Which oscillator is characterized by a split capacitor in its tank circuit?
RC phase shift oscillator Colpitts oscillator Wien bridge oscillator None of the above
Answer (Detailed Solution Below) Option 2 : Colpitts oscillator
Colpitts oscillator:
The Colpitts oscillator consists of one inductor and one split capacitor in the tank circuit.
A capacitor with a center tap is used in the feedback system of the Colpitts oscillator
It is used for the generation of sinusoidal output signals with very high frequencies
RC phase shift oscillator :
The circuit diagram of the RC phase shift oscillator is shown below:
The frequency produced by the above phase shift oscillator is given by:
\(f = \frac{1}{{2\pi RC\sqrt 6 }}\)
Wein bridge oscillator:
The circuit diagram of the Wein bridge oscillator is shown below:
The frequency of oscillation is given by:
\({\omega _o} = \frac{1}{{RC}}\)
\({f_o} = \frac{{1}}{{2\pi RC}}\)
Type of Oscillator
Approximate frequency range
Crystal oscillator
Fixed frequency
Tickler feedback oscillator
Nearly fixed frequency
Wien bridge oscillator
1 Hz to 1 MHz
Phase-shift oscillator
1 Hz to 10 MHz
Hartley's oscillator
10 kHz to 100 MHz
Colpitt's oscillator
10 kHz to 100 MHz
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In which type of power amplifier does the output current flow for the entire cycle of input signal?
Class C Class AB Class B Class A
Answer (Detailed Solution Below) Option 4 : Class A
The transistor amplifier in which collector current flows for the entire cycle of input AC signal is called class A amplifier.
The transistor amplifier in which collector current flows for the half-cycle of an AC signal is called a class B amplifier.
The transistor amplifier in which collector current flows for less than half the cycle of an AC signal is called a class C amplifier
Power Amplifier
Conduction Angle
Maximum Efficiency
Figure of Merit
Class A
360°
50%
2
Class B
180°
78.5%
0.4
Class AB
180° - 360°
50 – 78.5%
0.4 – 2
Class C
< 180°
≥ 90°
< 0.25
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In an amplifier with the negative feedback, bandwidth is _________ and voltage gain is _________.
Bandwidth is decreased by a factor (1+Aβ) and voltage gain decreases Bandwidth is decreased by factor β and voltage gain remains same Bandwidth is increased by the factor (1+Aβ) and voltage gain is reduced Bandwidth remains the same and voltage gain increases
Answer (Detailed Solution Below) Option 3 : Bandwidth is increased by the factor (1+Aβ) and voltage gain is reduced
The feedback amplification factor is given by \({A_f} = \frac{A}{{1 + A\beta }}\)
where A is open-loop gain and βA is loop gain.
As feedback increases the gain decreases thereby bandwidth increases.
The negative feedback in amplifiers causes
1. reduced the voltage gain and increases the stability in gain
2. increases the bandwidth by the factor (1+Aβ) to maintain constant gain-bandwidth product
3. Reduces the distortion and noise in the amplifier
4. but the signal to noise ratio is not affected
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The crystal oscillator is a constant frequency oscillator due to:
Rigidity Vibrations Low Q High Q
Answer (Detailed Solution Below) Option 4 : High Q
One of the most important features of the crystal oscillator is its frequency stability as it has the ability to provide a constant frequency output under varying load conditions.
The stability of the crystal oscillator is closely related to its quality factor or Q.
High-Q crystal oscillator will oscillate at constant frequency because it produces oscillation only when it is nearer to its resonance frequency.
A typical Q for a crystal oscillator ranges from 104 to 106.
Important Points:
The crystal of crystal oscillator is usually made of the quartz material and provides a high degree of frequency stability and accuracy.
It uses a piezoelectric crystal and when an ac voltage is applied across a crystal it starts vibrating at the frequency of supply voltage this effect is known as piezoelectric effect and the crystal which exhibits this effect is known as piezoelectric crystals.
Conversely, when these crystals are placed under mechanical strain to vibrate, they produce an ac voltage
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What would be the computational value of feedback voltage in a negative feedback amplifier with A = 100, β = 0.03 and input signal voltage = 30 mV?
Answer (Detailed Solution Below) Option 3 : 0.09 V
Concept:
Output voltage signal is given by
V_{0} = A V_{i}
Feedback voltage is given by
V_{f} = β V_{0} = Aβ V_{i}
Calculation:
Given A = 100
β = 0.03
V_{i} = 30 mV = 0.03 V
V
_{f} = 100 × 0.03 × 0.03 = 0.09 V
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A Hartley oscillator is commonly used in ____.
