Analog Circuits: Feedback and Oscillators

Contents

List the drawbacks of Basic Amplifier 1

Describe the Structure of a Feedback Amplifier 1

Classify the Feedback Amplifier 3

Find the Open Loop Gain and Closed Loop Gain of a Feedback Amplifier 4

Recall the Loop Gain in a Negative Feedback Amplifier 5

Discuss the Stability of Gain in case of a Negative Feedback Amplifier 6

List the Merits and Demerits of a Negative Feedback Amplifier 7

Recall the effect of Negative Feedback on the Cut-off Frequencies of an Amplifier 8

Recall the effect of Negative Feedback on Harmonic distortion on an Amplifier 9

Describe the effect of Negative Feedback on Input and Output Impedance of an Amplifier 9

Recall Current Amplifier 10

Recall Voltage Amplifier 11

Compare the Current Amplifier and the Voltage Amplifier 12

Recall the types of Feedback Topologies 14

Describe the Series-Series Feedback connection 17

Find the values of various parameters used in the Series-series Feedback Network 18

Recall the Series-Shunt Feedback Connection 19

Find the following parameters in the Series-Shunt Feedback Network: i. Input Resistance ii. Output Resistance iii. Voltage Gain iv. Feedback Ratio 20

Recall the Shunt-Series Feedback Connection 22

Find the following parameters in the Shunt-Series Feedback Network: i. Input Resistance ii. Output Resistance iii. Voltage Gain iv. Feedback Ratio 23

Recall the Shunt-Shunt Feedback Connection 24

Determine the following parameters in the Shunt-Shunt Feedback Network: i. Input Resistance ii. Output Resistance iii. Voltage Gain iv. Feedback Ratio 25

Identify the Topology of a Feedback Circuit 26

Recall the basic concept of Oscillator 27

Recall the condition for the Oscillation 28

Classify the Oscillator 29

Describe the LC Oscillator and its types 30

Describe the working of following Oscillators: i. Hartley Oscillator ii. Crystal Oscillator iii. Colpitts Oscillator iv. Clapp Oscillator 31

Recall RC Oscillator 33

Describe the following RC Phase-shift Oscillators: i. RC Phase-shift Oscillator using Op-Amp and FET 34

Describe the following RC Phase-shift Oscillators: ii. RC Phase-shift Oscillator using BJT 35

Describe Wein-bridge Oscillator 36

Describe the following Oscillators: Relaxation Oscillator, Tuned-based Oscillator, and blocking Oscillator 37

List the drawbacks of Basic Amplifier

The basic Amplifier, which typically consists of a single amplifying device such as a transistor or vacuum tube, has several drawbacks, including:

  1. Limited gain: The gain of a basic Amplifier is limited by the properties of the amplifying device and the input/output impedance of the circuit. This can make it difficult to achieve the desired level of amplification for some applications.
  2. Nonlinear distortion: Basic Amplifiers can introduce nonlinear distortion, which can alter the shape of the output waveform and introduce harmonic distortion. This can be especially problematic in audio Amplifiers, where the distortion can affect the quality of the sound.
  3. Temperature sensitivity: Basic Amplifiers can be sensitive to changes in temperature, which can affect their performance and stability. This is especially true for vacuum tube Amplifiers, which require a warm-up period before they can operate properly.
  4. Noise: Basic Amplifiers can introduce noise into the Signal due to thermal noise, shot noise, and other sources. This noise can be especially problematic in low-level Signal applications, where it can mask the desired Signal.
  5. Power consumption: Basic Amplifiers can consume a significant amount of power, which can limit their usefulness in battery-powered or low-power applications.
  6. Biasing requirements: Basic Amplifiers often require careful biasing to ensure proper operation and prevent thermal runaway. This can make the circuit design more complex and increase the likelihood of errors or instability.

