bjt emitter stabilized bias elec 121. january 2004elec 1212 improved bias stability the addition of...
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![Page 1: BJT Emitter Stabilized Bias ELEC 121. January 2004ELEC 1212 Improved Bias Stability The addition of R E to the Emitter circuit improves the stability](https://reader030.vdocuments.site/reader030/viewer/2022033022/5697bf7b1a28abf838c839fb/html5/thumbnails/1.jpg)
BJT Emitter Stabilized Bias
ELEC 121
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January 2004 ELEC 121 2
Improved Bias Stability
The addition of RE to the Emitter circuit improves the stability of a transistor output
Stability refers to a bias circuit in which the currents and voltages will remain fairly constant over a wide range of temperatures and transistor forward current gain ()The temperature (TA or ambient temperature) surrounding the transistor circuit is not always constantTherefore, the transistor is not a constant value
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January 2004 ELEC 121 3
BJT Emitter Bias• Draw Equivalent Input circuit• Draw Equivalent Output circuit• Write necessary KVL and KCL Equations• Determine the Quiescent Operating Point
– Graphical Solution using Loadlines– Perform a Computational Analysis
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January 2004 ELEC 121 4
Emitter-Stabilized Bias Circuit
Adding an emitter resistor to the circuit between the emitter lead and ground stabilizes the bias circuit over Fixed Bias
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January 2004 ELEC 121 5
Base-Emitter Loop
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January 2004 ELEC 121 6
Equivalent Network
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January 2004 ELEC 121 7
Reflected Input impedance of RE
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January 2004 ELEC 121 8
Base-Emitter Loop
Applying Kirchoffs voltage law: - VCC + IB RB + VBE +IE RE = 0
Since: IE = ( + 1) IB
We can write: - VCC + IB RB + VBE + ( + 1) IB RE = 0
Grouping terms and solving for IB:
Or we could solve for IE with:
CC BEB
B E
V - VI =
R + (β+1)R
BCC E BE E E
R- V + I + V + I R = 0
( + 1)
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January 2004 ELEC 121 9
Collector-Emitter Loop
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January 2004 ELEC 121 10
Collector-Emitter Loop
Applying Kirchoff’s voltage law: - VCC + IC RC + VCE + IE RE = 0
Assuming that IE IC and solving for VCE: VCE = VCC – IC (RC + RE)
If we can not use IE IC the IC = IE and: VCE = VCC – IC (RC + RE)
Solve for VE: VE = IE RE
Solve for VC: VC = VCC - IC RC or
VC = VCE + IE RE
Solve for VB: VB = VCC - IB RB or
VB = VBE + IE RE
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January 2004 ELEC 121 11
Transistor Saturation
CC CE CSAT
C E
V - VI =
R + R
At saturation, VCE is at a minimum
We will find the value VCEsat = 0.2V
For load line analysis, we use VCE = 0
To solve for ICSAT, use the output KVL equation:
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January 2004 ELEC 121 12
Load Line Analysis
The load line end points can be calculated:
At cutoff:
At saturation:
C CE CC I = 0 mAV V |
CE
CCC V = 0V
C E
VI =
R + R|
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January 2004 ELEC 121 13
Emitter Stabilized Bias Circuit Example
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January 2004 ELEC 121 14
Design of an Emitter Bias CE Amplifier
Where .1VCC VE .2VCC
And .4VCC VC .6VCC
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January 2004 ELEC 121 15
Emitter Bias with Dual Supply
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January 2004 ELEC 121 16
Emitter Bias with Dual Supply
Input Output