EKT 214 ANALOG ELECTRONIC CIRCUIT II
SEM II 2012/2013
SOLUTION TUTORIAL 2 – FEEDBACK AMPLIFIERS and OSCILLATOR
QUESTION (Feedback)
1.
Define the feedback:
Answer:
1. The feedback process takes a portion of the output signal and returns it to the input to become part
of the input excitation.
Or
Feedback system, a portion of the output signal is fed back and combined with the input signal.
2.
State the advantages of negative Feedback
Answer:
i.
ii.
iii.
iv.
v.
3.
Gain Sensitivity – variations in gain is reduced.
Bandwidth Extension – larger than that of basic amplified.
Noise Sensitivity – may increase S-N ratio.
Reduction of Nonlinear Distortion
Control of Impedance Levels – input and output impedances can be increased or decreased.
Analyze and obtain the transfer function of the ideal closed-loop feedback system in Figure 1.
Answer:
The output signal is: So  AS
where A is the amplification factor.
Feedback signal is: S fb  S o
where is the feedback transfer function.
At summing node:
S  Si  S fb
Closed-loop transfer function or gain is
if
A  1 then A f 
Af 
A 1

A 
1
So
A

Si 1  A
Figure 1
EKT 214 ANALOG ELECTRONIC CIRCUIT II
SEM II 2012/2013
SOLUTION TUTORIAL 2 – FEEDBACK AMPLIFIERS and OSCILLATOR
4.
Analyze the four basic feedback topologies, based on the parameter to be amplified (voltage or
current) and the output parameter (voltage or current). List the closed-loop transfer function for
each circuit.
Answer:
First topology,
SERIES-SHUNT (Voltage amplifier)
Input signal Voltage and output signal Voltage
Output voltage is:
feedback voltage is:
Error voltage is:
Voltage gain transfer function:
Feedback transfer function:
Figure 2
Second topology,
SHUNT-SERIES (Current amplifier)
Input signal Current and output signal Current
Output current is:
feedback current is:
Error current is:
Current gain transfer function:
Feedback transfer function:
Figure 3
2
EKT 214 ANALOG ELECTRONIC CIRCUIT II
SEM II 2012/2013
SOLUTION TUTORIAL 2 – FEEDBACK AMPLIFIERS and OSCILLATOR
Third topology,
SERIES- SERIES (Transconductance amplifier)
Input signal Voltage and output signal Current
Output current is:
feedback voltage is:
Error voltage is:
Voltage gain transfer function:
Feedback transfer function:
Figure 4
Fourth topology,
SHUNT- SHUNT (Transresistance amplifier)
Input signal Current and output signal Voltage
Output voltage is:
feedback voltage is:
Error current is:
Current gain transfer function:
Feedback transfer function:
-
Figure 5
3
EKT 214 ANALOG ELECTRONIC CIRCUIT II
SEM II 2012/2013
SOLUTION TUTORIAL 2 – FEEDBACK AMPLIFIERS and OSCILLATOR
Summary of the four ideal topology
Table 1
5.
Design feedback amplifier to amplify the output signal of a microphone to meet the following
specifications, the microphone output signal = 10 mV, the output signal of the amplifier = 0.5 V,
The output resistance of the microphone RS = 5 kΩ, RL = 75 Ω, with op-amp with parameters Ri =
10 kΩ, Ro = 100 Ω, and a gain of Av = 104 is available.
Answer:
The closed-loop voltage gain:
Feedback transfer function:
Input resistance:
Output resistance:
The feedback for non-inverting amplifier resistor ratio is:
R1 + R2 must be much larger than Ro
Then we choose R1= 1 kΩ and R2 = 49 kΩ.
4
EKT 214 ANALOG ELECTRONIC CIRCUIT II
SEM II 2012/2013
SOLUTION TUTORIAL 2 – FEEDBACK AMPLIFIERS and OSCILLATOR
6.
Design feedback amplifier to provide a given current gain. Asume that a signal current source has a
output resistance RS = 10 kΩ, RL = 50 Ω, and a current gain of 10 is required. With op-amp
parameters Ri = 10 kΩ, Ro = 100 Ω, and a gain of Ai = 106 is available.
Answer:
The closed-loop current gain:
Feedback transfer function:
Input resistance:
Output resistance:
The feedback for non-inverting amplifier resistor ratio is:
R1 must be fairly small
Then we choose R1= 1 kΩ and R2 = 9 kΩ.
7.
Determine the input resistance of a shunt input connection and the output resistance of a series
output connection, for a feedback current amplifier. Consider a shunt–series feedback amplifier in
which the open-loop gain is Ai = 105 and the closed-loop gain is Aif = 50. Assume the input and
output resistances of the basic amplifier are Ri = 10 kΩ and Ro = 20 kΩ, respectively.
Answer:
The closed-loop current gain:
OR
Input resistance:
Output resistance:
5
EKT 214 ANALOG ELECTRONIC CIRCUIT II
SEM II 2012/2013
SOLUTION TUTORIAL 2 – FEEDBACK AMPLIFIERS and OSCILLATOR
QUESTION (Oscillator)
8.
Define the Oscillator:
Answer:
An oscillator is a circuit that produces a continuous or repetitive signal from a dc voltage without
the need of an input.
9.
List the types of Oscillator
Answer:
1.
2.
3.
10.
RC Oscillator
-
Wien Bridge Oscillator
-
Phase-Shift Oscillator
-
- Crystal Oscillator
LC Oscillator
Relaxation Oscillator
Discus the feedback Oscillator Principles on a positive feedback and the relaxation oscillator
Answer:

