BJT Emitter Stabilized Bias ELEC 121. January 2004ELEC 1212 Improved Bias Stability The addition of...
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Transcript of BJT Emitter Stabilized Bias ELEC 121. January 2004ELEC 1212 Improved Bias Stability The addition of...
BJT Emitter Stabilized Bias
ELEC 121
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
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
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
January 2004 ELEC 121 5
Base-Emitter Loop
January 2004 ELEC 121 6
Equivalent Network
January 2004 ELEC 121 7
Reflected Input impedance of RE
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)
January 2004 ELEC 121 9
Collector-Emitter Loop
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
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:
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|
January 2004 ELEC 121 13
Emitter Stabilized Bias Circuit Example
January 2004 ELEC 121 14
Design of an Emitter Bias CE Amplifier
Where .1VCC VE .2VCC
And .4VCC VC .6VCC
January 2004 ELEC 121 15
Emitter Bias with Dual Supply
January 2004 ELEC 121 16
Emitter Bias with Dual Supply
Input Output