Applying the BJT in Amplifier Design - U of S Engineeringengr 6 BJT...Title Microsoft PowerPoint -...

Microelectronic Circuits, Sixth Edition Sedra/Smith Copyright © 2010 by Oxford University Press, Inc.

C H A P T E R 6

Applying the BJT in

Amplifier Design


Obtaining a Voltage Amplifier

CCCCC RiVv −=


Obtaining a Voltage Amplifier

CCCCC RiVv −=

TBE Vv

SC eIi/=

TBE Vv

SCCCCE eIRVv/−=

Voltage Transfer Characteristics


Figure 6.32 Biasing the BJT amplifier at a point Q

located on the active-mode segment of the VTC.

Biasing the BJT to Obtain Linear Operation

TBE Vv

SCCCCE eIRVV/−=

TBE VV

SC eII/=

)()( tvVtv beBEBE +=


T

RC

T

CV

vV

V

V

RIA −=−=


Figure 6.34 Graphical construction for determining the VTC of the

amplifier circuit of Fig. 6.33(a).


Locating the bias point Q


Small-signal operation and models

TBE VV

SC eII/=

β/CB II =

CCCCCE RIVV −=

α/CE II =


The Collector Current and the Transconductance


Figure 6.38 Illustrating the definition of rπ

and re.

The Base current and

the Input Resistance at

the base

The Emitter current

and the Input

Resistance at the

Emitter


Figure 6.39 The amplifier circuit of Fig. 6.36(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.

Separating the Signal and the DC Quantities


Figure 6.40 Two slightly different versions of the hybrid-π 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).

The Hybrid-ππππ Model (npn and pnp)


The T Model (npn and pnp)


Application of the Small-Signal Equivalent Circuits


Figure 6.43 Signal waveforms in the circuit of Fig. 6.42. (continued)


Figure 6.44 (a) circuit; (b) dc analysis; (c) circuit with the dc sources

eliminated; (d) small-signal analysis using the T model for the BJT.

Input and output waveforms

for the circuit of Fig. 6.44.

Observe that this amplifier is

non-inverting, a property of

the grounded base

configuration


Figure 6.46 Performing signal analysis directly on the circuit

diagram with the BJT small-signal

model implicitly employed:

(a) Circuit for Example 6.14; (b)

Circuit for Example 6.16.


Figure 6.47 The hybrid-π small-signal model, in its two versions, with the resistance ro included (taking into account the early effect).

)||( 00

0

rRvgv

I

Vr

Cbem

C

A

−=

=


Exercise 6.41


Table 6.4 Small-Signal

Models of the BJT

Applying the BJT in Amplifier Design - U of S Engineeringengr 6 BJT...Title Microsoft PowerPoint -...

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