Analog Circuits: Tuned Amplifiers

Contents

**List the applications, advantages, and disadvantages of Tuned Amplifiers** 3

**Describe the concept of Resonance in Tuned Amplifiers** 4

**Derive an expression for Series Resonant Frequency** 5

**Determine the relationship between Q-factor, Bandwidth, and Resonant Frequency** 6

**Derive an expression for Parallel Resonant Frequency** 7

**Determine the relationship between Q-factor, Bandwidth, and Resonant Frequency** 9

**Describe Single Tuned Amplifier with circuit diagram** 9

**Determine Voltage gain & Frequency response of Single Tuned Amplifiers** 10

**List advantage, disadvantage and applications of Single Tuned Amplifiers** 12

**Describe Double Tuned Amplifier with circuit diagram** 13

**Determine Voltage gain and Frequency response of Double Tuned Amplifier** 14

**List advantage, disadvantage and applications of Double Tuned Amplifiers** 16

**Describe Staggered-tuned Amplifier with the help of Circuit diagram** 17

**Determine Voltage-gain & Frequency response of Staggered-tuned Amplifier** 18

**List advantage, disadvantage and applications of Staggered-tuned Amplifiers** 19

**Describe the term Neutralisation in Tuned Amplifiers** 20

**Describe Tuned Amplifiers**

Tuned amplifiers are a type of electronic amplifier that utilize a resonant circuit, also known as a tank circuit, to amplify a specific frequency range. The resonant circuit consists of an inductor and a capacitor, which resonate at a specific frequency called the resonant frequency. Tuned amplifiers are used in radio frequency (RF) communication systems, such as radios and televisions, to amplify the signal in a particular frequency band.

There are two main types of tuned amplifiers: parallel-tuned and series-tuned. In a parallel-tuned amplifier, the resonant circuit is connected in parallel with the amplifier’s input and output, while in a series-tuned amplifier, the resonant circuit is connected in series with the amplifier’s input and output. The choice of configuration depends on the application and the desired performance characteristics.

The performance of a tuned amplifier is characterized by its Q-factor, which is a measure of the selectivity of the resonant circuit. A high Q-factor indicates that the amplifier is highly selective and amplifies a narrow frequency range, while a low Q-factor indicates that the amplifier is less selective and amplifies a wider frequency range. The Q-factor of a tuned amplifier can be adjusted by varying the values of the inductor and capacitor in the resonant circuit.

Tuned amplifiers are widely used in RF communication systems due to their ability to provide high selectivity and gain in a specific frequency range. However, they are also susceptible to frequency drift and require careful tuning to maintain their performance.

**List the applications, advantages, and disadvantages of Tuned Amplifiers**

Applications of Tuned Amplifiers:

- In radio receivers for selective amplification of a particular frequency band.
- In audio systems to amplify a specific frequency range.
- In electronic oscillators to provide feedback for sustaining oscillations.
- In signal processing circuits for filtering and frequency discrimination.

Advantages of Tuned Amplifiers:

- Selective amplification of a particular frequency band without affecting other frequencies.
- High gain at the center frequency of the tuned circuit.
- Low noise and distortion.

Disadvantages of Tuned Amplifiers:

- Narrow bandwidth, which limits the range of frequencies that can be amplified.
- Sensitivity to temperature changes, component variations, and external interference, which can affect the resonant frequency of the tuned circuit.
- Limited dynamic range, which can cause distortion at high input levels.

**Describe the concept of Resonance in Tuned Amplifiers**

Resonance is a phenomenon in which an electrical circuit or mechanical system vibrates at its natural frequency in response to an external stimulus. In the case of tuned amplifiers, resonance occurs when the reactive elements in the tuned circuit (inductor and capacitor) are adjusted to a specific frequency, known as the resonant frequency, at which the circuit exhibits maximum impedance.

When an AC signal is applied to the input of a tuned amplifier, the voltage across the tuned circuit depends on the frequency of the input signal. At the resonant frequency, the voltage across the tuned circuit is maximum, and the impedance of the circuit is minimum. This allows the maximum amount of signal power to be transferred to the next stage of the amplifier, resulting in maximum amplification of the desired frequency.

The bandwidth of a tuned amplifier is defined as the range of frequencies around the resonant frequency at which the output power is at least half of the maximum power. The Q factor, or quality factor, of the tuned circuit is a measure of its selectivity and is defined as the ratio of the resonant frequency to the bandwidth. Higher Q circuits have a narrower bandwidth and better selectivity.

Overall, the concept of resonance is critical to the functioning of tuned amplifiers, as it allows for selective amplification of specific frequencies while rejecting unwanted frequencies.

**Derive an expression for Series Resonant Frequency**

To derive an expression for the series resonant frequency, let’s consider an RLC series circuit consisting of a resistor (R), inductor (L), and capacitor (C) connected in series. The resonant frequency is the frequency at which the circuit exhibits maximum impedance or minimum current.

