Ch(6) Small-Signal Amplifiers
In a small-signal amplifier F the input and the output
signals F small F in comparison to Ø
the dc bias Û the quiescent operating values of Ø
voltage Û current
Used the linear models F to predict the performance of
single-stage amplifiers at moderate frequencies
RC – COUPLED AMPLIFIERS
Each stage of a multistage amplifier F consists of F an
electronic two-port
biasing network Û coupling circuits
In RC-Coupled F the output of each transistor F coupled to
the next stage F RL Û C
Frequency-Response Curve
The voltage gain of one stage of an RC-coupled amplifier
Shown in Fig. 14.1
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Ch(6) Small-Signal Amplifiers
Gain F A F constant over F midfrequency range
A F falls of at F f1 Û f2
The bandwidth F defied by F f1 Û f2
At the half-power frequencies v
A1 = A 2 = Ao /
θ = ± 4 5o
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2
Ch(6) Small-Signal Amplifiers
A Generalized Amplifier
The input signals F vs Û zs F consist of F sinusoid of F
various amplitudes Û frequencies F shown as
Effective – value phasors
The one stage amplifier F characterized by F zi Û zo
A =Vo/Vi
AFcomplex function of F f
zL F transducer Û a following stage
the output F determined F when the frequency response of
the amplifier F known
Small-signal amplifier circuits F shown in the following
Fig.
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Ch(6) Small-Signal Amplifiers
RC – coupled amplifier F coupling F CC1 Û CC2 F with
their resistance
Our approach F replacing F complicated circuits by linear
models F respond in the same way to ac signals
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Ch(6) Small-Signal Amplifiers
Bias F RS Û RE Û CS Û CE Û VCC Û VDD
Appear in the circuit F don’t appear in the model
Unintentional elements F CW Û Cj
Appear in the model F don’t appear in the circuit
Source impedance F Rs
Load impedance F CL Û RL F in parallel
Small-Signal Amplifier Models
It is quite difficult F determination the values of individual
elements Fshown in Fig.
Specific values F depend upon factors v
a. Quiescent F (V Û I)
b. Operating temperature
c. Characteristics of the following stage
d. Configuration of the physical circuit
Replacing an actual amplifier by a linear circuit model that can
be analyzed is the goal of this analysis
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Ch(6) Small-Signal Amplifiers
The model of F FET Û BJT F similar and hold fairly
well for frequency from F Hz to MHz
General analysis for F series and parallel combinations of
FRÛC
If FωCR ≥ 10 F Zser ≅ R F within 0.5 %
If FωCR ≤ 0.1F Zpar ≅ R
When FR = 1/ωC F Zser Û Zpar F independent of
frequency
Gain F A F independent of frequency over the
midfrequency range F Fig. 14-1
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Ch(6) Small-Signal Amplifiers
Series capacitances F important F f < f1
Parallel capacitances F important F f > f2
P.B 14-1F cancelled
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