Chapter 4b: DC biasingBJTs

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Chapter 4b DC biasing-BJTs
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DC Bias with Voltage Feedback (Collector
Feedback)

• improve the stability of a bias circuit is to
  add a feedback path from collector to base.
• bias circuit the Q-point is only slightly
  dependent on the transistor Beta ?.

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Base Emitter Loop (Input Loop) Analysis

• KVL VCC IC?RC IBRB VBE IERE 0
• Note IC? IC IB -- but usually IB ltlt IC --
  so IC? ? IC
• Knowing IC ?IB and IE ? IC then VCC ?IB RC
  IBRB VBE ?IBRE 0
• Simplifying and solving for IB

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Collector Emitter Loop (Output Loop) Analysis
KVL VCC - IC?RC - VCE IERE 0 Since IC?
? IC and IC ????? VCC - ICRC - VCE ICRE
0 Solving for VCE VCE VCC - IC(RC RE)

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Example 15 For the collector feedback
configuration of the following figure,
determine a) IB b) IC c) VC
16V
3.6 k?
470 k?
0.51 k?
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Example 16 Determine the range of possible
values for VC for the following network using 1
M? potentiometer.
4.7 k?
VC
150 k?
1 M?
3.3 k?
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Transistor Saturation Level
Load Line Analysis
It is the same analysis as for the voltage
divider bias and the emitter-biased circuits.
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Miscellaneous Bias Configurations
CASE 1
RC1.2 k?
RB100 k?
VEE-9V
Determine VC and VB
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CASE 2
Vi
RB240 k?
Vo
RE2 k?
VEE -20 V
Determine VCEQ and IE
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RC2.4 k?
CASE 3
RE1.2 k?
Determine VCB and IB
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Transistor Switching Networks
Transistors with only the DC source applied can
be used as electronic switches.
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Switching Circuit Calculations
The transistor will switch between saturation and
cutoff regions. You must ensure that
Also consider the resistance across the
transistor at saturation The resistance
across the transistor in cutoff is theoretically
infinite, but it is calculated
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Switching Circuit Calculations
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Switching Time
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Example 17 Design the transistor inverter. The
specification as follows a) IC(sat) 8 mA b) ?
100 c) IB 120IBmax and use standard resistor
values
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PNP Transistors
- VCC
- RC
Do the mathematical analysis with 2 parts a)
base-emitter loop analysis. Find IB b)
collector-emitter loop analysis. Find VCE Make
useful of these relationship IE(?1)IB and IE
_at_IC
- RB

IC
VBE
-
IB
- RE
IE
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Example 18 Determine VC, VCE and IC for the
following network
IC
3.3 kW
510 kW
VC
VCE
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References
1. Thomas L. Floyd, Electronic Devices, Sixth
edition, Prentice Hall, 2002. 2. Robert
Boylestad, Electronic Devices and Circuit
Theory, Eighth edition, Prentice Hall,
2002. 3.Puspa Inayat Khalid, Rubita Sudirman,
Siti Hawa Ruslan, Modul Pengajaran Elektronik
1, UTM, 2002. 4. Website http//www2.eng.tu.ac
.th
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