Feedback Amplifier And Transistor Oscillator

Oscillator in a feedback amplifier and transistor oscillator provides an amplified output signal without taking any input signals. An oscillator's operation is a repeating process involving amplified input and output, which results in feedback and persistent operations. This ensures that information signals are transmitted back and forth in an electrical device without any gaps. The external signal supplies an alternating current that is self-sustaining, implying a single input lead with infinite outputs depending on feedback and frequency regulation. Feedback is the phenomenon of feeding a portion of the output signal back to the input circuit. The effect causes a dependency between the output and the input, allowing for effective control of the circuit's operation.

Key Terms: Feedback Amplifier, Transistor Oscillator, Negative Feedback, Positive Feedback, Transistor, Feedback, Oscillator, Types of Oscillator, Circuit

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What is Feedback Amplifier? 

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Feedback can be defined as the process of feeding a portion of the output signal back to the input circuit. The effect results in dependence between the output and the input and effective control can be obtained in the working of the circuit.

Feedback Amplifiers have been classified mainly into two types:

1. Negative Feedback
2. Positive Feedback

Feedback Network

Take a look at the circuit in the diagram above. Apart from the input signal, we can see that a portion of the output signal is returned to the input line. The feedback to the system is delivered in the form of a liquid that is either in-phase (positive feedback) or out-of-phase (negative feedback) with the initial input signal.

Positive or Regenerate Feedback

1. In positive feedback, the feedback energy (voltage or currents) is in phase with the input signal, assisting it.
2. Positive feedback enhances the amplifier's gain but also increases distortion, noise, and instability; as a result, positive feedback is rarely used in amplifiers.
3. Oscillators, on the other hand, use positive feedback.

Negative or Degenerate Feedback

1. The feedback energy (voltage or current) is out of phase with the input signal and acts as an antagonist in Negative Feedback.
2. Negative feedback lowers the amplifier's gain.
3. Negative feedback enhances bandwidth and improves input and output impedances, reducing distortion, noise, and instability.
4. Negative feedback is commonly employed in amplifiers because of these benefits.

Comparison between Positive and Negative Feedback

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Transistor: Structure and Function

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Three doped areas in a transistor form two p-n junction transistors between them. As illustrated in Fig A, there are two types of transistors:

• Transistor with n-p-n junction: A p-type semiconductor segment (base) separates two n-type semiconductor segments (emitter and collector).
• Transistor with a p-n-p junction: A n-type semiconductor (termed as the base) separates two p-type semiconductor segments (known as the emitter and collector).

Figure A shows schematic representations of an n-p-n and a p-n-p setup.

NPN and PNP Transistor

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Feedback Amplifier and Transistor Oscillator

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Now we know that sinusoidal input is delivered to an amplifier, which results in an amplified signal at the output. This means that external input is required to maintain an ac signal in the amplifier's output. We get ac output from an oscillator without any external input signal. In other words, an oscillator's output is self-sustaining. An amplifier is used to do this.

As seen in Figure A, a portion of the output power is returned to the input in phase with the starting power (this is known as positive feedback). Inductive coupling (via mutual inductance) or LC or RC networks can be used to provide feedback. Apart from the resonant circuit for obtaining oscillation at a specific frequency, other types of oscillators use different techniques of linking the output to the input (feedback network).

Feedback Amplifier and Oscillator

Consider the circuit depicted in Figure B in which feedback is provided via inductive coupling from one coil winding (T1) to another coil winding (T2). Because the coils T2 and T1 are coiled on the same core, their mutual inductance allows them to be inductively connected. The base-emitter junction is forward biased, similar to an amplifier, while the base-collector junction is reverse biased. For the sake of simplicity, the actual biassing circuits have been omitted.

Mutual Inductance

Current wave

Current wave not reaching maximum

Let's look at how oscillations are constructed.

• Assume switch S1 is turned on for the first time to apply suitable bias.
• In the transistor, a surge of collector current is evident.
• This current passes via coil T2, which has terminals 3 and 4 [Figure B].
• As seen in Figure B (1) this current does not reach maximum amplitude instantly but instead builds from X to Y. A current flow in the emitter circuit as a result of the inductive coupling between coils T2 and T1 (notice that this is the 'feedback' from input to output).
• This current (in T1; emitter current) likewise grows from X' to Y' as a result of the positive feedback [Figure B (2)].
• When the transistor becomes saturated, the current in T2 (collector current) linked in the collector circuit takes on the value Y.
• This indicates that the collector current has reached its maximum capacity and cannot be increased any further.
• The magnetic field around T2 stops growing since the collector current does not vary.
• There will be no further feedback from T2 to T1 once the field becomes static.
• The emitter current begins to decline if feedback is not maintained.
• As a result, collector current reduces as you move from Y to Z [Figure B (1)].
• A drop in collector current, on the other hand, causes the magnetic field around coil T2 to decay.
• Therefore, T1 now sees a fading field in T2 (in contrast to what it saw while the field was rising during the first start operation).
• The emitter current drops furthermore until it approaches Z′ when the transistor is turned off.
• This signifies that both I E and I C are no longer flowing.
• As a result, the transistor has returned to its initial condition (when the power was first switched on).
• The entire procedure is now repeated.
• To put it another way, the transistor is driven to saturation, then cut-off, and finally back to saturation.
• The constants of the tank circuit or tuned circuit (inductance L of coil T2 and Clinked in parallel to it) govern the time it takes to transition from saturation to cut-off and return.
• The frequency at which the oscillator will oscillate is determined by the resonance frequency (v) of this tuned circuit.