Describe the Structure of a Feedback Amplifier

A Feedback Amplifier is a type of electronic Amplifier that uses Feedback to control the gain and other performance parameters of the Amplifier. The Feedback loop consists of a Network of components that sense the output Signal and provide a portion of it back to the input of the Amplifier. The Feedback Signal is subtracted from the input Signal, resulting in a reduced error Signal that drives the Amplifier output to the desired value.

The basic structure of a Feedback Amplifier consists of the following components:

  1. Amplifier: The Amplifier is the main component that amplifies the input Signal to produce the output Signal. It can be any type of electronic Amplifier such as an operational Amplifier (op-amp), transistor Amplifier, or vacuum tube Amplifier.
  2. Feedback Network: The Feedback Network consists of a set of components that provide a portion of the output Signal back to the input of the Amplifier. It can be a Voltage divider Network, Resistor-Capacitor (RC) Network, or any other type of Network that provides Feedback.
  3. Feedback element: The Feedback element is the component that senses the output Signal and provides the Feedback Signal to the input of the Amplifier. It can be a Resistor, Capacitor, or any other type of Sensor.
  4. Summing point: The summing point is the point where the input Signal and the Feedback Signal are combined. It can be an op-amp or a simple node where the two Signals are added together.

The Feedback loop can be positive or negative depending on the nature of the Signal provided by the Feedback Network. In a negative Feedback Amplifier, the Feedback Signal is subtracted from the input Signal, resulting in a reduced error Signal that drives the Amplifier output to the desired value. This type of Feedback improves the stability, linearity, and frequency response of the Amplifier. In a positive Feedback Amplifier, the Feedback Signal is added to the input Signal, resulting in an increased output Signal that drives the Amplifier to saturation. This type of Feedback is used in oscillators and other applications where a sustained output Signal is required.

Overall, the Feedback Amplifier provides a powerful tool for controlling the performance parameters of electronic circuits and systems, and is widely used in a variety of applications such as audio Amplifiers, power Amplifiers, control systems, and Signal processing circuits.

Classify the Feedback Amplifier

Feedback Amplifiers can be classified based on several criteria, including the type of Feedback used, the Topology of the Amplifier, and the nature of the input and output Signals. Here are some common classifications:

  1. Based on type of Feedback:
  • Negative Feedback Amplifier: The Feedback Signal is subtracted from the input Signal, resulting in a reduced error Signal that drives the Amplifier output to the desired value. This type of Feedback improves the stability, linearity, and frequency response of the Amplifier.
  • Positive Feedback Amplifier: The Feedback Signal is added to the input Signal, resulting in an increased output Signal that drives the Amplifier to saturation. This type of Feedback is used in oscillators and other applications where a sustained output Signal is required.
  1. Based on Topology:
  • Voltage Amplifier: The Amplifier amplifies a Voltage Signal.
  • Current Amplifier: The Amplifier amplifies a Current Signal.
  • Transconductance Amplifier: The Amplifier amplifies a Voltage Signal and produces a proportional output Current.
  • TransResistance Amplifier: The Amplifier amplifies a Current Signal and produces a proportional output Voltage.
  1. Based on input and output Signals:
  • Inverting Amplifier: The output Signal is inverted with respect to the input Signal.
  • Non-inverting Amplifier: The output Signal is in phase with the input Signal.
  • Differential Amplifier: The Amplifier amplifies the difference between two input Signals.
  • Operational Amplifier (op-amp): A high-gain Voltage Amplifier with a differential input and single-ended output.

In practice, many Feedback Amplifiers combine different types of Feedback, topologies, and input/output configurations to achieve the desired performance characteristics.

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Find the Open Loop Gain and Closed Loop Gain of a Feedback Amplifier

The open-loop gain of a Feedback Amplifier is the gain of the Amplifier without any Feedback applied. It is denoted by Aol and is given by the ratio of the output Voltage to the input Voltage, when no Feedback is present.

