Chapter 3(ii) BJT (DC analysis)
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Chapter 3(ii)
BJT (DC analysis)
definitions BiasingBiasing refers to the DC voltages applied to a transistor in order to turn it on so that it can amplify the AC signal. The DC input establishes an operating or quiescent point called the Q-pointQ-point.
• Active or Linear Region OperationActive or Linear Region OperationBase–Emitter junction is forward biased
Base–Collector junction is reverse biased
• Cutoff Region OperationCutoff Region OperationBase–Emitter junction is reverse biased
• Saturation Region OperationSaturation Region OperationBase–Emitter junction is forward biasedBase–Collector junction is forward biased
DC biasing circuits
• Fixed-bias circuit• Emitter-stabilized bias circuit• Voltage divider bias circuit• DC bias with voltage feedback
Fixed-bias circuit
Base-Emitter LoopBase-Emitter Loop
From Kirchhoff’s voltage law:
Solving for the base current:
+VCC – IBRB – VBE = 0
B
BECCB R
VVI
Collector-Emitter LoopCollector-Emitter Loop
The collector current is given by:
From Kirchhoff’s voltage law:
BIIC
CCCCCE RIVV
DC analysis
Load Line for Fixed-bias circuitLoad Line for Fixed-bias circuit
• where the value of RBB sets the value of IBB
• where IBB and the load line intersect • that sets the values of VCECE and ICC
The Q-point is the particular operating point:
The end points of the load line are:
C
CCC RVI
VVCE 0
CCCE VV
mAIC 0
ICsat
VCEcutoff
Circuit Values Affect the Q-PointCircuit Values Affect the Q-Point
Increasing level of IB
Increasing level of RC
Decreasing value of VCC
Emitter-Stabilized Bias CircuitEmitter-Stabilized Bias CircuitAdding a resistor (RE) to the emitter circuit stabilizes the bias circuit.
Base-Emitter LoopBase-Emitter Loop Collector-Emitter LoopCollector-Emitter LoopDC analysis
From Kirchhoff’s voltage law :
0R1)I(-RI-V EBBBCC
0 RI-V-RI-V EEBEEECC
EB
BECCB 1)R(R
V-VI
Since IE = (b + 1)IB:
Solving for IB:
From Kirchhoff’s voltage law :
0 VRI V RI CCCCCEEE
Since IE IC:
)R (RI– V V ECCCCCE
Also:
EBEBRCCB
CCCCECEC
EEE
V V RI– V VRI - V V V V
RI V
Improved Biased StabilityImproved Biased Stability
Adding RE to the emitter improves the stability of a transistor.
Stability refers to a bias circuit in which the currents and voltages will remain fairly constant for a wide range of temperatures and transistor Beta () values.
Load Line for Emitter-bias circuitLoad Line for Emitter-bias circuit
The end points of the load line are:
EC
CCC RR
VI
VVCE 0
CCCE VV
mAIC 0
ICsat
VCEcutoff
Voltage Divider BiasVoltage Divider Bias
This is a very stable bias circuit.
The currents and voltages are almost independent of variations in .
There are two ways of analyzing the voltage divider bias circuit :-1. Exact analysis2. Approximate
analysis
Exact Analysis
21
22 RR
VRVE CCRTh
21 RRRTh
ETh
BEThB RR
VEI1
ECCCCCE RRIVV
Approximate analysis
Where IB << I1 and I2 and I1 I2 :
Where RE > 10R2:
From Kirchhoff’s voltage law:
21
CC2B RR
VRV
E
EE R
VI
BEBE VVV
EECCCCCE RI - RI - V V
)R (R-IVVII
ECCCCCE
CE
DC Bias with Voltage Feedback
Another way to improve the stability of a bias circuit is to add a feedback path from collector to base.
In this bias circuit the Q-point is only slightly dependent on the transistor beta, .
Base-Emitter loop
)R(RRVV
IECB
BECCB
From Kirchhoff’s voltage law:
0RI–V–RI–RI– V EEBEBBCCCC
Where IB << IC:
CBCC IIII
Knowing IC = IB and IE IC, the loop equation becomes:
0RIVRIRI– V EBBEBBCBCC
Solving for IB:
Collector-emitter loop
Applying Kirchoff’s voltage law:IERE + VCE + ICRC – VCC = 0
Since IC IC and IC = IB:IC(RC + RE) + VCE – VCC =0
Solving for VCE:VCE = VCC – IC(RC + RE)
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