Bipolar Junction Transistors (BJT

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Chapter 02 Bipolar Junction Transistors (BJTs)
2.1 Device Structure and Physical Operation 2.2
Current-Voltage Characteristics 2.3 BJT as an
Amplifier 2.4 BJT Circuits at DC 2.5 Biasing in
BJT Amplifier Circuits 2.6 Small-Signal
Operation and Models 2.7 Single-Stage BJT
Amplifier 2.8 The BJT internal Capacitances and
High-Frequency Model 2.9 Frequency Response of
the CE Amplifier
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• The Key point to solve the DC solutions

• If under active mode the terminal currents
• could be set to
• VBE 0.7 V (for npn)
• iB iB iC ? iB and iE
  (?1) iB
• then by KVL, solve iB at first, and then to
  solve the node voltages
• to verify the validity of the active
  operation.
• If under saturation mode
• Let VCE VC,sat 0.2 V at first
• the terminal voltage VBVE0.7 and
  VBVC0.5 (for npn)
• then by KCL, to solve the branch currents
  respectively.

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2.4 BJT Circuits at DC

• BJT operation as a switch

Hihg OFF ? vCVCC Low Saturation ?
vCVC,sat
10V
5V
Figure 5.32 A simple circuit used to illustrate
the different modes of operation of the BJT.
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• Assumption Let VBE 0.7 V of a
  conducting transistor
• VCE 0.2 V
  of a saturated transistor.
• neglect the Early effect.

Ex With the above assumptions, let ?100, find
out the terminal voltages and the branch
currents.
NPN Case
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Ex For a PNP BJT case. The minimum value of ?
is specified to be 30. To determine the
voltages at all nodes and the currents through
all branches.

• To verify which operation mode (active or
  saturation) is possible
• at first!

• Using active mode assumption,
• then we can find
• the transistor becomes
• in saturation mode operation
• ? violate the assumption
• in the active mode operation
• ? thus in saturation mode.

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Ex Assume ?100. To determine the voltages at
all nodes and the currents through all
branches.

• Using Thevenins theorem to find out the
  equivalent resistance
• Assume in active mode.

(12.78uA)
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Ex Assume ?100 for both Q1 and Q2. To determine
the node voltages And the branch currents.

• Assumed all in active mode.

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5.5 Biasing in BJT Amplifier Circuits

• Two obvious Bias schemes for BJT (Not so good,
  because of variation)

• Both result in wide variation in IC and hence in
  VCE
• Using a degenerating RE to stabilize the bias
  current.

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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• RE degeneration for Bias

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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• Using Two-Power-Supply Bias Scheme (with
  degeneration RE)

AC signal
To provide an AC signal blocking.
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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• Biasing Using a Collector-to-Base Feedback
  Resistor

• Suitable for the CE amplifier
• RB provides negative feedback, which helps to
  stabilize the
• bias point.

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• However, the value of RB determines the allowable
  signal
• swing at the collector.

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• Biasing Using a Constant-Current Source

• Advantages (a) the emitter current is
  independent of the values
• of ? and RB (b) RB can be made large to keep
  the input resistance
• high without adversely affecting bias stability.
  (c) to make the
• design simple.

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2.6 Small-Signal Operation and Models
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Say, if vbe ? 10 mV ? small signal
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Figure 5.49 Linear operation of the transistor
under the small-signal condition A small signal
vbe with a triangular waveform is superimposed on
the dc voltage VBE. It gives rise to a collector
signal current ic, also of triangular waveform,
superimposed on the dc current IC. Here, ic
gmvbe, where gm is the slope of the iCvBE curve
at the bias point Q.
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• Small-Signal Model Development

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• Small-Signal Models Hybrid-? Model and T Model
  

• From the above analysis, we find that we can
  separate
• the signal and the DC quantities to simplify the
  analysis.

Hybrid-? Model
Figure 5.51 Two slightly different versions of
the simplified hybrid-p model for the
small-signal operation of the BJT. The equivalent
circuit in (a) represents the BJT as a
voltage-controlled current source (a
transconductance amplifier), and that in (b)
represents the BJT as a current-controlled
current source (a current amplifier).
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Note both models can be viewed as (a)
voltage-controlled current source, and
(b) current-controlled current source types.
T Model
Figure 5.52 Two slightly different versions of
what is known as the T model of the BJT. The
circuit in (a) is a voltage-controlled current
source representation and that in (b) is a
current-controlled current source representation.
These models explicitly show the emitter
resistance re rather than the base resistance rp
featured in the hybrid-p model.
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• Steps to doing small-signal analysis

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Ex 5.14 Assume ?100. Find the small-signal
voltage gain vo/vi.

• Obey VBE0.7V
• in active mode.

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• Small-Signal Model accounting for the Early
  Effect

voltage-controlled current source
current-controlled current source
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2.7 Single-Stage BJT Amplifiers

• Basic Structure

• Constant-current drive
• RB for AC signal blocking.

Figure 5.59 Basic structure of the circuit used
to realize single-stage, discrete-circuit BJT
amplifier configurations.
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parameter dependent.
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• The Common-Emitter (CE) Amplifier

Coupling capacitors AC-short
Bypass capacitor AC-short
Figure 5.60 (a) A common-emitter amplifier using
the structure of Fig. 5.59.
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Figure 5.60 (b) Equivalent circuit obtained by
replacing the transistor with its hybrid-p model.
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Figure 5.60 (b) Equivalent circuit obtained by
replacing the transistor with its hybrid-p model.
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Figure 5.60 (b) Equivalent circuit obtained by
replacing the transistor with its hybrid-p model.
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Figure 5.60 (b) Equivalent circuit obtained by
replacing the transistor with its hybrid-p model.
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Figure 5.60 (b) Equivalent circuit obtained by
replacing the transistor with its hybrid-p model.
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• The Common-Emitter (CE) Amplifier with an Emitter
  Resistance

• With a degeneration resistance Re

Figure 5.61 (a) A common-emitter amplifier with
an emitter resistance Re.
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Figure 5.61 (b) Equivalent circuit obtained by
replacing the transistor with its T model.
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• Degeneration Resistance Effect

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• The Degeneration Effect

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2.8 BJT Internal Capacitance HF model
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• The High-Frequency Hybrid-? Model

Figure 5.67 The high-frequency hybrid-p model.
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Figure 5.68 Circuit for deriving an expression
for hfe(s) Ic/Ib.
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Figure 5.69 Bode plot for uhfeu.
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Under high-level injection
Figure 5.70 Variation of fT with IC.
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2.9 Frequency Response of the CE Amplifier

• Gain drop due to
• Blocking Effects of CC1, CC2, and CE
• Shorting Effects of C? and C?

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• High-Frequency Response (Omit CC1, CC2, and CE)

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• Low-Frequency Response (Omit C? and C?)

• The first pole (by CC1)

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• The second pole
• (by CE)

• The third pole
• (by CC2)

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• Special Design for fp2 gt fp1 gt fp3

(20dB/decade)
The End