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Thought of the Day. When you meet other people, look them in the eye, state your name clearly, and shake hands firmly. Small-Signal Amplifier. D.C. Kulshreshtha. 2. Equivalent Circuit Method. The transistor is replaced by its ac equivalent circuit model.
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Thought of the Day When you meet other people, look them in the eye, state your name clearly, and shake hands firmly. Electronic Amplifiers
Small-Signal Amplifier D.C. Kulshreshtha Electronic Amplifiers
2. Equivalent Circuit Method • The transistor is replaced by its ac equivalent circuit model. • Modelling of FET is much simpler than that of BJT, because of two reasons : • BJT is current operated devices • The output affects the input. • For BJT, we have either h-parameter model or hybrid-π model. Electronic Amplifiers
Hybrid Parameters Electronic Amplifiers
Complete h-parameter circuit model of a BJT. Electronic Amplifiers
Simplified h-parameter model of a BJT. Electronic Amplifiers
Hybrid-πCircuit Model Electronic Amplifiers
Vce = 0 means output shorted. Whole current flows through the short. Electronic Amplifiers
From the input of the circuit : Electronic Amplifiers
Equivalent circuit method of AC Analysis Electronic Amplifiers
Input Resistance Electronic Amplifiers
Output resistance Electronic Amplifiers
How Small is Small Peak-to-peak current should be less than 10 % of dc current. Electronic Amplifiers
Example : Analyze the CE amplifier circuit using h-parameter model of BJT. Electronic Amplifiers
Note: Since 1/hoe can be ignored, the h – parameter model is further simplified. Electronic Amplifiers
Solution Electronic Amplifiers
FET Amplifier Electronic Amplifiers
AC Analysis Electronic Amplifiers
FET Parameters Typical Values 500 kΩ 0.5 mS to 4 mS 80 to 200 Electronic Amplifiers
Determining gm Electronic Amplifiers
Example 7.8Calculate gm for a JFET with IDSS = 8 mA and Vp = 4 V, for dc bias point of (a) -0.5 V, and (b) -1.5 V Electronic Amplifiers
AC Circuit Model of an FET Electronic Amplifiers
AC Equivalent of FET Amplifier Electronic Amplifiers
Negative sign means that the output is 180° out of phase. Electronic Amplifiers
Maximum Voltage Gain : As Rac Electronic Amplifiers
This is much smaller than RS , hence it is completely bypassed. Electronic Amplifiers
Review • Equivalent Circuit Method. • Hybrid Parameters. • Hybrid-πCircuit Model. • How Small is Small. • FET Parameters. • Determining gm. • AC Equivalent of FET Amplifier. • Maximum Voltage Gain Electronic Amplifiers