Detailed Report on Amplifier Types and Feedback Mechanisms

This report provides a comprehensive analysis of various amplifier types, including voltage, current, transconductance, and transresistance amplifiers. It delves into the characteristics of each type, highlighting their input and output signal properties, impedance levels, and applications. The report extensively covers amplifier feedback topologies, such as series-shunt and shunt-series configurations, explaining how feedback mechanisms influence amplifier performance. It examines the effects of feedback on gain stability, input and output impedance, frequency response, and distortion levels. A numerical problem is included to illustrate the application of feedback concepts. The document also discusses the benefits of negative feedback, emphasizing its role in enhancing amplifier stability, reducing noise and distortion, and improving overall performance. The report concludes by summarizing the advantages of negative feedback in amplifier design and its impact on the trade-off between gain and other performance metrics.

Assignment
Voltage Amplifiers
 Characteristics of Voltage amplifiers
o The input signal is the voltage signal and the output signal is also the voltage
signal.
o This is a VCVS device which is Voltage-controlled voltage source.
o It has high input impedance and low 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 sampled by the feedback network and the
feedback signal xf is a voltage signal which is mixed with the input
signal voltage in series.
 Here the word “Series” is related to the connection at the input and the
the connection at the output is represented by the term “shunt” which
means parallel.
o Characteristics:
 This stabilizes the voltage gain.
 The series connection at the input gives higher input resistance.
 The shunt connection at the output gives lower output resistance.
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 topology
o Current-sampling shunt-mixing (shunt-series) topology
 The output current is sampled by the feedback network and the
feedback current signal can be mixed with the source current 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 offers the low
input resistance and the high output resistance.
Transconductance Amplifier
 Characteristics of a trans-conductance amplifier:
o The Input signal in this amplifier is a voltage signal and the output signal is
the 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 output impedance.
 Feedback topology
o This is series-series topology which means Current-sampling series-mixing.
 The output current is sampled by feedback network and the feedback
signal xf is a voltage that can be mixed with the source voltage in
series.
 There the term “Series” is related to the connection at the input and the
other “series” word is related to the connection at the output.
 Characteristics:
o This amplifier stabilizes the trans-conductance gain
o The trans-conductance amplifier gives high input resistance at the input post
of the amplifier and high output resistance at the output port of the amplifier.

Transresistance Amplifier
 Characteristics of trans-resistance amplifier:
o The input signal of a trans-resistance amplifier is a current signal and output
signal is a voltage signal.
o It is a CCVS device which stands for Current-controlled voltage source.
o This amplifier has low input impedance and low output impedance
 Feedback topology
o Voltage-sampling shunt-mixing (parallel-parallel) topology
 The output voltage signal is sampled by the feedback network and the
feedback signal xf is a current which can be mixed in parallel with the
source current.
 The term “Shunt” is related to the input connection and the term
“shunt” is also related to the 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 characteristic of gain of the amplifier with and without feedback is as
shown.
 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.