Amplifier Characteristics, Feedback Loops, and Performance Analysis

This report delves into the characteristics and performance of various amplifier types, including voltage, current, transconductance, and transresistance amplifiers. It explores the fundamental principles of each amplifier type, detailing their input and output signal characteristics, impedance levels, and applications. The report then examines different feedback topologies, such as series-shunt and shunt-series designs, illustrating how feedback impacts amplifier gain, input and output impedance, and overall stability. A numerical problem is solved to demonstrate the application of feedback formulas and the calculation of amplifier gain. The analysis includes the benefits of negative feedback, such as increased input impedance, decreased output impedance, improved frequency response, reduced distortion and noise, and enhanced stability. The report concludes by summarizing the advantages of negative feedback and the trade-offs involved, offering a comprehensive overview of amplifier design and performance optimization.

Assignment
Voltage Amplifiers
 Characteristics of Voltage amplifiers
o The input and output signal in the circuit are both voltage signal.
o This is a VCVS device which is Voltage-controlled voltage source.
o This circuit is characterised by its input impedance that is high and a
comparatively lower output impedance
 Amplification Feedback topology
o In this, the sampling is of Voltage and mixing is of series (series-shunt)
topology
 The output voltage signal is used as the source for the feedback
network and feedback signal xf is fed into the input through a series
connection
 Here the word “Series” is used to define the feedback signal collected
from the output signal sample and sent to the input and the feedback
circuit at output is represented by the term “shunt” which means
parallel.
o Characteristics:
 This circuit stabilizes the voltage gain.
 The resistance in series at the input gives higher resistance in input.
 The parallel resistance at the output gives lower resistance at output.
Current Amplifier
 Characteristics of Current Amplifiers:
o The input and output signals are both current.
o It is a CCCS source which stands for Current-controlled current source
o This amplifier has low input impedance
o This amplifier has high input impedance
 Amplifier Feedback loop design
o Current-sampling shunt-series design
 The feedback loop takes sample of the output current and generates the
feedback current that can be mixed with the input signal in parallel.
 The term “Shunt” is related to the input connection and the term
“series” is related to the output connection.
 Characteristics:
o This circuit stabilizes the current gain.

o In this current amplifier circuit, the input port of the amplifier is characterised
by low input impedance and the high output impedance.
Transconductance Amplifier
 Characteristics of a trans-conductance amplifier:
o The Input signal in this amplifier is characterised as voltage signal and the
output signal is characterised as current signal.
o This is a VCCS device which stands for Voltage-controlled current source.
o This amplifier provides high input impedance.
o This amplifier provides high impedance in its output.
 Feedback design
o This is series-series design which means Current-sampling series-mixing.
 The feedback network takes sample of the output and generates the
feedback voltage signal xf that is fed into the input signal voltage
through series connection.
 There the term “Series” is related to the resistance connected in the
input and output
 Characteristics:
o This amplifier stabilizes the trans-conductance gain
o The trans-conductance amplifier gives high input resistance and offers high
output resistance for the amplifier.

Transresistance Amplifier
 Characteristics of trans-resistance amplifier:
o The input of a trans-resistance amplifier is current and output signal is
characterised as a voltage signal.
o It is a CCVS device which stands for Current-controlled voltage source.
o This amplifier has low ohmage at both input and output
 Feedback loop design
o Voltage-sampling parallel-parallel design
 The feedback mechanism situated at output terminal takes sample of
output voltage and generates feedback current xf that is sent to the
input through parallel
 The term “Shunt” is refers to the parallel connected resistance at both
the input and output connection.
 Characteristics:
o This amplifier stabilizes the trans-resistance gain.
o The input of the trans-resistance amplifier provides low input resistance and
the output provides low output resistance.
Effect of Feedback on Amplifier Performance
Characteristic Voltage Series Current Series Current Shunt Voltage Shunt
Feedback Signal Voltage Voltage Current Current
Sampled Signal Voltage Current Current Voltage
Feedback Gain(B) Feedback
voltage/output
voltage
Feedback
voltage/output
current
Feedback
Current/Input
Current
Feedback
Current/Output
Voltage
Open Loop Gain Av= Vo/Vi Gm=Io/Vi Ai=Io/Ii Rm=Vo/Ii
D 1+BAv 1+BGm 1+BAi 1+BRm
Af Av/D Gm/D Ai/D Rm/D
Rif RiD RiD Ri/D Ri/D
Rof Ro/D RoD RoD Ro/D
Numerical Problem

To convert the decibels into magnitude, we use the formula
gain in dB = 20 log (A)
Putting the values
20 log (A) = 99
log (A) = 99/20 = 4.5
A = 10 (99/20)
A = 104.5
A = 31623
B = Vf/Vo = 10/100 = 0.1
Av = A/(1+AB) = 31623/(1+31623*0.1) = 9.997
Av = 9.997
Shunt-Shunt Negative Feedback Circuit

Performance Analysis of Feedback on Gain
|Af| < |Ao|, If the absolute value of the denominator of the transfer function is more than unity,
there is a negative feedback. Negative feedback is quite useful because it tends to make
system self-regulating. The gain obtained from the negative feedback is rather less than the
gain obtained from the amplifiers without feedback. Despite the loss of the gain, it is possible
to achieve a high inputs impedance, low output impedance, more stable amplifier gain and
higher cut-off frequency with feedback circuits. The thermal changes, changes in parameters
in time and the effect of the noises are reduced in conjunction with the increase of the
stability. Benefits of negative feedback can be summarized as:
 Increasing input impedance. (It can be provided with suitable feedback)
 Decreasing output impedance. (It can be provided with suitable feedback)
 Better Frequency response. Frequency range is extended resulting from band-width
increases. The gain characteristics of the amplifier with and without feedback are
shown in the diagram.
 The distortion and noise at the output can be minimized with feedback. The factor of
(1+ βA) provides the significant improvement by way of substantially reducing both
input noise and the non-linear distortion which is occurred in output. However, it
should be noted that total gain decreases. More stages can be added to the amplifier to
increase the gain but those stages can cause the noise.
 Increasing stability. The gain of the feedback circuit is independent of the thermal
changes and the parameter changes in time.

Conclusion
The benefit of the negative feedback is that it improves the sensitivity of the open
loop gain of the amplifier. The negative feedback also improves the bandwidth of the
amplifier hence improving the frequency response of the amplifier. There trade-off is
that there is some loss in the gain of the amplifier.