Comprehensive Analysis of Feedback Amplifier Circuit Designs

This report provides a detailed analysis of various feedback amplifier topologies, including series-shunt, shunt-sequence, transconductance, and transresistance amplifiers. It explores the impact of feedback on amplifier performance, such as gain, input and output impedance, and frequency response. The report also covers the benefits of negative feedback, including increased stability, reduced distortion, and improved bandwidth. Furthermore, it includes a numerical problem demonstrating the calculation of amplifier gain and analyzes the performance of a series-shunt negative feedback circuit in the frequency domain. The report concludes with a summary of the advantages of negative feedback and provides several illustrative diagrams.

Assignment
Voltage Amplifier/Series-shunt Feedback
Series-shunt amps are designed to increase a source power indicator moreover supply a
productivity power indicator. The power amp is fundamentally a VCVS device. The input
impedance of this source has to be elevated, as well as the productivity impedance should be
very short.
As at the input side, the connection is sequence and at the output side, the connection is
parallel, therefore, this kind of amplifier response topology is called because sequence-shunt
views.
Current Amplifier/Shunt-Sequence Feedback
Here, the output physical quantity is current; therefore the view network must be able to
sample the current. The view indicator must be in the type of in order that it can get
combined in parallel by the input supply current. Therefore the amplifier response topology
which suits our requirement is the current-merging current-sampling topology. Since the link
next to the input is shunt as well as the connection next to the productivity is sequence,
therefore the feedback topology is moreover recognized as shunt-sequence response. This
topology stabilizes the current increase and also provides a low key resistance furthermore a
high productivity resistance. These are two attractive properties in support of a current amp.

Transconductance Amp/Series-Series Feedback
This amplifier is known as trans-conductance amplifier since the gain of the amplifier gives
the ratio of current to voltage. Inside this amp, voltage is the input indicator and current is the
productivity signal. Since the series link exists next to both the input furthermore the
productivity side, the feedback topology is also recognized the same as series-series
feedback.
Transresistance Amplifier/Shunt-Shunt Feedback
In trans-resistance amps the input indicator is current as well as the productivity signal is
power. It pursues that the suitable response topology is of the current-blending power-
sampling form. The existence of the equivalent (or force) link by the side of equally the input
as well as the productivity creates this feedback topology recognized like shunt-shunt
response too.

Effect of Response on Amplifier Performance
Characteristic Power Sequence Current
Sequence
Current Shunt Voltage Shunt
Feedback Signal Power Power Current Current
Sampled Signal Power Current Current Power
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
Xo = A Xi
Xf = B Xo
Xi = Xs – Xf
Xo = A(Xs - Xf)
Xo = A Xs – A Xf
X o = A Xs – AB Xo
Xo (1 + AB) = A Xs
Xo/Xs = A/(1 + AB)
To convert the decibels into magnitude, we use the formula
gain in dB = 20 log (gain in numerals)
Putting the values
99 = 20 log (A)
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
Series-Shunt Negative Feedback Circuit
Performance Analysis of Feedback on Gain
The analysis in the frequency domain will be performed to determine the -3dB gain
bandwidth of the circuit by using the "Analysis-Setup ... -AC Sweep (Enabled)" command.
Choose "AC Sweep Type" = Decade for a logarithmic representation of the frequency on X
axis. The Sweep Parameters are:
- Pts/decade =101; -Start Freq =1
- End Freq = 100meg
In other words, the simulation will be done in the 1Hz-100MHz band, with a simulation step
of 101 points per decade. After entering the simulation parameters, press the "OK" button and
close the "Analysis Setup" menu by using the "Close" command. Run the simulation using
the "Analysis - Simulate" command (or directly by pressing the F11 key).
After running the simulation, the "Orcad Pspice A / D Demo" program is opening to
graphically represent the gain versus frequency characteristic. To draw this graph, run the
"Trace-Add Trace" command. If you want to view the voltage gain according to the
frequency you should choose:
V (Q2: c) / V (Q1: b).
Tick the Toggle Cursor and determine the lower limit Fj and the upper limit FS frequencies
values for the analyzed circuit (at 1/√2 of the maxim gain).
Conclusion

|Af| < |Ao|, If the complete assessment of the denominator of the transport role 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 increase of the amp through and without response is as
shown.
 The distortion and noise at the output can be minimized with feedback. The factor of
(1+ βA0) 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 increase of the feedback route is sovereign of the thermal
changes and the parameter changes in time.