v = 12π√LC

The tank or tuned circuit is linked at the collector side of the circuit shown in Fig B. Thus, it's called a tuned collector oscillator. It's called a tuned base oscillator if the tuned circuit is on the base side. Other types of tank circuits (such as RC) or feedback circuits produce oscillators such as Colpitt's oscillator, Hartley oscillator, and RC-oscillator.

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Types of Oscillator

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There are many different types of oscillators, however, they can be divided into two groups: Harmonic Oscillators (sometimes called Linear Oscillators) and Relaxation Oscillators.

• The energy transfer in a harmonic oscillator is always from active to passive components, and the frequency of oscillations is determined by the feedback path.
• The energy is exchanged between the active and passive components in a relaxation oscillator, and the frequency of oscillations is governed by the charging and discharging time constants involved in the process.
• Harmonic oscillators create low-distortion sine-wave outputs, and on the other hand relaxation oscillators produce non-sinusoidal (sawtooth, triangular, or square) waveforms.

The main types of Oscillators include:

1. RC Oscillators
2. Wien Bridge Oscillator
3. RC Phase Shift Oscillator
4. LC Oscillators
5. Hartley Oscillator
6. Colpitts Oscillator
7. Clapp Oscillator
8. Crystal Oscillators

Wien Bridge Oscillators

• Wien-Bridge Oscillator is a phase-shift oscillator based on a Wien-Bridge network with four arms connected in a bridge configuration.
• Two of the arms are completely resistive, while the other two are made up of resistors and capacitors.
• One arm includes a series resistor and capacitor (R1 and C1), while the other has a parallel resistor and capacitor (R2 and C2).
• This means that the network's two arms function exactly like a high pass or low pass filter, simulating the behaviour of the circuit.
• The reactance of the capacitors C1 and C2 will be substantially lower at high frequencies in this circuit, resulting in the voltage V0 becoming zero as R2 is shorted.
• The reactance of the capacitors C1 and C2 will thus become extremely high at low frequencies.
• However, because the capacitor C1 is behaving as an open circuit, the output voltage V0 will remain at zero only in this instance.
• In the case of low and high frequencies, the Wien-Bridge network exhibits this type of behaviour, making it a lead-lag circuit.

Colpitts Oscillator

• Colpitt oscillator was named after physicist Edwin Colpitts, who invented it in 1918.
• It encourages preferable frequency stability when compared to the Hartley oscillator's functioning principle.
• A two-capacitor, one-inductor variant of an oscillator tank circuit.
• It can be connected in series, which allows the inductor to be positioned in parallel with the capacitors.

Hartley Oscillator

• A tank circuit with two inductors and one capacitor is known as a Hartley Oscillator.
• The inductors are connected in a combined series, while the capacitor is parallel to the inductor series.
• The planet was discovered in 1915 and named after an American physicist called Ralph Hartley.
• It uses frequencies ranging from 20 kHz to 20 MHz in most cases.

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Things To Remember

• An amplifier with a feedback lane between the output and the input is known as a feedback amplifier.
• A positive feedback system is used in an oscillator. The oscillator, which also serves as an amplifier, generates an output frequency by positive feedback.
• The feedback energy (voltage or currents) in positive feedback is in phase with the input signal and so aids it.
• The feedback energy (voltage or current) in negative feedback is out of phase with the input signal and consequently opposes it.
• The feedback in an oscillator is in phase with the oscillator (positive feedback). The gain is less than one if the feedback voltage is in the opposite phase (negative feedback), and it can never act as an oscillator. It will be a gain-reduced amplifier. Negative feedback, on the other hand, minimizes noise and distortion in an amplifier, which is a useful attribute.
• The voltage feedback (Vfb) from the output voltage (Vo ) should be such that it becomes Vo after amplification (A) for steady oscillations. If fraction ′ is feedback, then Vfb = Vo.′ and its value A(vo.′) should be equal to Vo after amplification. This suggests that A′ = 1 is the criterion for maintaining steady oscillations. Barkhausen's Criteria is the name for this.

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