Transistor Circuit DC Bias
Part 1
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DC Biasing Circuits
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Fixed-Bias Circuit
Emitter-Stabilized Bias Circuit
Collector-Emitter Loop
Voltage Divider Bias Circuit
DC Bias with Voltage Feedback
Miscellaneous Bias Circuits
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Maximum Power Curve
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Fixed-Bias Circuit
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DC Equivalent circuit
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Base-Emitter (Input) Loop
Using Kirchoff’s voltage law: – VCC + IBRB + VBE = 0
Solving for IB:
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VCC - VBE
IB =
RB
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Collector-Emitter (Output) Loop
Since:
IC =  IB
Using Kirchoff’s voltage law: VCE – VCC – IC RC
Because:
VCE = VC - VE
Since VE = 0V, then:
VC = VCE
Also:
VBE = VB - VE
with VE = 0V, then:
VB = VBE
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BJT Saturation Regions
When the transistor is operating in the
Saturation Region, the transistor is
conducting at maximum collector
current (based on the resistances in
the output circuit, not the spec sheet
value) such that:
VCC - VCE
RC
where VCE = 0.2 V
ICsat =
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Determining Icsat
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Determining Icsat for the fixed-bias configuration
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Load Line Analysis
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Load Line Analysis
The end points of the line are : ICsat and VCEcutoff
For load line analysis, use VCE = 0 for ICSAT, and IC = 0 for VCEcutoff
ICsat:
VCEcutoff:
VCC
|VCE  0V
RC
VCE = VCC |IC  0mA
ICsat =
Where IB intersects with the load line we have the Q point
Q-point is the particular operating point:
• Value of RB
• Sets the value of IB
• Where IB and Load Line intersect
• Sets the values of VCE and IC.
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Circuit values effect Q-point
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Circuit values effect Q-point (continued)
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Circuit values effect Q-point (continued)
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DC Fixed Bias Circuit Example
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Load-line analysis
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Fixed-bias load line
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Movement of Q-point with increasing levels of IB
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Effect of RC on the load line and Q-point
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Effect of VCC on the load line and Q-point
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Example
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Emitter Stabilized Bias
Emitter-Stabilized Bias Circuit
Adding a resistor to the emitter circuit (between the emitter lead and ground) stabilizes
the bias circuit
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Improved Bias Stability
The addition of 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 forward current gain
()
The temperature surrounding the transistor circuit is not always constant
Therefore, the transistor  is not a constant value
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Base-Emitter Loop
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Equivalent Network
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Reflected Input impedance of RE
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Base-Emitter Loop
Applying Kirchoffs voltage law:
Since:
- VCC + IB RB + VBE +IE RE = 0
IE = ( + 1) IB
We can write:
- VCC + IB RB + VBE + ( + 1) IB RE = 0
VCC - VBE
IB =
RB + (β+1)RE
Grouping terms and solving for IB:
Or we could solve for IE with:
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 RB 
- VCC + IE 
 + VBE + IE RE = 0
(

+
1)


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Collector-Emitter Loop
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Collector-Emitter Loop
Applying Kirchoff’s voltage law:
- VCC + IC RC + VCE + IE RE = 0
Assuming that IE  IC and solving for VCE: IC = VCC – VCE – (RE + RC)
Solve for VE:
V E = IE R E
Solve for VC:
VC = VCC - IC RC
or
VC = VCE + IE RE
Solve for VB:
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VB = VCC - IB RB
or
VB = VBE + IE RE
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Transistor Saturation
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:
ICSAT =
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VCC - VCE
RC + RE
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Load Line Analysis
The load line end points can be calculated:
At cutoff:
VCE  VCC | IC = 0 mA
At saturation:
IC =
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VCC
| VCE = 0V
RC + R E
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Emitter Stabilized Bias Circuit Example
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Load Line For The Emitter-bias Configuration.
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