The impedance of the RLC series circuit is given by the following equation:

Z = R + j(XL – XC)

Where:

Z is the impedance of the circuit,

R is the resistance,

XL is the inductive reactance,

XC is the capacitive reactance, and

j is the imaginary unit (âˆš(-1)).

At the resonant frequency, the inductive reactance (XL) and the capacitive reactance (XC) are equal in magnitude but opposite in sign. Mathematically, XL = -XC.

Substituting this condition into the impedance equation, we have:

Z = R + j(XL – XC)

Z = R + j(0)

Z = R

At the resonant frequency, the impedance of the circuit is purely resistive, and there is no reactive component. Therefore, the series resonant frequency can be determined by equating the inductive and capacitive reactances and solving for the frequency.

XL = XC

Ï‰L = 1/Ï‰C

Where:

Ï‰ is the angular frequency in radians per second,

L is the inductance in Henry (H), and

C is the capacitance in Farad (F).

Simplifying the equation, we have:

Ï‰ = 1/âˆš(LC)

The series resonant frequency (fr) is given by:

fr = Ï‰ / (2Ï€)

Substituting the expression for Ï‰, we get:

fr = (1/âˆš(LC)) / (2Ï€)

Simplifying further, we can express the series resonant frequency as:

fr = 1 / (2Ï€âˆš(LC))

This is the expression for the series resonant frequency of an RLC series circuit. It represents the frequency at which the inductive and capacitive reactances cancel each other out, resulting in maximum impedance or minimum current in the circuit.

**Determine the relationship between Q-factor, Bandwidth, and Resonant Frequency**

The relationship between Q-factor, bandwidth, and resonant frequency in a tuned circuit can be expressed mathematically as follows:

Q = f_{resonant} / bandwidth

where Q is the quality factor, f_{resonant} is the resonant frequency, and bandwidth is the width of the frequency range over which the circuit resonates.

Alternatively, we can rearrange the equation to solve for one of the other variables:

f_{resonant} = Q x bandwidth

or

bandwidth = f_{resonant} / Q

This equation tells us that the higher the Q-factor of the tuned circuit, the narrower the bandwidth of the circuit. This means that the circuit will resonate at a very specific frequency, and will not respond as strongly to frequencies outside of this narrow range.

Conversely, if we want a wider bandwidth, we can either decrease the Q-factor or increase the resonant frequency. However, doing so will also decrease the selectivity of the circuit, which means it will not be able to filter out unwanted frequencies as effectively.

**Derive an expression for Parallel Resonant Frequency**

To derive an expression for the parallel resonant frequency, let’s consider the following parallel RLC circuit:

Where:

- R1 and R2 are the resistors in parallel.
- L is the inductor.
- C is the capacitor.

The total impedance of the circuit is:

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Z = R1 || R2 + jÏ‰L + 1/(jÏ‰C)

Where “||” represents the parallel combination.

We want to find the frequency at which the circuit is resonant, i.e., the frequency at which the impedance is purely resistive. This happens when the imaginary part of the impedance is zero, so:

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jÏ‰L + 1/(jÏ‰C) = 0

Multiplying both sides by jÏ‰C:

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jÏ‰L jÏ‰C + 1 = 0

Solving for Ï‰:

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jÏ‰L jÏ‰C = -1 Ï‰^{2} = 1/(LC)

Therefore, the parallel resonant frequency is:

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f_{res} = 1 / (2Ï€ âˆš(LC))

where f_{res} is the frequency of resonance, L is the inductance, and C is the capacitance.

**Determine the relationship between Q-factor, Bandwidth, and Resonant Frequency**

The relationship between Q-factor, bandwidth, and resonant frequency for a parallel resonant circuit is given as:

Q-factor = Resonant frequency / Bandwidth

Bandwidth = Resonant frequency / Q-factor

Resonant frequency = 1 / [2 * pi * sqrt(L * C)]

where L is the inductance of the circuit, C is the capacitance of the circuit.

From these equations, we can see that the Q-factor and resonant frequency are inversely proportional to the bandwidth. This means that a circuit with a high Q-factor will have a narrow bandwidth and a high resonant frequency, while a circuit with a low Q-factor will have a wider bandwidth and a lower resonant frequency.

**Describe Single Tuned Amplifier with circuit diagram**

A single tuned amplifier is a type of tuned amplifier that consists of an LC circuit tuned to the desired frequency of the input signal, and an amplifier stage. The circuit diagram of a single tuned amplifier is shown below:

The input signal is applied to the inductor L, which is in series with the capacitor C. The LC circuit acts as a filter that passes the desired frequency while rejecting other frequencies. The resistor R is the load of the amplifier and the output signal is taken across it. The amplifier stage is placed between the LC circuit and the load resistor R.

The performance of a single tuned amplifier can be improved by using negative feedback. A portion of the output signal is fed back to the input of the amplifier with opposite polarity to the input signal. This reduces distortion and improves the amplifier’s frequency response.

