Test: Types of Oscillator - 1 - UPSC MCQ

Crystal Oscillator:

• 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 crystal oscillator, the crystal is suitably cut and mounted between two metallic plates as shown in the figure.

Rs → Low value

Ls → Large value

Cs → small value

Cp = Capacitance of electrode

• In reality, the crystal behaves like a series RLC circuit
• The Inductor L, capacitor C, and Resistor R are the analogous of mass, the compliance (reciprocal of spring constant), and the viscous damping factor of the mechanical system.
• Due to the presence of Cp, the crystal will resonate at two different frequencies.

(1) Series resonant frequency - 

(2) Parallel resonant frequency - 

Concept of Buffer:

• A Buffer is an op-amp circuit whose output voltage straight away follows the input voltage. i.e. output voltage equivalent to the input voltage.
• The Op-amp circuit does not provide any amplification thus, its voltage gain is unity.
• The voltage follower is used as a buffer amplifier, isolation amplifier, unity gain amplifier as the output follows the input.
• The voltage follower provides no alternation or no amplification but only buffering.

Characteristics:

• High input impedance and Low output impedance hence low loading effect and used in Electronics measuring instruments
• Current Gain & power gain high
• Voltage gain unity

Operational Amplifier (OP-AMP):

• OP-AMP is a two terminal device whose output is either +V_s or -V_s.
• + terminal is non-inverting terminal and - terminal is inverting terminal.
• When the voltage at inverting terminal is greater than the non-inverting terminal, the output is -V_s.
• When the voltage at non-inverting terminal is greater than the inverting terminal, the output is +V_s.

​Characteristics of an ideal OP-AMP:

• Infinite input resistance
• Zero output resistance
• Infinite voltage gain
• Infinite slew rate
• Zero off-set voltage
• Infinite CMRR

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

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

Concept:

For sinusoidal oscillations to begin, the voltage gain of the Wien Bridge circuit must be equal to or greater than 3, i.e.

Av ≥ 3.

For a non-inverting op-amp configuration, this value is set by the feedback resistor network and is given as:

 = 3 or more

Analysis:

For the given circuit, the voltage gain (non-inverting) will be:

Since the voltage gain is less than 3, sinusoidal oscillation cannot start.

OP-AMP

Operational amplifiers are linear devices that have all the properties required for nearly ideal DC amplification and are therefore used extensively in signal conditioning, filtering, or to perform mathematical operations such as add, subtract, integration and differentiation.

• It is a high voltage gain direct coupled amplifier.
• It can be used to perform mathematical operations on analog signals, hence it is called an operational amplifier.
• Op-amp is available as IC 741.
• The open-loop voltage gain of an ideal Op-amp is infinite.
• Op-amp consists of two terminals inverting(-ve) and non-inverting(+ve).

Maximum gain is limited to the open-loop gain of the op-amp but, for a linear amplifier, we need to add resistive elements in feedback known as inverting operational amplifiers.

At a frequency that is somewhat less than 10 Hz, the gain is flat down to DC and very high (10⁵).

The typical open-loop gain of an op-amp can be drawn as:

Hence the gain of an operational amplifier will be maximum at 1 Hz

So option (1) is the correct answer.

In a crystal oscillator, the frequency(f) of the oscillator is inversely proportional to the thickness(t) of the crystal,

%change in the frequency is,

So, f will increase by 1%

Concept:

LC Oscillator:

• It is a radio frequency oscillator.
• It consists of a Common Emitter amplifier which a phase shift of 180⁰.
• LC feedback network of three impedances together provides an additional phase shift of 180⁰ at F₀. So the total phase shift becomes 360⁰ and oscillations occur at F0.

​Crystal Oscillator:

• It is a fixed-frequency oscillator.
• The frequency of oscillations is highly stable.
• It is an LC oscillator in which a quartz crystal is used in place of an inductor.

RC phase shift Oscillator:

• It consists of a Common Emitter amplifier which a phase shift of 1800.
• It also consists of a feedback network having three high pass RC circuits which also provide an additional phase shift of 180⁰ at F_0.
• The total phase becomes 360⁰ and oscillations occur at F₀.

From the above-mentioned points, we can say that Crystal Oscillator is the most stable oscillator and have a higher quality factor than the LC oscillator.

Hence option (B) is true and option (A) is false.

In phase shift oscillator,

• The total phase shift becomes 360⁰ and oscillations occur at F₀ only,
• Hence Phase-shift oscillators don't have the widest range of frequency and therefore used in Audio Range frequencies.

So option (C) is also wrong.

Wien-Bridge Oscillators:

• They are low-frequency oscillators that are used to generate audio and sub-audio frequencies ranging between 20 Hz to 20 kHz. 
• They provide stabilized, low distorted sinusoidal output over a wide range of frequencies which can be selected using decade resistance boxes.

Hence option (D) is also wrong.