Voltage Amplifier Feedback Analysis

The assignment content discusses various types of amplifiers, including voltage amplifiers with series-shunt feedback, current amplifiers with shunt-series feedback, transconductance amplifiers with series-series feedback, and transresistance amplifiers with shunt-shunt feedback. The effects of feedback on amplifier performance are also discussed, including the impact on gain, input impedance, output impedance, frequency response, distortion, and noise. The assignment also includes a numerical problem and a simulation exercise to analyze the performance of a series-shunt negative feedback circuit.

Assignment
Voltage Amplifier/Series-shunt Feedback
Series-shunt amplifiers are designed to amplify a source voltage signal and provide an output
voltage signal. The voltage amplifier is essentially a VCVS device. The input impedance of
this source has to be high, and the output impedance should be very low.
Since at the input side, the connection is series and at the output side, the connection is
parallel, therefore, this kind of amplifier feedback topology is called as series- shunt
feedback.
Current Amplifier/Shunt-Series Feedback
Here, the output physical quantity is current; therefore the feedback network should be able
to sample the current. The feedback signal should be in the form of so that it can get mixed in
parallel with the input source current. Therefore the amplifier feedback topology which suits
our requirement is the current-mixing current-sampling topology. Since the connection at the
input is shunt and the connection at the output is series, therefore the feedback topology is
also known as shunt-series feedback. This topology stabilizes the current gain and also
provides a low input resistance and a high output resistance. These are two desirable
properties for a current amplifier.

Transconductance Amplifier/Series-Series Feedback
This amplifier is known as trans-conductance amplifier since the gain of the amplifier gives
the ratio of current to voltage. In this amplifier, voltage is the input signal and current is the
output signal. Since the series connection exists at both the input and the output side, the
feedback topology is also known as series-series feedback.
Transresistance Amplifier/Shunt-Shunt Feedback
In trans-resistance 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) 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 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.