Chapter 6a: BJT Transistor Modeling

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Chapter 7: BJT Transistor Modeling


*Topic objectivesAt the end of the course you will be able to Understand about the small signal analysis of circuit network using re model and hybrid equivalent model Understand the relationship between those two available model for small signal analysis


*INTRODUCTION:TRANSISTOR MODELING To begin analyze of small-signal AC response of BJT amplifier the knowledge of modeling the transistor is important. The input signal will determine whether it’s a small signal (AC) or large signal (DC) analysis. The goal when modeling small-signal behavior is to make of a transistor that work for small-signal enough to “keep things linear” (i.e.: not distort too much) [3] There are two models commonly used in the small signal analysis: a) re model b) hybrid equivalent model


*How does the amplification be done?Conservation; output power of a system cannot be large than its input and the efficiency cannot be greater than 1 The input dc plays the important role for the amplification to contribute its level to the ac domain where the conversion will become as η=Po(ac)/Pi(dc) Simply speaking…


*DisadvantagesRe model Fails to account the output impedance level of device and feedback effect from output to input Hybrid equivalent model Limited to specified operating condition in order to obtain accurate result


*I/p coupling capacitor  s/c Large values Block DC and pass AC signal Bypass capacitor  s/c Large valuesDC supply  “0” potentialVoltage-divider configuration under AC analysisRedraw the voltage-divider configuration after removing dc supply and insert s/c for the capacitors O/p coupling capacitor  s/c Large values Block DC and pass AC signal


*Modeling of BJT begin HERE!


*AC bias analysis : 1) Kill all DC sources 2) Coupling and Bypass capacitors are short cct. The effect of there capacitors is to set a lower cut-off frequency for the cct. 3) Inspect the cct (replace BJTs with its small signal model:re or hybrid). 4) Solve for voltage and current transfer function, i/o and o/p impedances.


*IMPORTANT PARAMETERS Input impedance, Zi Output impedance, Zo Voltage gain, Av Current gain, Ai Input Impedance, Zi(few ohms  M) The input impedance of an amplifier is the value as a load when connecting a single source to the I/p of terminal of the amplifier.


*Two port system -determining input impedance ZiThe input impedance of transistor can be approximately determined using dc biasing because it doesn’t simply change when the magnitude of applied ac signal is change.


*Demonstrating the impact of Zi


*Example 6.1: For the system of Fig. Below, determine the level of input impedance


*Output Impedance, Zo (few ohms  2M) The output impedance of an amplifier is determined at the output terminals looking back into the system with the applied signal set to zero.


*Example 6.2: For the system of Fig. below, determine the level of output impedance


*Example 6.3: For the system of Fig. below, determine Zo if V=600mV, Rsense=10k and Io=10A


*Example 6.4: Using the Zo obtained in example 6.3, determine IL for the configuration of Fig below if RL=2.2 k and Iamplifier=6 mA.


*Voltage Gain, AV DC biasing operate the transistor as an amplifier. Amplifier is a system that having the gain behavior. The amplifier can amplify current, voltage and power. It’s the ratio of circuit’s output to circuit’s input. The small-signal AC voltage gain can be determined by:


*By referring the network below the analysis are:


*Example 6.5: For the BJT amplifier of fig. below, determine: a)Vi b) Ii c) Zi d) Avs


*Current Gain, Ai This characteristic can be determined by:


*re TRANSISTOR MODEL employs a diode and controlled current source to duplicate the behavior of a transistor. BJT amplifiers are referred to as current-controlled devices. Common-Base Configuration Common-base BJT transistor re model re equivalent cct.


*isolation part, Zi=reZo  Therefore, the input impedance, Zi = re that less than 50Ω. For the output impedance, it will be as follows;


*The common-base characteristics






*Example 6.6: For a common-base configuration in figure below with IE=4mA, =0.98 and AC signal of 2mV is applied between the base and emitter terminal: Determine the Zi b) Calculate Av if RL=0.56k c) Find Zo and Ai

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Last Updated: 8th March 2018

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