The closed-loop gain of a Feedback Amplifier is the gain of the Amplifier with Feedback applied. It is denoted by Acl and is given by the ratio of the output Voltage to the input Voltage, when Feedback is present.

The closed-loop gain is related to the open-loop gain and the Feedback factor β by the following formula:

Acl = Aol / (1 + Aol * β)

where β is the Feedback factor, defined as the fraction of the output Signal that is fed back to the input.

To find the open-loop gain, one can measure the gain of the Amplifier at a certain frequency with no Feedback applied. This can be done by applying a small input Signal and measuring the resulting output Signal. The ratio of the output Voltage to the input Voltage is the open-loop gain.

To find the closed-loop gain, one can measure the gain of the Amplifier with Feedback applied. This can be done by connecting the Feedback Network and applying an input Signal. The output Signal is measured and the ratio of the output Voltage to the input Voltage is the closed-loop gain.

In practice, the closed-loop gain is usually preferred because it is more stable and predictable than the open-loop gain, which can vary with temperature, component variations, and other factors. The closed-loop gain can also be adjusted by changing the Feedback factor, allowing the Amplifier to be customized for different applications.

Recall the Loop Gain in a Negative Feedback Amplifier

In a negative Feedback Amplifier, the loop gain is the gain of the Amplifier with Feedback. It is the ratio of the output Signal to the input Signal in the presence of Feedback.

The loop gain of a negative Feedback Amplifier can be expressed as:

Loop gain = Output Signal / Input Signal

The loop gain of a negative Feedback Amplifier is typically less than the open loop gain of the Amplifier, which is the gain of the Amplifier without Feedback. This is because the Feedback Signal is subtracted from the input Signal before it is amplified, which reduces the overall gain of the Amplifier.

The loop gain of a negative Feedback Amplifier is an important characteristic that determines the stability and performance of the Amplifier. A high loop gain can result in instability and oscillation, while a low loop gain can result in poor performance and a reduced Signal-to-noise ratio.

The loop gain of a negative Feedback Amplifier can be controlled by adjusting the Feedback factor, which is the ratio of the Feedback Signal to the output Signal. By adjusting the Feedback factor, it is possible to optimise the loop gain for a particular application.

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Discuss the Stability of Gain in case of a Negative Feedback Amplifier

In a negative Feedback Amplifier, the stability of the gain depends on the loop gain of the Amplifier. The loop gain is the gain of the Amplifier with Feedback, and it is the ratio of the output Signal to the input Signal in the presence of Feedback.

If the loop gain of a negative Feedback Amplifier is high, the Amplifier is prone to oscillation and instability. This is because a high loop gain can cause the Amplifier to amplify small perturbations in the Signal, resulting in oscillation.

On the other hand, if the loop gain of a negative Feedback Amplifier is low, the Amplifier is stable and the gain is well-controlled. This is because a low loop gain results in a small amplification of perturbations in the Signal, which reduces the risk of oscillation.

The loop gain of a negative Feedback Amplifier can be controlled by adjusting the Feedback factor, which is the ratio of the Feedback Signal to the output Signal. By adjusting the Feedback factor, it is possible to optimize the loop gain for a particular application and ensure the stability of the gain.

In general, negative Feedback Amplifiers are more stable than Amplifiers without Feedback because the Feedback Signal helps to stabilize the Amplifier against changes in load impedance, temperature, and other variables that can affect the Amplifier’s performance.

List the Merits and Demerits of a Negative Feedback Amplifier

Here are some merits of a negative Feedback Amplifier:

1. Improved stability: Negative Feedback Amplifiers are generally more stable than Amplifiers without Feedback because the Feedback Signal helps to stabilize the Amplifier against changes in load impedance, temperature, and other variables that can affect the Amplifier’s performance.

2. Reduced distortion: Negative Feedback Amplifiers tend to have lower distortion than Amplifiers without Feedback because the Feedback Signal helps to cancel out nonlinearities in the Amplifier.

