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UNIT 5- RECTIFIERS, AMPLIFIERS AND OSCILLATORS
Half and full wave rectifiers - Capacitive and inductive filters - ripple factor- PIV-rectification efficiency - RC coupled amplifier positive and negative feedback - Barkhausen criterion for oscillations - RC and LC oscillators.
PART A
- What is meant by rectification. Rectification is the process of conversion of alternating current to direct current. Rectification is performed by a diode that allows current to flow in one direction but not in the opposite direction.
- What is a rectifier and list its types? A rectifier is a circuit that is used for converting AC supply into unidirectional DC supply. Types: Half-wave rectifier and Full-wave rectifier.
- What is a Half-wave rectifier? The half wave rectifier is a type of rectifier which converts half of the AC input signal (positive half cycle) into pulsating DC output signal and the remaining half signal (negative half cycle) is blocked or lost. In half wave rectifier circuit, we use only a single diode.
- What is a Full-wave rectifier? The full wave rectifier is a type of rectifier which converts the full AC input signal (positive half cycle and negative half cycle) to pulsating DC output. Here more than one diode is used.
- Define and explain Peak Inverse Voltage Peak inverse voltage is the maximum reverse voltage that can be applied to the PN junction without damage to the junction. If the reverse voltage across the junction exceeds to its peak inverse voltage, the junction may be destroyed due to excessive heat. Also may be called Reverse Breakdown Voltage.
- Define Ripple factor? The purpose of a rectifier is to convert ac into dc. The output of a rectifier is pulsating in nature i.e. it contains ac as well as dc components. This ac component is called ripple which is removed using a filter
circuit. The ratio of RMS value of ac component present in the waveform to the dc component in the waveform is called as ripple factor. It is a dimensionless quantity and denoted by γ..
- Discuss the advantages and disadvantages of full wave rectifier with center tapped transformer Advantages: High rectifier efficiency. Low power loss. Low ripples Disadvantages: High cost The center tapped transformers are expensive and occupy a large space.
- What are the advantages of bridge rectifier? The main advantage of this bridge circuit is that it does not require a special centre tapped transformer, thereby reducing its size and cost. It can be used in application with floating output terminals, no output terminal is grounded.
- What is the value of ripple factor for a half wave rectifier & full wave bridge rectifier?
For Half wave rectifier:
For Full Wave Rectifier: IDC = 2Imax / π, Irms = Im/√ 2 Ripple factor for Half wave rectifier is 1.21 and For Full Wave Rectifier is 0.48.
- State the importance of filters in a rectifier circuit also names the different types of filters. The circuit which converts the pulsating DC in to pure DC is called filter. As the name specifies it filters the oscillations in the signal and provides a pure DC at the output. The electronic reactive elements like capacitor and inductors are used to do this work. Generally they are
compact as it uses only resistors and capacitors Offers a constant gain over a wide frequency band.
- List four applications of RC Coupled amplifier. RF Communications Optical Fiber Communications Public address systems as pre-amplifiers Controllers Radio or TV Receivers as small signal amplifiers
- Define positive and negative feedback? Positive feedback: If the feedback voltage (or current) is so applied as to increase the input voltage (i.e it is in phase with it), then it is called positive feedback. Negative feedback: If the feedback voltage (or current) is so applied as to reduce the input voltage (i.e it is 180 out of phase with it), then it is called negative feedback.
- Mention the advantages of negative feedback? The advantages of negative feedback are higher fidelity and stabilized gain, increased bandwidth; less distortion and reduced noise and input and output impedances can be modified as desired.
- Negative feedback is preferred to other methods of modifying Amplifier characteristics. Why? Negative feedback is preferred to other methods of modifying Amplifier Characteristics because it has the advantages of reduction in distortion, stability in gain, increased bandwidth etc.
- What is an oscillator An oscillator is a circuit which produces a continuous, repeated, alternating waveform without any input. They basically convert unidirectional current flow from a DC source into an alternating waveform which is of the desired frequency, as decided by its circuit components.
- State the Barkhausen criterion for an oscillator. The Barkhausen stability criterion is a mathematical condition to determine when a linear electronic circuit will oscillate. The two important and necessary conditions are (i).The loop gain is equal to unity in absolute magnitude, that is, |A| = 1 (ii).The phase shift around the loop is zero or an integer multiple of 2π.∠A = 0 or 360°
- Explain the concept of positive feedback.
When the feedback energy (voltage or current) is in phase with the input signal and thus aids it, it is called positive feedback. Both amplifier and feedback network introduce a phase shift of 180°. The result is a 360° phase shift around the loop, causing the feedback voltage Vf to be in phase with the input signal Vin.
