Short Notes: MOSFET Biasing & Amplifiers- 3

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Multistage,Feedback&PowerAmpli?ers
This section covers multistage ampli?ers, feedback mechanisms, and power ampli?ers in ana-
log circuits, focusing on their design, characteristics, and applications.
1. MultistageAmpli?ers
Multistageampli?erscascademultipleampli?erstagestoachievehighergain,improvedband-
width, or speci?c impedance characteristics.
• Purpose: Increase overall gain, match input/output impedances, or enhance bandwidth.
• Con?gurations:
– Common Emitter (CE)-CE: High voltage gain, used in audio ampli?ers.
– CE-Common Collector (CC): High gain with low output impedance.
– Cascode (CE-CB): High gain, wide bandwidth, high output impedance.
• GainCalculation: TotalvoltagegainA
v
=A
v1
·A
v2
·...·A
vn
,whereA
vi
isthegainof
thei-th stage (loading effects must be considered).
• Bandwidth: Limitedbythestagewiththesmallestbandwidth;high-frequencyresponse
may degrade due to interstage coupling capacitances.
2. FeedbackAmpli?ers
Feedback ampli?ers use a portion of the output signal fed back to the input to modify perfor-
mance.
• TypesofFeedback:
– Negative Feedback: Reduces gain, improves stability, linearity, and bandwidth.
Gain with feedback: A
f
=
A
1+ßA
, where A is open-loop gain and ß is the feed-
back factor.
– PositiveFeedback: Increases gain, used in oscillators, may cause instability.
• FeedbackTopologies:
– Voltage-Series: Reduces output impedance, increases input impedance.
– Voltage-Shunt: Reduces output impedance, decreases input impedance.
– Current-Series: Increases both input and output impedance.
– Current-Shunt: Decreases input impedance, increases output impedance.
• AdvantagesofNegativeFeedback:
– Bandwidth: BW
f
=BW(1+ßA).
– Reduced distortion and noise.
– Stabilized gain against parameter variations.
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Page 2


Multistage,Feedback&PowerAmpli?ers
This section covers multistage ampli?ers, feedback mechanisms, and power ampli?ers in ana-
log circuits, focusing on their design, characteristics, and applications.
1. MultistageAmpli?ers
Multistageampli?erscascademultipleampli?erstagestoachievehighergain,improvedband-
width, or speci?c impedance characteristics.
• Purpose: Increase overall gain, match input/output impedances, or enhance bandwidth.
• Con?gurations:
– Common Emitter (CE)-CE: High voltage gain, used in audio ampli?ers.
– CE-Common Collector (CC): High gain with low output impedance.
– Cascode (CE-CB): High gain, wide bandwidth, high output impedance.
• GainCalculation: TotalvoltagegainA
v
=A
v1
·A
v2
·...·A
vn
,whereA
vi
isthegainof
thei-th stage (loading effects must be considered).
• Bandwidth: Limitedbythestagewiththesmallestbandwidth;high-frequencyresponse
may degrade due to interstage coupling capacitances.
2. FeedbackAmpli?ers
Feedback ampli?ers use a portion of the output signal fed back to the input to modify perfor-
mance.
• TypesofFeedback:
– Negative Feedback: Reduces gain, improves stability, linearity, and bandwidth.
Gain with feedback: A
f
=
A
1+ßA
, where A is open-loop gain and ß is the feed-
back factor.
– PositiveFeedback: Increases gain, used in oscillators, may cause instability.
• FeedbackTopologies:
– Voltage-Series: Reduces output impedance, increases input impedance.
– Voltage-Shunt: Reduces output impedance, decreases input impedance.
– Current-Series: Increases both input and output impedance.
– Current-Shunt: Decreases input impedance, increases output impedance.
• AdvantagesofNegativeFeedback:
– Bandwidth: BW
f
=BW(1+ßA).
– Reduced distortion and noise.
– Stabilized gain against parameter variations.
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3. PowerAmpli?ers
Power ampli?ers deliver signi?cant power to a load, typically for applications like audio sys-
tems or RF transmission.
• ClassesofOperation:
– Class A: Conduction for full 360? r cycle, high linearity, low ef?ciency (? ˜ 25-
50%).
– Class B: Conduction for 180? r cycle, higher ef?ciency (? ˜ 78.5%), used in push-
pull con?gurations.
– ClassAB: Conduction between 180? r and 360? r, balances ef?ciency and linearity.
– ClassC: Conduction < 180? r, high ef?ciency (? > 80%), used in RF applications.
• KeyParameters:
– Power Output: P
out
=
V
2
L(rms)
R
L
, whereV
L(rms)
is the RMS load voltage andR
L
is the
load resistance.
– Ef?ciency: ? =
Pout
P
DC
, whereP
DC
is the DC power from the supply.
– Distortion: MinimizedinClassA,higherinClassB/Cduetocrossoverorharmonic
distortion.
• DesignConsiderations:
– Transformer coupling for impedance matching.
– Heat dissipation using heat sinks for high-power transistors.
4. Applications
• Multistage: Audio preampli?ers, RF ampli?ers.
• Feedback: Operational ampli?ers, control systems.
• PowerAmpli?ers: Loudspeakers, RF transmitters, motor drivers.
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