3. Increased bandwidth: Negative Feedback Amplifiers often have a wider bandwidth than Amplifiers without Feedback because the Feedback Signal helps to reduce the phase shift in the Amplifier.

4. Improved Signal-to-noise ratio: Negative Feedback Amplifiers often have a higher Signal-to-noise ratio than Amplifiers without Feedback because the Feedback Signal helps to reduce noise in the Amplifier.

Here are some demerits of a negative Feedback Amplifier:

1. Reduced gain: Negative Feedback Amplifiers typically have a lower gain than Amplifiers without Feedback because the Feedback Signal reduces the overall gain of the Amplifier.

2. Increased complexity: Negative Feedback Amplifiers are typically more complex than Amplifiers without Feedback because they require additional components, such as a Feedback Network, to implement the Feedback loop.

3. Increased cost: Negative Feedback Amplifiers may be more expensive to design and manufacture than Amplifiers without Feedback due to the additional components required.

4. Reduced power efficiency: Negative Feedback Amplifiers may be less power efficient than Amplifiers without Feedback because the Feedback loop consumes additional power.

Overall, the merits and demerits of a negative Feedback Amplifier depend on the specific application and the desired performance characteristics. In some cases, the improved stability, reduced distortion, and increased bandwidth of a negative Feedback Amplifier may outweigh the reduced gain and increased complexity, while in other cases, the simplicity and higher gain of an Amplifier without Feedback may be more desirable.

Recall the effect of Negative Feedback on the Cut-off Frequencies of an Amplifier

In a negative Feedback Amplifier, the negative Feedback Signal is subtracted from the input Signal before it is amplified, which can affect the cut-off frequencies of the Amplifier. The cut-off frequencies of an Amplifier are the frequencies at which the gain of the Amplifier falls to a specified level, such as -3 dB or -20 dB.

Negative Feedback can reduce the gain of an Amplifier at high frequencies, which can increase the high-frequency cut-off frequency of the Amplifier. This is because the negative Feedback Signal is phase-inverted with respect to the input Signal and is subtracted from the input Signal before it is amplified. This can reduce the overall gain of the Amplifier at high frequencies, resulting in a higher high-frequency cut-off frequency.

On the other hand, negative Feedback can increase the gain of an Amplifier at low frequencies, which can decrease the low-frequency cut-off frequency of the Amplifier. This is because the negative Feedback Signal is subtracted from the input Signal before it is amplified, which can reduce the phase shift in the Amplifier at low frequencies. This can result in a higher gain at low frequencies and a lower low-frequency cut-off frequency.

Overall, the effect of negative Feedback on the cut-off frequencies of an Amplifier depends on the characteristics of the Amplifier and the Feedback Network. By adjusting the Feedback factor, which is the ratio of the Feedback Signal to the output Signal, it is possible to optimize the cut-off frequencies of a negative Feedback Amplifier for a particular application.

Recall the effect of Negative Feedback on Harmonic distortion on an Amplifier

In a negative Feedback Amplifier, the negative Feedback Signal is subtracted from the input Signal before it is amplified, which can affect the harmonic distortion of the Amplifier. Harmonic distortion is the presence of harmonics, or frequencies that are integer multiples of the fundamental frequency, in the output Signal of an Amplifier.

Negative Feedback can reduce the harmonic distortion of an Amplifier by canceling out nonlinearities in the Amplifier. This is because the negative Feedback Signal is phase-inverted with respect to the input Signal and is subtracted from the input Signal before it is amplified. The negative Feedback Signal is proportional to the output Signal, which contains the harmonic components generated by the Amplifier. By subtracting the negative Feedback Signal from the input Signal, the harmonic components are canceled out, resulting in a lower level of harmonic distortion in the output Signal.