- Compare Positive and negative feedback POSITIVE FEEDBACK NEGATIVE FEEDBACK When the feedback energy (voltage or current) is in phase with the input signal and thus aids , it is called positive feedback.
When the feedback energy (voltage or current) is out of phase with the input signal and thus opposes it, it is called negative feedback. The positive feedback increases the gain of the amplifier
The negative feedback decreases the gain of the amplifier It has the disadvantages of increased distortion and instability.
The distortion is less and the stability is more.
Positive feedback is employed in Oscillators Negative feedback is employed in amplifiers.
- Give the over all classification of oscillators? Classification Based on the Feedback Mechanism: Positive Feedback Oscillators ,Negative Feedback Oscillators Classification Based on the Shape of the Output Waveform: Sine Wave Oscillators ,Square or Rectangular Wave oscillators, Sweep Oscillators Classification Based on the Nature of the Frequency : Fixed Frequency Oscillators ,Variable or Tunable Frequency Oscillators Classification Based on the type of the Frequency Control Used: RC Oscillators ,LC Oscillators Crystal Oscillators Classification Based on the Frequency of the Output Signal: Low-Frequency Oscillators, Audio Oscillators , Radio Frequency Oscillators , High-Frequency Oscillators ,Very High-Frequency Oscillators ,Ultra High- Frequency Oscillators
- List four applications of oscillators. Oscillators are a cheap and easy way to generate specific Frequency of a signal. Quartz watches (which uses a crystal oscillator) Used in various audio systems and video systems Used in various radio, TV, and other communication devices Used in computers, metal detectors, stun guns, inverters, ultrasonic and radio frequency applications.
- Write down the advantages and disadvantages of Colpitt’s oscillator Advantages: (i) Colpitt’s oscillator generates high frequencies ranges between 1 MHz and 500 MHz.
generated a back-emf which results in the flow of current through the circuit in the same direction as that of before. This current flow through the circuit continues until the electromagnetic field collapses which result in the back-conversion of electromagnetic energy into electrical form, causing the cycle to repeat. However, now the capacitor would have charged with the opposite polarity, due to which one gets an oscillating waveform as the output.
PART – B
- Explain about half wave rectifier and derive the expression for Idc Irms , and ripple factor. A half wave rectifier is a type of rectifier which allows only half cycle (either positive half cycle or negative half cycle) of the input AC signal while the half cycle is blocked. For example, if the positive half cycle is allowed then the negative half cycle is blocked. Similarly, if the negative half cycle is allowed then the positive half cycle is blocked. However, a half wave rectifier will not allow both positive and negative half cycles at the same time. Therefore, the half cycle (either positive or negative) of the input signal is wasted. HALF WAVE RECTIFIER OPERATION The half wave rectifier is the simplest form of the rectifier. We use only a single diode to construct the half wave rectifier. The half wave rectifier is made up of an AC source, transformer (step-down), diode, and resistor (load). The diode is placed between the transformer and resistor (load).
Circuit diagram AC source The AC source supplies Alternating Current to the circuit. The alternating current is often represented by a sinusoidal waveform. Transformer Transformer is a device which reduces or increases the AC voltage. The step-down transformer
reduces the AC voltage from high to low whereas the step-up transformer increases the AC voltage from low to high. In half wave rectifier, we generally use a step-down transformer because the voltage needed for the diode is very small. Applying a large AC voltage without using transformer will permanently destroy the diode. So we use step-down transformer in half wave rectifier. However, in some cases, we use a step-up transformer.In the step-down transformer, the primary winding has more turns than the secondary winding. So the step-down transformer reduces the voltage from primary winding to secondary winding. Diode A diode is a two terminal device that allows electric current in one direction and blocks electric current in another direction. Resistor A resistor is an electronic component that restricts the current flow to a certain level. Half wave rectifier operation Positive half cycle:
During the positive half cycle of the input signal the anode of the diode becomes positive with respect to the cathode and hence the diode D conducts. For an ideal diode, the forward voltage drop is zero. So the whole-input voltage will appear across load resistance RL. Negative half cycle:
During negative half cycle of the input signal, the anode of the diode becomes negative with respective to the cathode and hence the diode D does not contact. For an ideal diode the impedance by the diode is infinity. So the whole input voltage appears across the diode D. Hence the voltage drop across R, is zero. The final output voltage waveform on the secondary side (DC) of Half Wave Rectifier
- Describe Centre tapped Full wave rectifier in detail? A full wave rectifier is a type of rectifier which converts both half cycles of the AC signal into pulsating DC signal.
