2 BJT Transistor Circuit Analysis Large signal DC analysisSmall signal equivalentAmplifiersChapter 13: Bipolar Junction Transistors
3 Circuit with BJTs Our approach: Operating point - dc operating point Analysis of the signals - the signals to be amplifiedCircuit is divided into: model for large-signal dc analysis of BJT circuitbias circuits for BJT amplifiersmall-signal models used to analyze circuits forsignals being amplifiedRemember !
4 Large-Signal dc Analysis: Active-Region Model Important: a current-controlled current source models thedependence of the collector current on the base currentVCBreverse bias?VBEforward bias?The constrains for IB and VCE must be satisfy to keep BJT in the active-mode
5 Large-Signal dc Analysis: Saturation-Region Model VCBforward bias?VBEforward bias?
6 Large-Signal dc Analysis: Cutoff-Region Model VCBreverse bias?VBEreverse bias?If small forward-bias voltage of up to about 0.5 V are applied, the currents are often negligible and we use the cutoff-region model.
7 Large-Signal dc Analysis: characteristics of an npn BJT
8 Large-Signal dc Analysis Procedure: (1) select the operation mode of the BJT(2) use selected model for the device to solve the circuitand determine IC, IB, VBE, and VCE(3) check to see if the solution satisfies the constrains forthe region, if so the analysis is done(4) if not, assume operation in a different region andrepeat until a valid solution is foundThis procedure is very important in the analysis and designof the bias circuit for BJT amplifier.The objective of the bias circuit is to place the operating point inthe active region.Bias point – it is important to select IC, IB, VBE, and VCEindependent of the b and operation temperature.Example 13.4, 13.5, 13.6
9 Large-Signal dc Analysis: Bias Circuit VBB acts as a shortcircuit for ac signalsFrom Example 13.6Remember: that the Q point should be independent of the b(stability issue)VBB & VCC provide this stability, however this impractical solutionOther approach is necessary to solve this problem-resistor network
10 Large-Signal dc Analysis: Four-Resistor Bias Circuit Solution of the bias problem:31Thevenin equivalent42Equivalent circuit for active-region modelInputOutput
11 Small-Signal Equivalent Circuit Small signal equivalent circuit for BJT:Thevenin equivalentand
12 Common Emitter Amplifier First perform DC analysis to find small-signal equivalent parameters at the operating point.Find voltage gain:Find input impedance:
13 Common Emitter Amplifier Find current gainFind power gain:Find output impedance:
14 Problem 13. 13: Suppose that a certain npn transistor has VBE = 0 Problem 13.13: Suppose that a certain npn transistor has VBE = 0.7V for IE =10mA. Compute VBE for IE = 1mA. Repeat for IE = 1µA. Assume that VT = 26mV.
15 Problem 13. 14: Consider the circuit shown in Figure P13. 14 Problem 13.14: Consider the circuit shown in Figure P Transistors Q1 and Q2 are identical, both having IES = 10-14A and β = 100. Calculate VBE and IC2. Assume that VT = 26mV for both transistors. Hint: Both transistors are operating in the active region. Because the transistors are identical and have identical values of VBE, their collector currents are equal.
16 Problem 13. 50: The transistors shown in Figure P13 Problem 13.50: The transistors shown in Figure P13.50 operate in active region and have β = 100, VBE=0.7V. Determine IC and VCE for each transistor.I1IE2VBE
17 Problem 13. 52: Analyze the circuit of Figure P13 Problem 13.52: Analyze the circuit of Figure P13.52 to determine IC and VCE.IICI1IBIE
18 Problem 13. 45: Analyze the circuits shown in Figure P13 Problem 13.45: Analyze the circuits shown in Figure P13.45 to determine I and V. For all transistors, assume that β = 100 and |VBE| = 0.7V in both the active and saturation regions. Repeat for β = 300.
20 Problem 13. 67: Consider the emitter-follower amplifier of Figure P13 Problem 13.67: Consider the emitter-follower amplifier of Figure P Draw the dc circuit and find ICQ. Next, determine the value of rπ. Then, calculate midband values for Av, Avoc, Zin, Ai, G and Z0.I1
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