Overall, the effect of negative Feedback on the harmonic distortion of an Amplifier depends on the characteristics of the Amplifier and the Feedback Network. By adjusting the Feedback factor, which is the ratio of the Feedback Signal to the output Signal, it is possible to optimize the harmonic distortion of a negative Feedback Amplifier for a particular application. Negative Feedback Amplifiers tend to have lower harmonic distortion than Amplifiers without Feedback because the Feedback Signal helps to cancel out nonlinearities in the Amplifier.

Describe the effect of Negative Feedback on Input and Output Impedance of an Amplifier

In a negative Feedback Amplifier, the negative Feedback Signal is subtracted from the input Signal before it is amplified, which can affect the input and output impedances of the Amplifier. The input impedance of an Amplifier is the impedance of the Amplifier at the input terminal, and the output impedance of an Amplifier is the impedance of the Amplifier at the output terminal.

Negative Feedback can increase the input impedance of an Amplifier because the Feedback Network presents a high impedance to the input Signal. This can make the Amplifier more sensitive to the input Signal and reduce the loading effect of the Amplifier on the source.

On the other hand, negative Feedback can decrease the output impedance of an Amplifier because the Feedback Network presents a low impedance to the output Signal. This can reduce the loading effect of the Amplifier on the load and improve the power transfer between the Amplifier and the load.

Overall, the effect of negative Feedback on the input and output impedances of an Amplifier depends on the characteristics of the Amplifier and the Feedback Network. By adjusting the Feedback factor, which is the ratio of the Feedback Signal to the output Signal, it is possible to optimize the input and output impedances of a negative Feedback Amplifier for a particular application.

Recall Current Amplifier

A Current Amplifier is a type of electronic Amplifier that amplifies the Current of an input Signal rather than the Voltage. Current Amplifiers are used in a variety of applications, including instrumentation, medical devices, and telecommunications.

There are several types of Current Amplifiers, including operational Amplifiers, transconductance Amplifiers, and Current mirrors. Operational Amplifiers are widely used as Current Amplifiers because of their high gain and versatility. Transconductance Amplifiers are specialized Current Amplifiers that are used to amplify small Signals in high-precision applications. Current mirrors are simple circuits that are used to replicate the Current of an input Signal.

Current Amplifiers are typically used to amplify low-level Signals, such as those produced by Sensors or Transducers. They are often used in conjunction with other electronic components, such as Resistors, Capacitors, and Inductors, to form circuits that perform a specific function.

The performance of a Current Amplifier is typically characterized by its gain, bandwidth, and linearity. The gain of a Current Amplifier is the ratio of the output Current to the input Current, and it is typically expressed in amperes per ampere (A/A). The bandwidth of a Current Amplifier is the range of frequencies over which the Amplifier can operate, and it is typically expressed in Hertz (Hz). The linearity of a Current Amplifier is a measure of how closely the Amplifier follows a linear transfer function, and it is typically expressed as a percentage of deviation from linearity.

Recall Voltage Amplifier

A Voltage Amplifier is a type of electronic Amplifier that amplifies the Voltage of an input Signal rather than the Current. Voltage Amplifiers are used in a variety of applications, including audio systems, power supplies, and communication systems.

There are several types of Voltage Amplifiers, including operational Amplifiers, transconductance Amplifiers, and Voltage regulators. Operational Amplifiers are widely used as Voltage Amplifiers because of their high gain and versatility. Transconductance Amplifiers are specialized Voltage Amplifiers that are used to amplify small Signals in high-precision applications. Voltage regulators are circuits that are used to maintain a constant output Voltage despite changes in the input Voltage or load conditions.

Voltage Amplifiers are typically used to amplify low-level Signals, such as those produced by Sensors or Transducers. They are often used in conjunction with other electronic components, such as Resistors, Capacitors, and Inductors, to form circuits that perform a specific function.

