Chapter 5' Bipolar Junction Transistors 5'4 5'6

1
Chapter 5. Bipolar Junction Transistors(5.4
5.6)

• Hanyang University
• ASIC Lab.

2
5.4 BJT Circuits At DC

• Example 5.4)

Figure 5.34
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5.4 BJT Circuits At DC

• Example 5.5) Not in active mode

Figure 5.35
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5.4 BJT Circuits At DC

• Example 5.6) cut-off

Figure 5.36 (a) circuit (b) analysis with the
order of the analysis steps indicated by circled
numbers.
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5.4 BJT Circuits At DC

• Example 5.7) pnp BJT

Figure 5.37 (a) circuit (b) analysis with the
steps indicated by circled numbers.
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5.4 BJT Circuits At DC

• Example 5.8)

Figure 5.38 (a) circuit (b) analysis with the
steps indicated by the circled numbers.
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5.4 BJT Circuits At DC

• Example 5.9) pnp BJT

Figure 5.39 (a) circuit (b) analysis with steps
numbered.
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5.4 BJT Circuits At DC

• Example 5.10)

Figure 5.40
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5.4 BJT Circuits At DC

• Example 5.11)

Figure 5.41
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5.4 BJT Circuits At DC

• Example 5.12)

Figure 5.42 (a) circuit (b) analysis with the
steps numbered.
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5.5 Biasing in BJT Amplifier Circuits

• - The biasing problem is that of establishing a
  constant dc current in the emitter of the BJT
•    this current has to be calculable,
  predictable, insensitive to variations in
  temperature and to the large variations in the
  value of ß among transistors of the same type

Figure 5.43 Two obvious schemes for biasing the
BJT (a) by fixing VBE (b) by fixing IB. Both
result in wide variations in IC and hence in VCE
and therefore are considered to be bad. Neither
scheme is recommended.
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5.5 Biasing in BJT Amplifier Circuits

• 5.5.1 The Classical Discrete-Circuit Bias
  Arrangement

Figure 5.44 Classical biasing for BJTs using a
single power supply (a) circuit (b) circuit
with the voltage divider supplying the base
replaced with its Thévenin equivalent.
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5.5 Biasing in BJT Amplifier Circuits

• - To make IE insensitive to temperature ß
  variation, we design the circuit to satisfy
  the following two constraints
•   i) VBBVBE insensitive to small variationsin
  VBE
•                                                   
                   (around 0.7v)
•    ii) RERB/(ß1) insensitive to variations in
  ß
• - RE provides a negative feedback action that
  stabilizes the bias current (CH 8)
• for some reason, IE increases

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5.5 Biasing in BJT Amplifier Circuits

• 5.5.2 A two-power-supply verson of the classical
  bias arrangement
•   
•  To make IE insensitive to temperature ß
  variation

Figure 5.45 Biasing the BJT using two power
supplies. Resistor RB is needed only if the
signal is to be capacitively coupled to the base.
Otherwise, the base can be connected directly to
ground, or to a grounded signal source, resulting
in almost total b-independence of the bias
current.
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5.5 Biasing in BJT Amplifier Circuits

• 5.5.3 Biasing Using a Collector-to-Base Feedback
  Resistor
•    
• To make IE insensitive to temperature ß
  variation
• - Bias stability is achieved by the negative
  feedback action of RB

Figure 5.46 (a) A common-emitter transistor
amplifier biased by a feedback resistor RB. (b)
Analysis of the circuit in (a).
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5.5 Biasing in BJT Amplifier Circuits

• 5.5.4 Biasing using a constant-current source
•                     
• IE is independent of the value of ß RB since
  the BJT is biased using a constant current source
  I  (The constant current source I can be easily
  implemented using another BJT(CH 6))

Figure 5.47 (a) A BJT biased using a
constant-current source I. (b) Circuit for
implementing the current source I.
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5.6 Small-Signal Operation and Models

• - DC conditions

Figure 5.48 (a) The operation of the transistor
as an amplifier. (b) The circuit of (a) with
the signal source vbe eliminated for dc (bias)
analysis.
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5.6 Small-Signal Operation and Models

• 5.6.1 The collector current iC transconductance
  gm
• The total instantaneous base-emitter voltage is
• - small-signal approximation - (usually vbe lt
  10mv)
•  gt the transistor behaves as a
  voltage-controlled current source
•   transconductance gt the slope of the iC-vBE
  characteristic curve at iCIC

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5.6 Small-Signal Operation and Models

• 5.6.2 The base current the input resistance at
  the base
• The small-signal input resistance between base
  and emitter, looking into
• the base
• 5.6.3 The emitter current the input resistance
  at the emitter
• the small-signal input resistance between base
  and emitter, looking into
• the emitter
• emitter resistance

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5.6 Small-Signal Operation and Models

• 5.6.4 Voltage gain

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5.6 Small-Signal Operation and Models

• 5.6.5 Separating the signal and the DC quantities
• DC voltage source ? short
• DC current source ? open
• lt The expression for the current increments ic,
  ib, and ie obtained
• when a small signal vbe is applied gt

Figure 5.50 The amplifier circuit of Fig. 5.48
(a) with the dc sources (VBE and VCC) eliminated
(short circuited). Thus only the signal
components are present. Note that this is a
representation of the signal operation of the BJT
and not an actual amplifier circuit.
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5.6 Small-Signal Operation and Models

• 5.6.6 The Hybrid-p Model the most widely used
  model for the
• 
  BJT
•  
•  
• lt The simplified hybrid-p model for the
  small-signal operation of the BJT gt
•      - a few tens of ohms, rx rp
•      - rx can usually b neglected in
  low-frequency applications, but not in
  high-frequency

Figure 5.51 (a) a transconductance amplifier
(b) a current amplifier
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5.6 Small-Signal Operation and Models

• 5.6.7 The T model           
•                           

Figure 5.52 Two slightly different versions of
what is known as the T model of the BJT. (a)
voltage-controlled current source (b)
current-controlled current source
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5.6 Small-Signal Operation and Models

• 5.6.8 Application of the small-signal equivalent
  circuits
• - The availability of the small-signal BJT
  circuit models makes the
• analysis of transistor amplifier circuits
  a systematic process
• - DC operating point is determined
•         gt the model parameters are calculated
•         gt the small-signal equivalent circuits
•                 (usually hybrid-p model)
•         gt analyze to determine Av, Rin, .

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5.6 Small-Signal Operation and Models

• Example 5.14)

Figure 5.53 a) circuit (b) dc analysis (c)
small-signal model.
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5.6 Small-Signal Operation and Models
Example 5.16)
Figure 5.55 (a) circuit (b) dc analysis (c)
small-signal model (d) small-signal analysis
performed directly on the circuit.
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5.6 Small-Signal Operation and Models

• - Table 5.4 Summary