microelectronic circuits, kyung hee univ. fall, 2015 4.6...

Microelectronic Circuits, Kyung Hee Univ. Fall, 2015

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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.

Microelectronic Circuits, Kyung Hee Univ. Fall, 2015

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4.6.2. Characterizing Amplifiers

Microelectronic Circuits, Kyung Hee Univ. Fall, 2015

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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)

Microelectronic Circuits, Kyung Hee Univ. Fall, 2015

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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.

Microelectronic Circuits, Kyung Hee Univ. Fall, 2015

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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.

Microelectronic Circuits, Kyung Hee Univ. Fall, 2015

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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.

Microelectronic Circuits, Kyung Hee Univ. Fall, 2015

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Figure 6.54 Illustrating the need for a unity-gain buffer amplifier.

Microelectronic Circuits, Kyung Hee Univ. Fall, 2015

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Microelectronic Circuits, Kyung Hee Univ. Fall, 2015

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Microelectronic Circuits, Kyung Hee Univ. Fall, 2015

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Microelectronic Circuits, Kyung Hee Univ. Fall, 2015

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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.

Microelectronic Circuits, Kyung Hee Univ. Fall, 2015

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