**Determine Voltage gain & Frequency response of Single Tuned Amplifiers**

To determine the voltage gain and frequency response of a single tuned amplifier, we need to analyze the circuit and derive the relevant equations.

The circuit diagram of a single tuned amplifier is as follows:

Where:

The output can be obtained from the coupling capacitor CC as shown above or from a secondary winding placed at L.

A single-tuned amplifier is a type of tuned amplifier circuit that uses a single-tuned circuit for the input and output stages. The input stage of the amplifier is tuned to the frequency of the input signal, while the output stage is tuned to the frequency of the output signal. The tuning circuit consists of a resonant circuit, typically an LC circuit.

The voltage gain and frequency response of a single-tuned amplifier depend on the values of the components in the tuning circuit, as well as the load resistance and the characteristics of the amplifier.

To determine the voltage gain of a single-tuned amplifier, we can use the following formula:

Av = -RL / R1 * sqrt(L / C) * (1 / (R1 + R2 + RL + Rf))

where Av is the voltage gain, RL is the load resistance, R1 is the resistance of the input stage, R2 is the resistance of the output stage, Rf is the feedback resistance, L is the inductance of the tuning circuit, and C is the capacitance of the tuning circuit.

To determine the frequency response of a single-tuned amplifier, we can plot the gain versus frequency curve using the following formula:

Av = -RL / R1 * sqrt(L / C) * (1 / sqrt((1 – (f / fc)^2)^2 + (BW / fc)^2))

where f is the frequency, fc is the resonant frequency of the tuning circuit, and BW is the bandwidth of the amplifier.

The plot of the gain versus frequency curve typically shows a peak at the resonant frequency, which is the frequency at which the gain is maximum. The bandwidth of the amplifier is the range of frequencies over which the gain is greater than or equal to 70.7% of the maximum gain.

In practical circuits, the frequency response of a single-tuned amplifier may be affected by factors such as component tolerances, parasitic capacitances and inductances, and the non-ideal characteristics of the amplifier. Therefore, it is important to carefully design and test the circuit to ensure that it meets the desired performance specifications.

**List advantage, disadvantage and applications of Single Tuned Amplifiers**

Single tuned amplifiers are commonly used in various applications, such as radio receivers, communication systems, and audio amplifiers. They have some advantages and disadvantages, which include:

Advantages:

- Single tuned amplifiers are relatively simple in design, making them cost-effective and easy to implement.
- They can provide a high level of selectivity and sensitivity, allowing for efficient filtering of specific frequencies.
- Single tuned amplifiers have a high Q-factor, which provides a narrow bandwidth and improves their frequency response.

Disadvantages:

- Single tuned amplifiers are sensitive to component tolerances and can be affected by temperature changes, making them less reliable.
- They have a limited frequency range due to their narrow bandwidth, which can be a drawback in some applications.
- Single tuned amplifiers can be susceptible to noise and interference, affecting their overall performance.

Applications:

- Single tuned amplifiers are commonly used in radio frequency (RF) systems, such as radio receivers and transmitters.
- They are used in audio amplifiers, especially in high-fidelity systems, to provide efficient filtering and amplification of specific frequencies.
- Single tuned amplifiers are used in communication systems, such as telecommunication and satellite systems, to filter and amplify specific signals.

**Describe Double Tuned Amplifier with circuit diagram**

A double tuned amplifier is a type of tuned amplifier that uses two tuned circuits to achieve a higher level of selectivity and gain than a single tuned amplifier. The circuit diagram of a double tuned amplifier consists of two tuned circuits, each with a capacitor and an inductor in series. The two tuned circuits are connected in parallel, with the input signal being applied across one tuned circuit, and the output signal being taken from the other tuned circuit.

The double tuned amplifier is commonly used in radio frequency (RF) applications such as high frequency amplifiers, radio transmitters, and receivers. It provides a high degree of selectivity, allowing only a narrow range of frequencies to pass through the circuit, while attenuating unwanted frequencies. The double tuned amplifier also provides a high level of gain, making it suitable for use in high frequency circuits where low signal levels are present.

One advantage of the double tuned amplifier is that it provides a higher level of selectivity than a single tuned amplifier. This means that it is able to filter out unwanted frequencies more effectively, leading to a clearer output signal. However, this increased selectivity can also be a disadvantage in some applications, where a wider range of frequencies needs to be passed through the circuit.

Another advantage of the double tuned amplifier is its high gain, which makes it suitable for use in low signal level applications. However, this high gain can also lead to instability and oscillation in the circuit, particularly at high frequencies.

In summary, the double tuned amplifier is a type of tuned amplifier that uses two tuned circuits to provide a high level of selectivity and gain. It is commonly used in high frequency RF applications, and provides advantages such as high selectivity and gain, but can also have disadvantages such as instability and oscillation at high frequencies.

**Determine Voltage gain and Frequency response of Double Tuned Amplifier**

In a double tuned amplifier, two resonant circuits are used to improve the selectivity of the amplifier. The circuit diagram of a double tuned amplifier is shown below: