Common-Emitter Amplifiers
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 seminar class Active In SP Posts: 5,361 Joined: Feb 2011 16-02-2011, 09:44 AM   178211 Ch09-49.ppt (Size: 1.97 MB / Downloads: 163) Common-Emitter Amplifiers Objectives • Describe AV, AI, AP associated with the three amplifier configurations. • Describe the input/output voltage and current phase relationships. • Calculate the ac emitter resistance of a transistor. • Discuss two roles of the capacitors in the circuits. • Derive the ac equivalent circuit for a given amplifier. • Explain how the voltage gain drift due to temperature occurs. • Discuss the relationship between the load resistance and voltage gain of a CE amplifier. • Calculate Zin(base) and Zin for a CE amplifier • Discuss the effects of swamping on the ac characteristics of a CE amplifier • List and describe the four ac h-parameters Common-emitter input-output phase relationship. AC Emitter Resistance Graphical determination of ac emitter resistance. The determination of ac beta AC Model of A BJT Roles of Capacitors in Amplifiers 1. A coupling capacitor passes an ac signal from one amplifier to another, while providing dc isolation between the two. 2. A bypass capacitor is used to “short circuit” an ac signal to ground while not affecting the dc operation of the circuit Coupling capacitors in a multistage amplifer. AC coupling DC isolation Capacitive vs. direct coupling Bypass capacitors Typical common-emitter amplifier signals. Deriving the CE ac-equivalent circuit Voltage Gain of CE CE Current Gain 1. The input ac current is divided between the transistor and the biasing network. 2. The output collector current is divided between the collector resistor and the load. Power Gain The Effects of Loading Calculating Amp.Input Impedance Procedure: 1. Do dc analysis 2. Find r’e for each stage 3. Find rC for each stage 4. Using r’e and rC to find Av for each stage The swamped CE amplifier and its ac equivalent ckt The Effect of Swamping on Zin Gain stabilization The Hybrid Equivalent Model Six Circuit-Parameter Models for Two-Port Systems Equations for Hybrid Model Equivalent Circuit for Hybrid Model h-Parameters h11 = hi = Input Resistance h12 = hr = Reverse Transfer Voltage Ratio h21 = hf = Forward Transfer Current Ratio h22 = ho = Output Admittance h-Parameters for CE Amp. • hie = the base input impedance • hfe = the base-to-collector current gain • hoe = the output admittance • hre = the reverse voltage feedback ratio Hybrid Model for CE Configuration h-parameters of 2N3904 Hybrid Model without hre and hoe Determining h-Parameter Values Use geometric means if given max. and min. values Summary • AC concepts • Roles of capacitors in amplifiers • Common-emitter ac equivalent circuit • Amplifier gain • Gain and impedance calculations • Swamped amplifiers • h-parameters