16-02-2011, 09:44 AM
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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