Analog and Pulses Circuits
Transistors )BJTs(
Eng.Saleh M. ALshehri
2/4/2018 1
Text book:
electronic-devices-9th-
edition-by-floyd
:رابط تحميل الكتاب
https://hristotrifonov.files.wordpress.
com/2012/10/electronic-devices-9th-
edition-by-floyd.pdf
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Chapter-4
BJT
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BJT Structure:
• The BJT has three regions called the emitter, base, and collector.
• Between the regions are junctions as shows.
• Three dopped semiconductor regions by two pn.
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BJT Operation
1. In normal operation, the base-emitter is forward-biased and the
base-collector is reverse-biased.
2. For the npn type shown, the collector is more positive than the base,
which is more positive than the emitter.
3. For the pnp type, the voltages are reversed to maintain the
forward-reverse bias.
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BJT Currents:
The direction of normal current is in the direction of the arrow on the emitter terminal.
The emitter current is the sum of the collector current and the small base current. That
is,
IE = IC + IB.
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BJT Characteristics:
• The collector characteristic
curves show the three
transistor currents.
• The first region is the
saturation region.
• As VCE is increased, IC
increases until B.
• The region between points
B and C, which is the active
region.
• After C, is the breakdown
region.
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BJT Characteristics
• The collector
characteristic curves
show the relationship
of the three transistor
currents.
• βDC is the ratio of
collector current to
base current.
• Family of IC versus VCE
curves for several values
of IB (IB1< IB2< IB3, etc.)
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Example:
What is the βDC for the transistor
shown?
1. Choose a base current near the
center of the range – in this case IB3
which is 30 mA.
2. in this case, Ic = 5.0 mA. Calculate the
ratio:
𝑰𝒄 5.0 mA
βDC = = = 167
𝑰𝑩 30 uA
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Cutoff
• the transistor (BJT), cutoff is
the condition in which there
is no base current IB=0.
• Collector leakage current
(ICEO) is so small and is
usually neglected.
• Base-emitter and base-
collector junctions are
reverse-biased.
• ideal cutoff where IC = 0
and VCE =VCC.
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Saturation:
• In a BJT, saturation is the
condition in which there is
maximum collector current.
• IB increases due to increasing
VBB, IC also increases.
IC= βDC*IB
• VCE decreases due to the
increased voltage drop across RC.
VCE =VCC – IC*RC
• Base-emitter and base-collector
junctions are forward-biased.
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DC Load Line
• The DC load line represents the
circuit that is external to the
transistor.
• The bottom of the load line is ideal
cutoff where IC =0 and VCE =VCC.
• The top of the load line is at
saturation where IC= IC(sat)
and VCE =VCE(sat).
• It is drawn by connecting the
saturation and cutoff points.
• The transistor characteristic curves
are shown
• In between cutoff and saturation
along the load line is the active
region of the transistor’s
operation.
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THE BJT AS AN AMPLIFIER:
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THE BJT AS AN AMPLIFIER
• Transistor amplifies current
because the collector current is
equal to the base current
multiplied by the current gain, β
Ic= IB*β
• When add ac voltage, Vs, on the
dc bias voltage VBB .
• The VCC is connected to the
collector through the collector
resistor, RC.
• The ac collector current produces
an ac voltage across RC, thus
producing an amplified, but
inverted.
• This amplification voltage gain is
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Common-Base (CB) Amplifier
The CB gets high voltage
gain with a maximum
current gain of 1.
input resistance=~ Zero .
Suitable because the
sources tend to have very
low-resistance outputs.
The B is the common
terminal and is at ac
ground because of
capacitor C2.
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(CB) amplifier
Voltage Gain.
Input Resistance.
Output Resistance.
Current Gain.
Power Gain.
see at page- 299
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