Analysis of Voltage Amplifiers with Series-Shunt Feedback

This report delves into the analysis of voltage amplifiers, specifically examining the series-shunt feedback configuration. It begins by defining different amplifier types, including voltage, current, transconductance, and transresistance amplifiers, and their corresponding feedback topologies. The report then explores the impact of feedback on amplifier characteristics such as gain, input impedance, and output impedance. A numerical problem is presented to illustrate gain calculations, followed by a detailed analysis of a series-shunt negative feedback circuit using frequency domain analysis. The simulation results, including gain versus frequency characteristics, are discussed, leading to a conclusion that highlights the benefits of negative feedback, such as increased stability, improved frequency response, and reduced distortion and noise, while also acknowledging the trade-off of reduced gain. The report concludes by summarizing the advantages of negative feedback in amplifier circuits.

Assignment
Voltage Amplifier/Series-shunt Feedback
Voltage amplifiers are intended to amplify an input voltage signal and provide an output
voltage signal. The voltage amplifier is essentially a voltage-controlled voltage source. The
input impedance is required to be high, and the output impedance is required to be low.
Because of the series connection at the input and the parallel or shunt connection at the
output, this feedback topology is also known as series- shunt feedback.
Current Amplifier/Shunt-Series Feedback
The output quantity of interest is current; hence the feedback network should sample the
output current. The feedback signal should be in current form so that it may be mixed in
shunt with the source current. Thus the feedback topology suitable for a current amplifier is
the current-mixing current-sampling topology. Because of the parallel (or shunt) connection
at the input, and the series connection at the output, this feedback topology is also known as
shunt-series feedback. This topology not only stabilizes the current gain but also results in a
lower input resistance, and a higher output resistance, both desirable properties for a current
amplifier.
Transconductance Amplifier/Series-Series Feedback

In transconductance amplifiers the input signal is a voltage and the output signal is a current.
It follows that the appropriate feedback topology is the voltage-mixing current-sampling
topology. The presence of the series connection at both the input and the output gives this
feedback topology the alternative name series-series feedback.
Transresistance Amplifier/Shunt-Shunt Feedback
In transresistance amplifiers the input signal is current and the output signal is voltage. It
follows that the appropriate feedback topology is of the current-mixing voltage-sampling
type. The presence of the parallel (or shunt) connection at both the input and the output
makes this feedback topology also known as shunt-shunt feedback.

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) Vf/Vo Vf/Io If/Io If/Vo
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 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+ 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 gain of the feedback circuit is independent of the thermal
changes and the parameter changes in time.