The feedback oscillator relies on a positive feedback of the output to maintain the
oscillations.
A and  are functions of frequency and thus the equation may be written as
6
EKT 214 ANALOG ELECTRONIC CIRCUIT II
SEM II 2012/2013
SOLUTION TUTORIAL 2 – FEEDBACK AMPLIFIERS and OSCILLATOR

The relaxation oscillator makes use of an RC timing circuit to generate a non-sinusoidal
signal such as square wave.
11.
Explain the design Criteria for oscillators to be sustain
Answer:
For oscillation to be sustained there is two conditions must met
i.
|A | equal to unity or slightly |A|=1.
ii.
Total phase shift, of the loop gain must be 0°
or 360°.
Also this called Barkhaussen criterion.
12.
a) Categorize Wien-Bridge Oscillator. b) draw the schematic circuit and show the combined
voltage divider and a lead-lag circuit. c) Drive to prove that the unity-gain condition in the
feedback loop is met when
.
Answer:
a) Wien-Bridge Oscillator is a low frequency oscillator which ranges from a few kHz to 1 MHz.
b) Schematic of this oscillator is
c)
== >
== >
7
EKT 214 ANALOG ELECTRONIC CIRCUIT II
SEM II 2012/2013
SOLUTION TUTORIAL 2 – FEEDBACK AMPLIFIERS and OSCILLATOR
13.
Determine the resonant frequency for the Wien-bridge oscillator in Figure 12. Also, calculate the
, of the JFET is 500Ω when
setting for Rf assuming the internal drain-source resistance
oscillations are stable..
Answer:
The resonant frequency is:
The closed-loop gain must be 3.0 for oscillations to be sustained
Input resistance is composed of R3 (the source resistor) and
:
Substituting Ri and rearranging to solve for Rf:
14.
Determine the frequency of oscillationfor a phase shift circuit shown in Figure 13. Also, calculate
the value of Rf to operate as an oscillator.
Answer:
The frequency of oscillation is:
The closed-loop gain must be 29 for oscillations to be sustained
8
EKT 214 ANALOG ELECTRONIC CIRCUIT II
SEM II 2012/2013
SOLUTION TUTORIAL 2 – FEEDBACK AMPLIFIERS and OSCILLATOR
15.
Determine the frequency of oscillation of the circuit in Figure 14. Also determine the value 0f R1 to
make the frequency 20 kHz?
Answer:
The frequency of oscillation is:
To make
16.
For a sawtooth voltage-controlled oscillator circuit in Figure 14.
a.
Find the amplitude and the frequency of the sawtooth output, assume that the forward PUT
voltage VF is approximately 1V.
b.
Sketch the output waveform.
Answer:
a)The gate voltage for the PUT to turn on is:
The peak to peak amplitude is:
To determine the frequency
b) The period of the output waveform is determined by:
9