Circuit Diagram of Full wave Rectifier
The rectifier circuit consists of centre tapped step-down transformer two diodes and they are connected using a centre tapped transformer. Thus, this type of rectifier where centre tapping is provided is called centre tap rectifier. The load resistor is connected, and the output voltage is obtained across this resistor.
Working of Centre-Tap Full Wave Rectifier
positive half cycle of the input AC signal As shown in the figure, an ac input is applied to the primary coils of the transformer. This input makes the secondary ends P1 and P2 become positive and negative alternately. For the positive half of the ac signal, the secondary point D1 is positive, GND point will have zero volt and P2 will be negative. At this instant diode D1 will be forward biased and diode D2 will be reverse biased. The diode D1 will conduct and D2 will not conduct during the positive half cycle. Thus the current flow will be in the direction P1- D1-C-A-RLoad-GND. Thus, the positive half cycle appears across the load resistance RLOAD. During the negative half cycle, the secondary ends P1 becomes negative and P2 becomes positive. At this instant, the diode D1 will be negative and D2 will be positive with the zero reference point being the ground, GND. Thus, the diode D2 will be forward biased and D1 will be reverse biased. The diode D will conduct and D1 will not conduct during the negative half cycle. The current flow will be in the direction P2-D2-C-A-RLoad-GND.
- Draw the circuit diagram and explain the working of full wave bridge rectifier with output filter and derive the expression of average output current and ripple factor. Discuss its advantages over center tap full wave rectifier.
Full Wave Bridge Rectifier uses four individual rectifying diodes connected in a closed loop “bridge” configuration to produce the desired output The single secondary winding is connected to one side of the diode bridge network and the load to the other side as shown below
As the current flowing through the load is unidirectional, so the voltage developed across the load is also unidirectional the same as for the previous two diode full- wave rectifier, therefore the average DC voltage across the load is 0.637Vmax. However in reality, during each half cycle the current flows through two diodes instead of just one so the amplitude of the output voltage is two voltage drops ( 2 x 0.7 = 1.4V ) less than the input VMAX amplitude. The ripple frequency is now twice the supply frequency (e.g. 100Hz
The AC component of this signal is coupled to the second stage of the RC coupled amplifier through the coupling capacitor CC and thus appears as an input at the base of the second transistor Q2. This is further amplified and is passed-on as an output of the second stage and is available at the collector terminal of Q2 after being shift by 180o in its phase. This means that the output of the second stage will be 360o out-of-phase with respect to the input, which in turn indicates that the phase of the input signal and the phase of the output signal obtained at stage II will be identical. Further it is to be noted that the cascading of individual amplifier stages increases the gain of the 16 overall circuit as the net gain will be the product of the gain offered by the individual stages. However in real scenario, the net gain will be slightly less than this, due to the loading effect. In addition, it is important to note that by following the pattern exhibited by Figure 1, one can cascade any number of common emitter amplifiers but by keeping in mind that when the number of stages are even, the output will be in-phase with the input while if the number of stages are odd, then the output and the input will be out-of-phase. The frequency response of a RC coupled amplifier (a curve of amplifier’s gain v/s frequency), shown by Figure 2, indicates that the gain of the amplifier is constant over a wide range of mid- frequencies while it decreases considerably both at low and high frequencies. This is because, at low frequencies, the reactance of coupling capacitor CC is high which causes a small part of the signal to couple from one stage to the other. Moreover for the same case, even the reactance of the emitter capacitor CE will be high due to which it fails to shunt the emitter resistor RE effectively which in turn reduces the voltage gain.
On the other hand, at high frequencies, the reactance of CC will be low which causes it to behave like a short circuit. This results in an increase in the loading effect of the next stage and thus reduces the voltage gain. In addition to this, for this case, the capacitive reactance of the base-emitter junction will be low. This results in a reduced voltage gain as it causes the base current to increase which inturn decreases the current amplification factor β. However, in mid-frequency range, as the frequency increases, the reactance of CC goes on decreasing which would lead to the increase in gain if not compensated by the fact that the reduction in reactance leads to an increase in the loading effect. Due to this reason, the gain of the amplifier remains uniform/constant throughout the mid- frequency band. Advantages of RC Coupled Amplifier Cheap, economical and compact as it uses only resistors and capacitors. Offers a constant gain over a wide frequency band. Disadvantages of RC Coupled Amplifier Unsuitable for low-frequency amplification. Low voltage and power gain as the effective load resistance (and hence the gain) is reduced due to the fact that the input of each stage presents a low resistance to its next stage. Moisture-sensitive, making them noisy as time elapses. Poor impedance matching as it has the output impedance several times larger than the device at its end- terminal (for example, a speaker in the case of a public address system). Narrow bandwidth when compared to JFET amplifier. Applications of RC Coupled Amplifier
- RF Communications.