The performance of a Voltage Amplifier is typically characterized by its gain, bandwidth, and linearity. The gain of a Voltage Amplifier is the ratio of the output Voltage to the input Voltage, and it is typically expressed in volts per volt (V/V). The bandwidth of a Voltage Amplifier is the range of frequencies over which the Amplifier can operate, and it is typically expressed in Hertz (Hz). The linearity of a Voltage Amplifier is a measure of how closely the Amplifier follows a linear transfer function, and it is typically expressed as a percentage of deviation from linearity.

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Compare the Current Amplifier and the Voltage Amplifier

Current Amplifiers and Voltage Amplifiers are two types of electronic Amplifiers that are used to amplify Signals in different ways. Here are some key differences between Current Amplifiers and Voltage Amplifiers:

The main difference between a Voltage Amplifier and a Current Amplifier is, Voltage Amplifier amplifies the Voltage whereas the Current Amplifier amplifies the Current. In other words, we can say the Voltage Amplifier provides higher Voltage gain whereas the Current Amplifier provides higher Current gain.

You can better understand the difference if you have a clear knowledge of Amplifier gain. The gain is basically the ratio of output and input. So the Voltage gain is the ratio of output Voltage and input Voltage. Similarly, the Current gain is the ratio of output Current and input Current.
So, in a Current Amplifier, the high Current gain implies that a small change in the input Current will cause a large change in the output Current. Also, a large amount of Current will flow in the output circuit when a small amount of Current flows in the input circuit.

Similarly, in a Voltage Amplifier, the high Voltage gain implies that a small change in the input Voltage will cause a large change in the output Voltage. Also, a large amount of Voltage will produce in the output when a small amount of Voltage is applied to the input. Voltage Amplifiers are mostly made up of Voltage control semiconductor devices such as Field Effect Transistors or FETs whereas Current Amplifiers are mostly made up of Current-controlled semiconductor devices such as Bipolar Junction Transistors or BJTs.
Other key differences are,

1. Voltage Amplifiers have very high impedance so they circulate low Current. Generally, the Voltage Amplifiers provide high Voltage gain and unity Current gain. On the other hand, Current Amplifiers have low impedance so they circulate high Current. So, the Current Amplifiers provide high Current gain and unity Voltage gain.
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2. Voltage Amplifiers produce low power loss due to their high impedance and low Current flow, on the other hand, Current Amplifiers produce high power loss due to high Current flow.

3. Voltage Amplifiers are most suitable for audio Signal amplification and driving loudspeakers whereas Current Amplifiers are most suitable for Signal processing, pre-amplification, increase the power of Transducer, etc.
4. Generally, the output and input Voltage are almost equal for a Current Amplifier but input and output Current differs. On the other hand, input and output Current is almost equal for a Voltage Amplifier but input and output Voltage differs.

5. Due to the high impedance, Capacitance effect or miller effect occurs in Voltage Amplifiers but in Current Amplifiers, Capacitance effect does not happen more.

6. Current Amplifiers are very sensitive to changes in input Current whereas Voltage Amplifiers are very sensitive to changes in Voltage.

7. Leakage Current mostly occurs in Voltage Amplifier circuits due to Capacitance effects but in Current Amplifier leakage Current happens in a very smaller amount than the normal Current.

8. When Voltage Amplifiers are used with very high-frequency Signals, noise or unwanted Signals occur but in the case of a Current Amplifier this happens in a very small amount.

Recall the types of Feedback Topologies

There are several types of Feedback topologies that are used in electronic Amplifiers to control the gain and other performance characteristics of the Amplifier. Here are some common types of Feedback topologies:

1. Voltage Feedback: In a Voltage Feedback Amplifier, the Feedback Signal is taken from the output of the Amplifier and is compared to the input Signal. The difference between the input and output Signals is used to control the gain of the Amplifier.

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2. Current Feedback: In a Current Feedback Amplifier, the Feedback Signal is taken from the output of the Amplifier and is compared to the input Signal. The difference between the input and output Signals is used to control the gain of the Amplifier.