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| SEC. 9-1 BASE-BIASED
AMPLIFIER Good coupling occurs when the reactance of the coupling capacitor is much smaller than the resistance at the lowest frequency of the ac source. In a basebiased amplifier, the input signal is coupled into the base. This produces an ac collector voltage. The amplified and inverted ac collector voltage is then coupled to the load resistance. SEC. 9-2 EMITTER-BIASED AMPLIFIER Good bypassing occurs when the reactance of the coupling capacitor is much smaller than the resistance at the lowest frequency of the ac source. The bypassed point is an ac ground. With either a VDB or a TSEB amplifier, the ac signal is coupled into the base. The amplified ac signal is then coupled to the load resistance. SEC. 9-3 SMALL-SIGNAL OPERATION The ac base voltage has a dc component and an ac component. These set up dc and ac components of emitter current. One way to avoid excessive distortion is to use small-signal operation. This means keeping the peak-to-peak ac emitter current less than one-tenth of the dc emitter current. SEC. 9-4 AC BETA The ac beta of a transistor is defined as the ac collector current divided by the ac base current. The values of the ac beta usually differ only slightly from the values of the dc beta. When troubleshooting, you can use the same value for either beta. On data sheets, hFE is equivalent to β dc, and hfe is equivalent to β . SEC. 9-5 AC RESISTANCE OF THE EMITTER DIODE The base-emitter voltage of a transistor has a dc component VBEQ and an ac component vbe. The ac base-emitter voltage sets up an ac emitter current of ie. The ac resistance of the emitter diode is defined as vbe divided by ie. With mathematics, we can prove that the ac resistance of the emitter diode equals 25 mV divided by dc emitter current. SEC. 9-6 TWO TRANSISTOR MODELS As far as ac signals are concerned, a transistor can be replaced by either of two equivalent circuits: the π model or the T model. The π model indicates that the input impedance of the base is β r'e. SEC. 9-7 ANALYZING AN AMPLIFIER The simplest way to analyze an amplifier is to split the analysis into two parts: a dc analysis and an ac analysis. In the dc analysis, the capacitors are open. In the ac analysis, the capacitors are shorted and the dc supply points are ac grounds. SEC. 9-8 AC QUANTITIES ON THE DATA SHEET The h parameters are used on data sheets because they are easier to measure than r' parameters. The r. parameters are easier to use in analysis because we can use Ohm’s law and other basic ideas. The most important quantities are the data sheet are hfe and hie. They can be easily converted into >β and r'e. |