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1、Figure 4-7 Amplitude modulation with a diode.Figure 4-6 The tuned circuit ? lters out the modulating signal and carrier harmonics, leaving only the carrier and sidebands.4-2 Amplitude ModulatorsAmplitude modulators are gen erally one of two types: low level or high level. Low-level modulators gen er
2、ateAM with small sig nals and thus must be amplii ed con siderably if they are to be tran smitted. High-level modulators produce AM at high power levels, usually in the i nal amplii er stage of a transmitter. Although the discrete component circuits discussed in the follow ing sect ions are still us
3、ed to a limited exte nt, keep in mind that today most amplitude modulators and demodulators are in in tegrated-circuit form.Low-Level AMLow-level AMDiode Modulator. One of the simplest amplitude modulators is the diode modulator Diode modulator described in Sec. 4-1. The practical implementation sho
4、wn in Fig. 4-7 consists of a resistive mixing network, a diode rectii er, and an LC tuned circuit. The carrier (Fig. 4-8b) is applied to one in put resistor and the modulat ing sig nal (Fig. 4-8 a) to the other. The mixed signals appear across. This network causes the two signals to be linearly mixe
5、d, i.e., algebraically added. If both the carrier and the modulating signal are sine waves, the waveform resulting at the junction of the two resistors will be like that shown inFig. 4-8(c), where the carrier wave is riding on the modulating signal. This signal is not AM. Modulatio n is a multiplica
6、ti on process, not an additi on process.The composite waveform is applied to a diode rectii er. The diode is connected so that it is forward-biased by the positive-go ing half-cycles of the in put wave. During the n egative portions of the wave, the diode is cut off and no sig nal passes. The curre
7、nt through the diode is a series of positive-going pulses whose amplitude varies in proportion to the amplitude of the modulating signal see Fig. 4-8( d).Figure 4-8 Waveforms in the diode modulator. ( a) Modulating signal. ( b) Carrier.(c) Linearly mixed modulating signal and carrier. ( d) Positive-
8、going signal after diodeD1. (e) Am output signal.(a)These positive-go ing pulses are applied to the parallel-t uned circuit made up of L and C, which are resonant at the carrier freque ncy. Each time the diode con ducts, a pulse of current l ows through the tuned circuit. The coil and capacitor repe
9、atedly exchange energy, causing an oscillation, or“ ringing, ” at the resonant frequency. The oscillation ofthe tuned circuit creates one n egative half-cycle for every positive in put pulse. High- amplitude positive pulses cause the tuned circuit to produce high- amplitude n egative pulses. Low-amp
10、litude positive pulses produce corresp onding low-amplitude n egative pulses. The resulting waveform across the tuned circuit is an AM signal, as Fig. 4-8( e) illustrates. The Q of the tuned circuit should be high eno ugh to elim in ate the harm onics and produce a clea n sine wave and to i lter out
11、 the modulati ng sig nal, and low eno ugh that its ban dwidth accommodates the sideba ndsge nerated.# Chapter4Figure 4-9 Simple transistor modulator.AMThis signal produces high-quality AM, but the amplitudes of the signals are criticalto proper operation. Because the nonlinear portion of the diode s
12、 characteristic curveoccurs only at low voltage levels, sig nal levels must be low, less tha n a volt, to produceAM. At higher voltages, the diode curre nt resp onse is n early lin ear. The circuit works best with millivolt-level sig nals.Tran sistor Modulator. An improved version of the circuit jus
13、t described is show n Transistor modulator in Fig. 4-9. Because it uses a tran sistor in stead of the diode, the circuit has gain. The emitter-base junction is a diode and a nonlinear device. Modulation occurs as described previously, except that the base curre nt con trols a larger collector curre
14、nt, and therefore the circuit amplii es. Rectii cation occurs because of the emitter-base junction.This causes larger half-s ine pulses of curre nt in the tuned circuit. The tuned circuit oscillates (rin gs) to gen erate the missi ng half-cycle. The output is a classic AM wave.Dif erential Ampli? er
15、. A differential amplii er modulator makes an excellent ampli- Differential ampli?er modulator tude modulator. A typical circuit is show n in Fig. 4-10(a). Tran sistors Q1 and Q2 formthe differential pair, and Q is a constant-current source. Transistor Q3 supplies a i xed emitter current IE to Q1 an
16、d Q2, one-half of which l ows in each transistor. The output is developed across the collector resistors R1 and R?.The output is a function of the differe nee betwee n in puts Vi and V2； that is, Vout 5 A(V? 2 Vi), where A is the circuit gain. The amplii er can also be operated with a sin gle in put. When this is done, the other in put is gro un ded or set to zero. InFig. 4-10(a), if Vi is zero, th
