size, the weight, and especially the high cost of audio transformation has led to their virtual elimination from audio circuitry. For RF amplifiers they retain some advantages Transformer coupling also makes it easy to couple balanced inputs or loads to the amplifier. Balanced lines have equal impedance from each conductor to ground. They are often used with antennas; ordinary television twin-lead is an example of a balanced line that should be transformer coupled to the first stage in a television receiver

Amplifier Classes Amplifiers are classified according to the portion of the input cycle during which the active device conducts current. This is called the conduction angle and is expressed in degrees

Neutralization Device and stray capacitance tend to reduce gain and cause instability as frequency increases.c are must be taken to separatei nputs and outputs to avoid feedback, but often a transistor or tube will itself introduce sufficient feedback to cause oscillations to take place. Sometimes this type of feedback can be cancelled by a process called neutralization. Neutralization is accomplished by deliberately feeding back a portion of the output signal to the input in such a way that it has the same amplitude as the unwanted feedback but the opposite phase. since the device capacitances vary from component to component, careful adjustment is necessary Frequency Multipliers frequency multipliers operate at lower efficiencies than straightthrough amplifiers, they are used at low power levels Most multipliers operate at the second or third harmonic of the input frequency and are known as doublers or triplers, respectively. They are more efficient than multipliers operating at hi gher-order harmonics. Multipliers can be used in cascadei if greaterm ultiplication is required 2.4 Radio-FrequencyO scillators RF oscillators do not differ in principle from those used at lower frequencies, but the practical circuits are quite different. While low-frequency oscillators usually use RC circuits in the frequency-determining section, tC circuits are more common at radio frequencies. In addition, many RF oscillators are crystal controlled.

Any amplifier can be made to oscillate if a portion of the output is fed back to the input in such a way that the following criteria, known as the Barkhqusen criteria, are satisfied: 1. The gain around the loop must be equal to one. (If it is initially greater than one, it will become equal to one when oscillations start, due to some process such as transistor saturation; otherwise the output voltage would continue to increase without limit.) 2. The phase shift around the loop must total either 0o or some integer multiple of 360o at the operating frequency (and not at other frequencies Taken together, these statements simply mean that at the operating frequency, an input signal will be amplified then fed back in phase and with suffrcienr amplitude that it will maintain its value at the output without any further input. The initial signal needed to start the process can be noise or a transient caused by switching on the power to the oscillator circuit. LC Oscillators Oscillators whose frequency is controlled by a resonant circuit using inductance rnJ capacitanc All of these oscillator types can be implemented with either rrerting or noninverting amplifiers and will be shown both ways. Hertley Oscillator This oscillator type can be recognized by its use of a tapped *luctor, part of a resonant circuit, to provide feedback.

colpitts Oscillator The colpitts oscillator uses a capacitive voltage divider instead of a tapped inductor to provide feedback The operating frequency is determined by the inductor and the series combination of Cl and C2.

Like the Hartley, the Colpitts oscillator can be configured for an amplifier with power gain but no voltage gain

Clepp Oscillator The Clapp oscillator is a variation of the Colpitts circuit, designed to swamp device capacitances for greater stability. The feedback fraction is found in the same way as for the Colpitts oscillator

varactor-Tuned Oscillators The frequency of an LC oscillator can be changed by varying, or tuning, either the rductive or the capacitive element in a tuned circuit. Inductors are typically tuned by moving a ferrite core into or out of the coil; this is known as slug tuning. variable capacitors usually have two sets of plates that can be interleaved to a greater or lesser extent. Varactors are a more convenient substitute for variable capacitors in many appllications. Essentially,a varactor is a reverse-biaseds ilicon diode The variation of capacitance with given approximately by voltage is not linear for a varactor. It is given approximately by

Crystal-Controlled Oscillators Crystal oscillators achieve greater stability by using a small slab of quartz as a mechanical resonator, in place of an LC tuned circuit. Quartz is a piezoelectric material: deforming it mechanically causes the crystal to generate a voltage, and applying a voltage to the crystal causes it to deform. Like any rigid body, the crystal slab has a mechanical resonant frequency. If it is pulsed with voltage at that frequency, it will vibrate. From the outside, the crystal will have the appearance

of an electrical resonant circuit. Crystal oscillators offer great accuracy and stability at the price of fixed frequency operation. The temperature dependence of the frequency of an oscillator can be given by equation