ELECS - 7) BJT
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Transcript of ELECS - 7) BJT
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Bipolar Junction Transistors
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Introduction
The analysis or design of a transistor amplifier requires
a knowledge of both the dc and ac response of the
system
The improved output ac power level is the result of atransfer of energy from the applied dc supplies
The analysis or design of any electronic amplifier
therefore has two components: the dc portion and theac portion
the superposition theorem is applicable and the
investigation of the dc conditions can be totally
separated from the ac response
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DC Level of Operation
The dc level of operation of a transistor is
controlled by a number of factors, including:
1. the range of possible operating points on the
device characteristics. Once the desired dccurrent and voltage levels have been defined, anetwork must be constructed that will establish
the desired operating point
2. Each design will also determine the stability of the system, that is, how sensitive the system is
to temperature variations
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Important Basic Relationships for a
Transistor
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BJT DC Operating Point
The term biasing is the term for the application of dc voltages toestablish a fixed level of current and voltage.
For transistor amplifiers the resulting dc current and voltageestablish an oper ating point on the characteristics that define theregion that will be employed for amplification of the applied signal.
Since the operating point is a fixed point on the characteristics, it is
also called the qui esc ent point (abbrev iat ed Q- point ). It meansquiet, still, inactive.
The biasing circuit can be designed to set the device operation atany of these points or others within the acti ve regi on. Themaximum ratings are indicated on the characteristics by ahorizontal line for the maximum collector current ICmax and a
vertical line at the maximum collector-to-emitter voltage VCEmax.The maximum power constraint is defined by the curve PCmax inthe same figure. At the lower end of the scales are the cutoff region, defined by IBµ0 , and the saturation region, defined byVCE VCEsat.
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Operating Point Using Load Line
The BJT device could be biased to operate outside
these maximum limits, but the result of such operation
would be either a considerable shortening of thelifetime of the device or destruction of the device.
Confining ourselves to the acti ve regi on , one can select
many different operating areas or points.
The chosen Q- point of t en depend s on the intendeduse of the circuit. Still, we can consider some
differences among the various points shown in Fig. 4.1
to present some basic ideas about the operating point
and, thereby, the bias circuit.
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DC Load Line
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Linear, Cut-off, Saturation
Operation in the cut-off, saturation, and linear regions of
the BJT characteristic are provided as follows:
1. Linear -regi on oper ati on:
Baseemitter junction forward biasedBasecollector junction reverse biased
2. C ut off -regi on oper ati on:
Baseemitter junction reverse biased
3. Sat ur ati on-regi on oper ati on:
Baseemitter junction forward biased
Basecollector junction forward biased
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Cut-Off
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Saturation
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DC Bias
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Cut-off and Saturation
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BJT Biasing
For the BJT to be biased in its linear or active
operating region the following must be true:
1. The baseemitter junction must be forward-
biased (p-region voltage more positive ) ,with aresulting forward-bias voltage of about 0.6 to 0.7V.
2. The basecollector junction must be reverse-
biased (n-region more positive ) , with the reverse-bias voltage being any value within the maximum
limits of the device.
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Graphical Analysis
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DC Load Line
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Linear Operation
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LINEAR OPERATION
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Troubleshooting Transistors
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DMM Test Of a Properly Functioning
Transistor
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FIXED-BIAS NETWORK
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Emitter Stabilized
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Summary of Transistor Bias Circuits
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