Radio Receivers Radio Transmitters TV Receivers TV Transmitters
Answer (Detailed Solution Below) Option 1 : Radio Receivers
Hartley Oscillator:
In a Hartley oscillator the positive output freed-back is inductively coupled by tank circuit consisting inductor coil with center tap.
The tank circuit of Hartley Oscillator is shown:
Applications
The Hartley oscillator is to produce a sine wave with the desired frequency
Hartley oscillators are mainly used as radio receivers
The Hartley oscillator is Suitable for oscillations in RF (Radio-Frequency) range, up to 30MHZ
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In the phase-shift oscillator, the operating frequency is determined by
Resistance only Capacitance only LC combinations RC combinations
Answer (Detailed Solution Below) Option 4 : RC combinations
In a phase shift oscillator, the frequency-determining elements are R and C
Important:
RC phase shift oscillator as shown in the figure is used to invert the input for 180° phase difference.
The single R-C network gives the 60° phase shift
In RC phase shift oscillator the inverting amplifier produces 180° phase shift.
The RC Phase shift Oscillator: has a fixed frequency and is used at lower frequencies
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A circuit using op-amp shown below has
Voltage series feedback Voltage shunt feedback Current shunt feedback Current series feedback
Answer (Detailed Solution Below) Option 2 : Voltage shunt feedback
Concept-
Identification of Feedback topology-
1. Identify the feedback network/element.
2. If at the output side, feedback is connected to the output of the circuit directly, name it as 'voltage sampling', or else 'current sampling'
3. If at the input side, feedback is connected to the input given to the circuit directly, name it as 'shunt mixing ' or else 'series mixing'
Analysis:
Step-1: feedback element is both R_{1} Resistor
Step – 2: The feedback element is directly connected to output so voltage sampling.
Step – 3: The feedback element is directly connected to input so shunt mixing.
Hence, Voltage-shunt is the right answer.
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Crystal oscillator uses
silicon crystal germanium crystal crystal diode piezo-electric quartz crystal
Answer (Detailed Solution Below) Option 4 : piezo-electric quartz crystal
Crystal oscillator circuit works on the principle of the inverse piezoelectric effect.
The applied electric field will produce a mechanical deformation across some materials.
It utilizes the vibrating crystal’s mechanical resonance, that is made with a piezoelectric material for generating an electrical signal of a particular frequency.
Crystal oscillator uses piezo-electric quartz crystal and it is also known as stable oscillator.
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The oscillator that gives good frequency stability is _____
Harley Oscillator Colpitts Oscillator Crystal oscillator RC phase shift oscillator
Answer (Detailed Solution Below) Option 3 : Crystal oscillator
Crystal Oscillator :
A crystal oscillator is the most stable frequency oscillator.
Advantages :
The crystal oscillator is possible to obtain a very high precise and stable frequency of oscillators
It has very-low-frequency drift due to change in temperature and other parameters
The Q is very high
It has automatic amplitude control
Disadvantages :
These are suitable for high-frequency application
Crystals of low fundamental frequencies are not easily available
Hartley and Colpitts's oscillators are LC oscillators.
LC oscillators are unstable oscillators.
Phase shift oscillator is suitable for oscillations in AF range up to 1 kHz
Crystals like quartz have high-quality factors, Q (range: 104 - 106 ). The high-quality factor will result in high-frequency stability.
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In the given circuit, which type of feedback configuration is used?
Series - Shunt feedback Shunt - Shunt feedback Series - Series feedback Shunt - Series feedback
Answer (Detailed Solution Below) Option 3 : Series - Series feedback
Concept:
There are 4 possible combinations for Voltage and Current with which we can sample at the output and mix the feedback to the input.
Sampling :
On the output side, we will take a sample of output since we want to check the behavior of output and we don't want to disturb the output when we take the sample.
That's why when the voltage is sampled, it is in parallel (as the voltage is the same in parallel ) and current in series (as the current is the same in series ).
Mixing :
In the mixing end, we want to affect the signal that is provided to the amplifier since that is the actual fundament of giving the feedback.
So, the voltage will be in series and the current will be in parallel. So that they can change the input and effect the change in output.
