Ch(6) Small-Signal Amplifiers
FREQUENCY RESPONSE
Frequency response F divided into three frequency ranges
High frequency Û Mid frequency Û Low frequency
& H.F.R F parallel capacitances F must be considered
& M.F.R F capacitances F can be neglected
& L.F.R F series capacitances CC F must be considered
Midfrequency Gain
FET
The general model Ø
replaced by the purely resistive circiuts Ø
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Ch(6) Small-Signal Amplifiers
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Ch(6) Small-Signal Amplifiers
BJT
For stability Ö RB << β RE
RB >> rπ
The ideal condition Ø
The circuit stable and simple to analyze
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Ch(6) Small-Signal Amplifiers
Low-Frequency Response
Below the midfrequencies
The susceptances of the parallel capacitors neglibly small Ø
The reactance of the coupling capacitance increasliy
important Ø
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Ch(6) Small-Signal Amplifiers
Coupling Capacitors
The general model Ö reduced to Ö simpler low frequency model
RG Ö retained Ö significant
RB Ö omitted Ö large compared to rπ
FET Û BJT Ö models are analogous Ö one analysis Ö serve
for both
The general circuit Ö (a) Ö replaced by Ø
Thevenin form Ö input circuit
OR
Norton form Ö output circuit
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Ch(6) Small-Signal Amplifiers
The two forms Ö are equivalents Ö CC Ö effectively shorted
The output of Ö Thevenin circuitØ
The low-frequency output VL related to the midfrequency
output by Ö complex factor dependent on Ø
frequency Û RC product
As frequencyÖ decreased Ö large fraction of Ö VT
appears Ö across CC Ö V at the output Ö reduced
The cutoff Ö half - power frequency Ø
Defined by
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Ö
Ch(6) Small-Signal Amplifiers
The behavior of Ö FET
Û BJT Ö predicted at low-
frequencyØ
The input voltageÖ Vgs Û Vbe Ö down to 70% of Vo at Ø
The output voltage Ö Vo Ö down to 70% of gm Vgs Ro Û gm
Vbe Ro Ö the corresponding amplifier gainÖ reduced
The overall Ö low-frequency voltage gain Ö for Ø
FET
The relative gain for Ö BJT OR FET
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Ch(6) Small-Signal Amplifiers
To predict the behavior of a given circuit determine Ø
& ω11 Û ω12 Ö the higher is
To design a circuit calculate Ø
& CC1 Û CC2
H-W P.P 14-3
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