Lecture 10 Bipolar Junction Transistor (BJT)

Outline Continue BJT Continue DC analysis More examples Introduction to AC signal analysis BJT

Example (1) A bipolar transistor having IS = 5×10-16 A is biased in the forward active region with VBE =750 mV. If the current gain (or β) varies from 50 to 200 due to manufacturing variations, calculate the minimum and maximum terminal currents of the device. BJT

Solution For a given VBE, the collector current remains independent of β The base current varies from IC/200 to IC/50: the emitter current experiences only a small variation because (β+ 1)/β is near unity for large β: BJT

Example (2) Determine the dc level of IB and VC for the BJT circuit

Solution For the dc mode, the capacitor assumes the open-circuit equivalence and RB =R1+R2 BJT

Example (3) Determine the dc bias voltage VCE and the current IC for the voltage-divider configuration shown in the figure. (use exact and approximate methods) BJT

Solution Using Exact Analysis Method BJT

Solution (cont’d) Return to the example: BJT

Solution (cont’d) BJT

Solution (cont’d) Using Approximate Analysis Method The condition that will define whether the approximate approach can be applied BJT

Solution (cont’d) Return to the example: Testing: BJT

Solution (cont’d) Now, compare between obtained results by the exact and approximate methods BJT

Example (4) Determine the voltage VCB and the current IB for the common-base configuration as shown in figure BJT

Solution Applying Kirchhoff’s voltage law to the input circuit yields Applying Kirchhoff’s voltage law to the output circuit gives BJT

Example (5) Given the device characteristics as shown in figure, determine VCC, RB, and RC for the fixed bias configuration BJT

Solution From the load line BJT

BJT Transistor Modeling A model is an equivalent circuit that represents the AC characteristics of the transistor A model uses circuit elements that approximate the behavior of the transistor There are two models commonly used in small signal AC analysis of a transistor: re model Hybrid equivalent model BJT

BJT AC Analysis BJT

The re Transistor Model BJTs are basically current-controlled devices; therefore the re model uses a diode and a current source to duplicate the behavior of the transistor Recall (from lecture 5): the ac resistance of a diode can be determined by the equation rac = 26 mV/ID, where ID is the dc current through the diode at the Q (quiescent) point One disadvantage to this model is its sensitivity to the DC level. This model is designed for specific circuit conditions BJT

Common-Base Configuration Input impedance: Output impedance: Voltage gain: Current gain: Forward-biased junction RL BJT

Example For a common-base configuration, as shown in figure, with IE = 4 mA, α = 0.98, and an ac signal of 2 mV applied between the base and emitter terminals: (a) Determine the input impedance. (b) Calculate the voltage gain if a load of 0.56 kΩ is connected to the output terminals. (c) Find the output impedance and current gain. BJT

Solution BJT

Lecture Summary Covered material Continue BJT DC analysis More examples Introduction to AC signal analysis Material to be covered next lecture Continue BJT analysis with AC signal BJT