The four basic feedback topologies are as shown:
i) Voltage-sampling voltage- mixing (series-shunt) topology.
ii) current-sampling voltage-mixing (series-series) topology.
iii) current-sampling current-mixing (shunt-series) topology.
iv) voltage-sampling current-mixing (shunt-shunt) topology.
Calculation:
In the given circuit, the feedback element is R_{3}
1 . As the feedback element is not directly connected to the output node,
it indicates current sampling.
2. Also the feedback element is not connected to the input mode, then it indicates series mixing.
Hence the given circuit has current-series feedback or Series - Series feedback.
Hence option (3) is the correct answer.
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Which of the following is correct about the gain (in dB) of a cascade amplifier?
Total gain is a sum of individual gains Total gain is a product of voltage and current gains Total gain is a product of individual gains Total gain is a ratio of individual gains
Answer (Detailed Solution Below) Option 1 : Total gain is a sum of individual gains
In a cascade amplifier, the overall gain is equal to product of gains of individual amplifiers.
In the question, the gain is given in dB, so overall gain will be the sum of individual gains.
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The frequency of oscillation of a tuned-collector oscillator having L = 30 μH and C = 300 pF is nearby
267 kHz 1677 kHz 1.68 kHz 2.67 MHz
Answer (Detailed Solution Below) Option 2 : 1677 kHz
Concept:
The frequency of oscillation of a tuned-collector oscillator is given as:
\(f=\frac{1}{2\pi \sqrt{LC}}\)
Calculation:
Given:
L = 30 μH and C = 300 pF
\(f=\frac{1}{2\pi \sqrt{LC}}\)
\(f=\frac{1}{2\pi \sqrt{30\times10^{-6}\times300\times10^{-12}}}\)
f = 1677kHz
Tuned-collector or Armstrong Oscillator :
It uses inductive feedback from the collector of a transistor to the base.
The LC circuit is in the collector circuit of the transistor.
The following circuit diagram shows the arrangement of a tuned base oscillator circuit:
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A Wein bridge oscillator produces 4 kHz. For identical networks if C = 0.01 microfarads then R = _______.
8 kΩ 4 kΩ 12.5 kΩ 1.58 kΩ
Answer (Detailed Solution Below) Option 2 : 4 kΩ
Concept :
The circuit diagram of the Wein bridge oscillator is shown below:
The phase angle criterion for oscillation is that the total phase shift around the circuit must be 0°.
The total phase shift around circuit 0° occurs when the bridge gets balanced, which is at resonance.
∴ The frequency of oscillation is given by:
\({\omega _o} = \frac{1}{{RC}}\)
\({f_o} = \frac{{1}}{{2\pi RC}}\)
Calculation :
Given:
f0 = 4 kHz
C = 0.1 microFarad
\({f_o} = \frac{{1}}{{2\pi RC}}\)
We can write this as:
\(4 kHz = \frac{{1}}{{2\pi R(0.1\times10^{-6})}}\)
\(R = \frac{{1}}{{2\pi \times 4\times10^{3}\times0.1\times10^{-6}}}\)
R = 4 kΩ
The RC Phase shift Oscillator : has a fixed frequency and used at lower frequencies
The Wein Bridge Oscillator : has variable frequency and used at lower frequencies
The Crystal Oscillator: Stable frequency Oscillator
The Colpitts Oscillator and Hartley Oscillators are used at higher frequencies.
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Another name for a unity gain amplifier is
Difference amplifier Comparator Single ended Voltage follower
Answer (Detailed Solution Below) Option 4 : Voltage follower
In voltage follower amplifiers the voltage gain is unity.
BJT in Common collector configuration can be used as a voltage follower as shown
Op-amp in negative feedback configuration can be used as a voltage follower as shown in figure:
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Which among the following oscillator's output depends on nonlinear characteristics of the circuit?
Relaxation oscillator Colpitts oscillator Wien Bridge oscillator Crystal oscillator
Answer (Detailed Solution Below) Option 1 : Relaxation oscillator
A relaxation oscillator is a nonlinear electronic oscillator circuit that produces a non-sinusoidal repetitive output signal, such as a triangle wave or square wave outputs by discharging of output voltages through the capacitor.
A Colpitts oscillator is used for the generation of sinusoidal output signals with very high frequencies.
A Wien bridge oscillator is a type of electronic oscillator that generates sine waves. It can generate a large range of frequencies.
The crystals generate electric oscillations by nature of the Piezo Electric effect.