- Optical Fiber Communications.
- Public address systems as pre-amplifiers
- Controllers.
- Explain the working of Wein Bridge shift oscillator?
One of the simplest sine wave oscillators which uses a RC network in place of the conventional LC tuned tank circuit to produce a sinusoidal output waveform, is the Wien Bridge Oscillator. The Wien
minimized.
- Some method of stabilizing the amplitude of the oscillations must be provided because if the voltage gain of the amplifier is too small the desired oscillation will decay and stop and if it is too large the output amplitude rises to the value of the supply rails, which saturates the op-amp and causes the output waveform to become distorted.
- With amplitude stabilization in the form of feedback diodes, oscillations from the oscillator can go on indefinitely.
- Frequency of oscillation F=1/2πRC
- Explain Colpitt’s oscillator and derive the expression for frequency of oscillation. Also explain the advantages and disadvantages. The Colpitts Oscillator, named after its inventor Edwin Colpitts is another type of LC oscillator design. In many ways, the Colpitts oscillator is the exact opposite of the Hartley Oscillator we looked at in the previous tutorial. Just like the Hartley oscillator, the tuned tank circuit consists of an LC resonance sub-circuit connected between the collector and the base of a single stage transistor amplifier producing a sinusoidal output waveform. The basic configuration of the Colpitts Oscillator resembles that of the Hartley Oscillator but the difference this time is that the centre tapping of the tank sub- circuit is now made at the junction of a "capacitive voltage divider" network instead of a tapped autotransformer type inductor as in the Hartley oscillator.
The Colpitts oscillator uses a capacitor voltage divider as its feedback source. The two capacitors, C and C2 are placed across a common inductor, L as shown so that C1, C2 and L forms the tuned tank circuit the same as for the Hartley oscillator circuit. The advantage of this type of tank circuit configuration is that with less self and mutual inductance in the tank circuit, frequency stability is improved along with a more simple design. As with the Hartley oscillator, the colpitts oscillator uses a single stage bipolar transistor amplifier as the gain element which produces a sinusoidal output. Consider the circuit below. The transistor amplifiers emitter is connected to the junction of capacitors, C1 and C2 which are
connected in series and act as a simple voltage divider. When the power supply is firstly applied, capacitors C1 and C2 charge up and then discharge through the coil L. The oscillations across the capacitors are applied to the base-emitter junction and appear in the amplified at the collector output. The amount of feedback depends on the values of C1 and C2 with the smaller the values of C the greater will be the feedback. The required external phase shift is obtained in a similar manner to that in the Hartley oscillator circuit with the required positive feedback obtained for sustained un-damped oscillations. The amount of feedback is determined by the ratio of C1 and C2. These two capacitances are generally “ganged” together to provide a constant amount of feedback so that as one is adjusted the other automatically follows. The frequency of oscillations for a Colpitts oscillator is determined by the resonant frequency of the LC tank circuit and is given as:1/2π√ where CT is the capacitance of C1 and C connected in series and is given as:
The configuration of the transistor amplifier is of a Common Emitter Amplifier with the output signal 180 out of phase with regards to the input signal. The additional 180 phase shift require for oscillation is achieved by the fact that the two capacitors are connected together in series but in parallel with the inductive coil resulting in overall phase shift of the circuit being zero or 360o. Resistors, R1 and R provide the usual stabilizing DC bias for the transistor in the normal manner while the capacitor acts as DC-blocking capacitors. The radio frequency choke (RFC) is used to provide a high reactance (ideally open circuit) at the frequency of oscillation, (ƒr) and a low resistance at DC.
- Explain Hartley oscillator and obtain the equation for oscillation? In a Hartley oscillator the oscillation frequency is determined by a tank circuit comprising of two inductors and one capacitor. The inductors are connected in series and the capacitor is connected across them in parallel. Hartley oscillators are commonly used in radio frequency (RF) oscillator applications and the recommended frequency range is from 20 KHz to 30MHz. Hartley oscillators can be operated at frequencies lower than 20 KHz, but for lower frequencies the inductor value need to be high and it has a practical limit. The circuit diagram of a typical Hartley oscillator is shown in the figure below.
