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Lecture Electrical Engineering: Lecture 24 - Dr. Nasim Zafar

It is shown that the use of the small signal transmission line equivalent circuit model is not restricted solely to the small signal a.c. analysis of semiconductor devices. Simple algorithms are described which allow it to be used to compute the d.c. and the large signal transient properties of semiconductor devices to any desired precision. | Dr. Nasim Zafar Electronics 1 - EEE 231 Fall Semester – 2012 COMSATS Institute of Information Technology Virtual campus Islamabad Application of the Small-Signal Equivalent Circuits: Examples Lecture No. 24 Contents: Examples: BJT as an Amplifier. Examples: Small-Signal Equivalent Circuit Models. Nasim Zafar 2 Lecture No. 24 Reference: Application of the Small-Signal Equivalent Circuits Chapter-5.6.8 Microelectronic Circuits Adel S. Sedra and Kenneth C. Smith. Nasim Zafar 3 Introduction The availability of the small-signal BJT circuit models makes the analysis of transistor amplifier circuits a systematic process. The process consists of the following steps: Determine the dc operating point of the BJT and in particular the dc collector current IC. Calculate the values of the small-signal model parameters: gm = IC ⁄ VT , rπ = β ⁄ gm, and re = VT /IE ≅ 1 ⁄ gm. 4 Nasim Zafar Introduction 3. Draw ac circuit path. Eliminate the dc sources by replacing each dc voltage source with a short circuit and each dc current source with an open circuit. Replace the BJT with one of its small-signal equivalent circuit models. Analyze the resulting circuit to determine the required quantities e.g., voltage gain, input resistance. Nasim Zafar 5 Nasim Zafar 6 DC Analysis of BJT Using simple constant-voltage drop model, assuming , irrespective of the exact value of currents. Assuming the device operates at the active region, we can apply the relationship between IB, IC, and IE, to determine the voltage VCE or VCB. DC Analysis of BJT The AC signal, vbe , is removed for dc bias analysis Nasim Zafar 7 (a) Transistor Amplifier Circuit (b) Circuit for DC Analysis Figure 5.48 The Hybrid-p Model A transistor hybrid-p model: - a voltage controlled current source. 8 Nasim Zafar Hybrid-Pi Model for the BJT The small-signal parameters are controlled by the Q-point and are independent of the geometry of the BJT. Transconductance: Input resistance: Rin Output resistance: 9 Nasim . | Dr. Nasim Zafar Electronics 1 - EEE 231 Fall Semester – 2012 COMSATS Institute of Information Technology Virtual campus Islamabad Application of the Small-Signal Equivalent Circuits: Examples Lecture No. 24 Contents: Examples: BJT as an Amplifier. Examples: Small-Signal Equivalent Circuit Models. Nasim Zafar 2 Lecture No. 24 Reference: Application of the Small-Signal Equivalent Circuits Chapter-5.6.8 Microelectronic Circuits Adel S. Sedra and Kenneth C. Smith. Nasim Zafar 3 Introduction The availability of the small-signal BJT circuit models makes the analysis of transistor amplifier circuits a systematic process. The process consists of the following steps: Determine the dc operating point of the BJT and in particular the dc collector current IC. Calculate the values of the small-signal model parameters: gm = IC ⁄ VT , rπ = β ⁄ gm, and re = VT /IE ≅ 1 ⁄ gm. 4 Nasim Zafar Introduction 3. Draw ac circuit path. Eliminate the dc sources by replacing each dc voltage source with a .