Crystal oscillators are known for their frequency stability, hence are used in transmitters as carrier signal generators and local oscillators.
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A current controlled current source (CCCS) has an input impedance of 10 Ω and output impedance of 100 kΩ. When this CCCS is used in a negative feedback closed loop with a loop gain of 9, the closed loop output impedance is
Answer (Detailed Solution Below) Option 4 : 1000 kΩ
Concept :
The current-controlled voltage source means input is current and output is the voltage.
Current controlled voltage source is given as \({V_{out}} = k{I_{in}}\)
\(\therefore K = \frac{{{V_{ous}}}}{{{I_{in}}}} = resistance\)
∴ It acts like a trans-resistance amplifier.
It has high input and high output impedances. CCCS is current-shunt-negative feedback amplifier.
Calculation :
Given that,
Input impedance = 10 Ω
Output impedance = 100 kΩ
loop again (Aβ) = 9
closed loop impedance = Z_{o} [1 + Aβ]
= 100 × 10^{3} [1 + 9]
= 1000 kΩ
Note -
Different feedback configuration:
Feedback
Input
Output
Ri
Ro
Voltage Series
Voltage
Voltage
High
Low
Current Series
Voltage
Current
High
High
Voltage Shunt
Current
Voltage
Low
Low
Current Shunt
Current
Current
Low
High
The current-controlled voltage source means input is current and output is the voltage.
Where
Ri = Input resistance
Ro = Output resistance
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Large Signal Amplifier is also known as
Voltage Amplifier Power Amplifier Current Amplifier Circuit Amplifier
Answer (Detailed Solution Below) Option 2 : Power Amplifier
Voltage amplifier and current amplifier are small signal amplifier.
The term small signal refers to voltage in millivolts and the collector current of milliamperes.
The power handled by small signal amplifiers is in milliwatts
The large Signal amplifier is also called as power amplifiers.
The collector current in power amplifiers is greater than 100 Ma
Power handled by the power amplifiers is in watts to kilowatts
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The negative feedback in an amplifier leads to which one of the following?
Increases in current gain Increases in voltage gain Decrease in voltage gain Decrease in bandwidth
Answer (Detailed Solution Below) Option 3 : Decrease in voltage gain
The feedback amplification factor is given by
\({A_f} = \frac{A}{{1 + A\beta }}\) ,
where A is open-loop gain and βA is the loop gain.
As feedback increases the gain decreases thereby bandwidth increases.
The negative feedback in amplifiers:
1. reduces the gain and increases the stability in gain.
2. increases the bandwidth to maintain the constant gain-bandwidth product.
3. reduces the distortion and noise in the amplifier.
4. does not affect the signal to noise ratio.
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Among the following, which is a fixed frequency oscillator?
Crystal oscillator Phase-shift oscillator Hartley oscillator Colpitt's oscillator
Answer (Detailed Solution Below) Option 1 : Crystal oscillator
Crystal Oscillator:
Crystal oscillators are fixed frequency oscillators with a high Q-factor.
It operates on the principle of the inverse piezoelectric effect in which alternating voltage applied across the crystal surfaces causes it to vibrate at its natural frequency.
It is these vibrations that eventually get converted into oscillations.
These oscillators are made of Quartz crystal Rochelle salt and Tourmaline.
Quartz is inexpensive, naturally available, and mechanically strong when compared to others.
In a crystal oscillator, the crystal is suitably cut and mounted between two metallic plates as shown in the figure.
Type of Oscillator
Approximate frequency range
Crystal oscillator
Fixed frequency
Tickler feedback oscillator
Nearly fixed frequency
Wien bridge oscillator
1 Hz to 1 MHz
Phase-shift oscillator
1 Hz to 10 MHz
Hartley's oscillator
10 kHz to 100 MHz
Colpitt's oscillator
10 kHz to 100 MHz
Hartley Oscillator:
In a Hartley oscillator, the positive output feed-back is inductively coupled by a tank circuit consisting of an inductor coil with a center tap.
The tank circuit of Hartley Oscillator is shown:
Colpitts oscillator:
The Colpitts oscillator consists of two capacitors and one inductor
A capacitor with a center tap is used in the feedback system of the Colpitts oscillator
It is used for the generation of sinusoidal output signals with very high frequencies
Phase Shift Oscillators :
Phase Shift and Wein-bridge oscillators are used to generate audio frequencies , i.e. frequencies in AF Range.
Commonly used phase shift oscillator using three lag circuit is as shown:
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