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4.6. Basic BJT Amplifier Configurations
Microelectronic Circuits, Kyung Hee Univ. Fall, 2015
2Table 4.4 Small-Signal Models of the BJT
Microelectronic Circuits, Kyung Hee Univ. Fall, 2015
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4.6.1. Three-Basic Configurations
Figure 4.48: The three basic configurations of BJT amplifier. The biasing arrangements are not shown.
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4.6.2. Characterizing Amplifiers
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4.6.3. The Common-Emitter (CE) Amplifier
• Of three configurations, the CE amplifier is most widely used.
• Figure 4.50(a) shows a common-emitter amplifier – with biasing arrangement omitted.• signal course (vsig)
• source resistance (Rsig)
• input resistance (Rin)
• gain (Avo)
• output resistance (Ro)
• transconductance (Gv)
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Common-Emitter Amplifier
Figure 4.50: (a) Common-Emitter Amplifier fed with a signal vsig
from a generator with a resistance Rsig. (b) The common-emitter amplifier circuit with the BJT
replaced with its hybrid-pi model.
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Characteristic Parameters of the CE Amplifier
• Replacing BJT with hybrid-pi model yields the expressions to right…
(eq6.69) input resistance:
out( |e put voltage:
(eq6.70) open-circuit volta
q6.
ge gain: |
) |
|
in
o m C o
vo m C o
R r
v g v R r
A g R r
XX
with neglected
(eq6.71) oper-circuit voltage gain:
(eq6.72) output resistance:
o
vo m C
vo m C
r
A g R
A g R
(eq4.69)
(eq4.70)
(eq4.71)
(eq4.72) 𝑅𝑂 = 𝑅𝐶||𝑟𝑜 ≅ 𝑅𝐶
Microelectronic Circuits, Kyung Hee Univ. Fall, 2015
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Characteristic Parameters of the CE Amplifier
• Three Observations
• The input resistance Rin = r = b/gm is moderate to low in value.
• The output resistance Ro = RC is moderate to high in value.
• The open-circuit voltage gain (Avo) can be high – making the CE configuration the workhorse in BJT amplifier design.
Microelectronic Circuits, Kyung Hee Univ. Fall, 2015
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Figure 6.51 Performing the analysis directly on the circuit with the BJT model used implicitly.
Microelectronic Circuits, Kyung Hee Univ. Fall, 2015
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Overall Voltage Gain
not open-loop
(eq6.74) amplifier input voltage:
(eq6.75) voltage gain:
(eq6.76) overal
|| ||
in i sig
sig
v m C L o
rR v v r
r R
A g R R r
l voltage gain: || || ov m C L o
sig sig
v rG g R R r
v r R
(eq4.74)
(eq4.75)
(eq4.76)
Microelectronic Circuits, Kyung Hee Univ. Fall, 2015
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Figure 6.52 The CE amplifier with an emitter resistance Re; (a) Circuit without bias details;
(b) Equivalent circuit with the BJT replaced with its T model.
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4.6.5. The Common-Base (CB) Amplifier
Figure 4.53: (a) CB amplifier with bias details omitted; (b) Amplifier equivalent circuit with the BJT represented by its T Model.
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Figure 6.54 Illustrating the need for a unity-gain buffer amplifier.
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4.6.7. Summary and Comparisons
• The CE configuration is one of the best suited for realizing the bulk of the gain required in an amplifier. Depending on the magnitude of the gain required, either a single stage or a cascade of two or three stages may be used.
• Including a resistor Re in the emitter lead of the CE stage provides a number of performance improvements at the expense of gain reduction.
• The low input resistance of the CB amplifier makes it useful only in specific applications.
• The emitter follower finds application as a voltage buffer for connecting a high resistance source to a low-resistance load.
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