Recall Lecture 7

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Recall Lecture 7

• Clipper
• Step 1 Find the clip value by doing KVL at the
  output branch
• Step 2 Set the conditions to know whether diode
  is on or off sketch your output waveform
• Clamper
• Step 1 Calculate value of VC by knowing which
  cycle it is charged to (what is the peak value of
  that cycle) then confirmed the polarity of the
  capacitor
• Step 2 performed KVL as diode is now reverse-
  biased to obtain equation of Vout . Sketch your
  output waveform

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Multiple Diode Circuits
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Final Exam SEM I 2013/2014
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DIODE ID VD
OFF 0 VD lt V?
ON ID gt 0 VD V?
REMEMBER THAT A pn junction diode will conduct
when the p-type material is more positive than
the n-type material
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OR GATE
Vo voltage across R
V1 V2 VO
D1 and D2 off no current flow, 0 0 0
D1 off, D2 on, current flow, Vo V2 V? 0 0 5V ( 1 ) 4.3V
D1 on, D2 off, current flow, Vo V1 V? 0 5V ( 1 ) 0 4.3V
Both on, using both loops will give the same equation 5V ( 1 ) 5V ( 1 ) 4.3V
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AND GATE
Vo node voltage
V1 V2 VO
Both on, using both loops will give the same equation 0 0 0.7
D1 on, D2 off 0 5V ( 1 ) 0.7
D1 off, D2 on 5V ( 1 ) 0 0.7V
Both are off open circuit no current flowing through R since no GND destination 5V ( 1 ) 5V ( 1 ) 5V
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Chapter 4Bipolar Junction Transistor
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REMEMBER THIS
Current flow in the opposite direction of the
electrons flow same direction as holes
I
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Transistor Structures

• The bipolar junction transistor (BJT) has three
  separately doped regions and contains two pn
  junctions.
• Bipolar transistor is a 3-terminal device.
• Emitter (E)
• Base (B)
• Collector (C)

• The basic transistor principle is that the
  voltage between two terminals controls the
  current through the third terminal.

• Current in the transistor is due to the flow of
  both electrons and holes, hence the name bipolar.

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Transistor Structures

• There are two types of bipolar junction
  transistor npn and pnp.
• The npn bipolar transistor contains a thin
  p-region between two n-regions.

• The pnp bipolar transistor contains a thin
  n-region sandwiched between two p-regions.

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3 Regions of Operation

• Active
• Operating range of the amplifier.
• Base-Emitter Junction forward biased.
• Collector-Base Junction reverse biased
• Cutoff
• The amplifier is basically off. There is voltage
  but little current.
• Both junctions reverse biased
• Saturation
• The amplifier is full on. There is little
  voltage but lots of current.
• Both junctions forward biased

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OPERATIONS - npn
ACTIVE MODE

VBE

• The base-emitter (B-E) junction is forward biased
  and the base-collector (C-B) junction is
  reverse-biased,.

-

• Since the B-E junction is forward biased,
  electrons from the emitter are injected across
  the B-E junction into the base ? IE
• Once in the base region, the electrons are
  quickly accelerated through the base due to the
  reverse-biased C-B region ? IC

iB

• Some electrons, in passing through the base
  region, recombine with majority carrier holes in
  the base. This produces the current ?IB

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TO ILLUSTRATE
E
B
C
- VBE

• Imagine the marbles as electrons
• A flat base region with gaps where the marbles
  may fall/trapped recombine
• A sloping collector region represents high
  electric field in the C-B region
• Hence, when enough energy is given to the
  marbles, they will be accelerated towards to base
  region with enough momentum to pass the base and
  straight fly to the collector

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MATHEMATICAL EXPRESSIONS
IC
IB
IE
IE IS e VBE / VT -1 IS e VBE / VT
Based on KCL IE IC IB
No. of electrons crossing the base region and
then directly into the collector region is a
constant factor ? of the no. of electrons exiting
the base region
IC ? IB
No. of electrons reaching the collector region is
directly proportional to the no. of electrons
injected or crossing the base region.
Ideally ? 1, but in reality it is between 0.9
and 0.998.
IC ? IE
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Based on KCL IE IC IB
IC ? IB
IC ? IE
IE ? IB IB IB( ? 1)
IE IB( ? 1)
Now With IC ? IB ? IB IC / ? Hence, IE
IC /? ( ? 1) IC IE ? / ? 1
Comparing with IC ? IE
? ? / ? 1
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OPERATIONS - pnp
IB
IC
FORWARD ACTIVE MODE
-

• The emitter base (E- B) junction is forward
  biased and the base-collector (B- C) junction is
  reverse-biased,.

VEB
IE

IE IS e VEB / VT -1 IS e VEB / VT
Notice that it is VEB
Based on KCL IE IC IB
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pnp Transistor- Active mode
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SUMMARY Circuit Symbols and Conventions
Based on KCL IE IC IB
npn bipolar transistor simple block diagram and
circuit symbol. Arrow is on the emitter terminal
that indicates the direction of emitter current
(out of emitter terminal for the npn
device) pnp bipolar transistor simple block
diagram and circuit symbol. Arrow is on the
emitter terminal that indicates the direction of
emitter current (into of emitter terminal for the
pnp device)
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BJT Current-Voltage CharacteristicIC versus VCE
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Common-Emitter Configuration - npn

• The Emitter is common to both input
  (base-emitter) and output (collector-emitter).
• Since Emitter is grounded, VC VCE
• With decreasing VC (VCE), the junction B-C will
  become forward biased too.
• The current IC quickly drops to zero because
  electrons are no longer collected by the
  collector

Node B
0V
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Characteristics of Common-Emitter - npn
NOTE VEC for PNP
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Examples

• EXAMPLE 1
• Given IB 6.0?A and IC510 ?A
• Determine ?, ? and IE

• EXAMPLE 2
• NPN Transistor
• Reverse saturation current Is 10-13A with
  current gain, ? 90. Based on VBE 0.685V,
  determine IC , IB and IE

• EXAMPLE 3
• PNP Transistor
• ? 60, IC 0.85mA
• Determine ?, IE and IB