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Download Ch(6) Small-Signal Amplifiers FREQUENCY RESPONSE Frequency
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Ch(6) Small-Signal Amplifiers FREQUENCY RESPONSE Frequency response F divided into three frequency ranges High frequency Û Mid frequency Û Low frequency & H.F.R F parallel capacitances F must be considered & M.F.R F capacitances F can be neglected & L.F.R F series capacitances CC F must be considered Midfrequency Gain FET The general model Ø replaced by the purely resistive circiuts Ø ١ Ch(6) Small-Signal Amplifiers ٢ Ch(6) Small-Signal Amplifiers BJT For stability Ö RB << β RE RB >> rπ The ideal condition Ø The circuit stable and simple to analyze ٣ Ch(6) Small-Signal Amplifiers Low-Frequency Response Below the midfrequencies The susceptances of the parallel capacitors neglibly small Ø The reactance of the coupling capacitance increasliy important Ø ٤ Ch(6) Small-Signal Amplifiers Coupling Capacitors The general model Ö reduced to Ö simpler low frequency model RG Ö retained Ö significant RB Ö omitted Ö large compared to rπ FET Û BJT Ö models are analogous Ö one analysis Ö serve for both The general circuit Ö (a) Ö replaced by Ø Thevenin form Ö input circuit OR Norton form Ö output circuit ٥ Ch(6) Small-Signal Amplifiers The two forms Ö are equivalents Ö CC Ö effectively shorted The output of Ö Thevenin circuitØ The low-frequency output VL related to the midfrequency output by Ö complex factor dependent on Ø frequency Û RC product As frequencyÖ decreased Ö large fraction of Ö VT appears Ö across CC Ö V at the output Ö reduced The cutoff Ö half - power frequency Ø Defined by ٦ Ö Ch(6) Small-Signal Amplifiers The behavior of Ö FET Û BJT Ö predicted at low- frequencyØ The input voltageÖ Vgs Û Vbe Ö down to 70% of Vo at Ø The output voltage Ö Vo Ö down to 70% of gm Vgs Ro Û gm Vbe Ro Ö the corresponding amplifier gainÖ reduced The overall Ö low-frequency voltage gain Ö for Ø FET The relative gain for Ö BJT OR FET ٧ Ch(6) Small-Signal Amplifiers To predict the behavior of a given circuit determine Ø & ω11 Û ω12 Ö the higher is To design a circuit calculate Ø & CC1 Û CC2 H-W P.P 14-3 ٨