MODULE 3

PART I. RECTIFIERS & POWER SUPPLIES

PART II. AMPLIFIERS & OSCILLATORS

BLOCK DIAGRAM OF DC POWER SUPPLY

  • The electric energy is available in the form of ac supply (230V, 50Hz in India) , but dc supply is essential for the operation of many electronic devices.

Block Diagram Explanation:

  • The transformer steps down the ac input into the desired voltage level.
  • The rectifier converts the ac into pulsating dc.
  • Filters are used for removing the ripples (ac components) in the output of the rectifier.
  • Finally a voltage regulator is used to maintain the output of the power supply at a constant level irrespective of changes in load current or input line voltage.

RECTIFIERS

  • Rectifiers are used for converting AC into DC.
  • The unidirectional property of a diode enables it to be used as rectifier.
  • It’s of two types :
  1. Half wave rectifier
  2. Full wave rectifier

Input & output wave forms of half-wave rectifier

  • A transformer is used at the input side to step down the voltage.
  • In the circuit, the secondary of transformer is connected to a single diode in series with a load resistance RL.
  • During the positive half cycle, the diode is forward biased, hence the diode conducts so that a current i flows through the load resistor and a voltage is developed across it.
  • Therefore an output voltage is developed during the positive half cycle.
    • During the negative half cycle, the polarity of voltage on the transformer secondary is reversed.
    • Therefore the diode is reverse biased and it does not conducts.
  • Thus there is no output in the negative half cycle of the input.
    • Half wave rectifier produces output voltage only in one half cycle of the input voltage. Hence the name half wave rectifier.
  • The output voltage is not a pure DC. But it is a pulsating dc which contains ac components called ripples.

Average and rms values of half wave rectifier

  • Let the instantaneous value of sinusoidal ac voltage at the secondary is v =Vm sin wt . where Vm = Maximum value of secondary voltage.

  • The average or dc value of half wave rectified output voltage is given by the relation :
  • Similarly the average or dc value of the load current is

    • The rms value of load current is :

    • The rms value of output voltage is :

PIV of a Diode in half- wave rectifier

  • During the negative cycle of the input, the diode is reverse biased.
  • There is no current flow through the load and hence no output voltage.
  • As a result, whole of the input voltage appears across the diode.
  • When the input voltage reaches its maximum value (Vm) in the negative half cycle, the voltage across the diode is also maximum.
  • The maximum reverse voltage that a diode can withstand is known as Peak Inverse Voltage(PIV).

For half wave rectifier, PIV =Vm

FULL WAVE RECTIFIER

  • A full wave rectifier is a circuit which conducts unidirectional current during the entire input cycle.
  • Thus a pulsating DC(DC having ac components) is obtained during both the half cycles of the input.
Two types of Full wave rectifier :
  1. Centre-tapped full wave rectifier.
  2. Bridge rectifier.

1. Centre-tapped full wave rectifier

  • A centre tapped transformer is used to step down the input voltage.
  • D1 and D2 diodes are connected to the secondary of the transformer.
  • The centre tap on the secondary winding of the transformer is usually taken as the ground or zero voltage reference point.
  • The voltage between the centre tap and either end of the secondary winding is half of the secondary voltage.

WORKING OF CENTRE TAPPED FULL WAVE RECTIFIER

a) During positive half cycle

b) During negative half cycle

  • It is important that, current flow through the load in the same direction during both the half cycles.

  • As a result, the output voltage developed across the load resistor is of same polarity and magnitude.

    1. BRIDGE RECTIFIER
    • In a bridge rectifier, it uses four diodes instead of two, but avoids the use of center tapped transformer.

WORKING OF A BRIDGE RECTIFIER

    • During the positive half cycle of the input, diodes D1 & D2 conducts and diodes D3 & D4 do not conduct.

    • There fore current flow through the secondary winding, diode D1, load resistor RL and diode D2.

    • During the negative half cycle, diodes D3 & D4 conducts while D1 & D2 do not conduct.
    • Hence the current flow through the secondary winding, diode D3, load resistor RL and Diode D4.
    • In both cases, current flow through the load in the same direction.

Average and rms values of full wave rectifier

The average or dc value of full wave rectified output voltage is given by the relation :

Where Vm = Maximum value of secondary voltage.

The average or dc value of full wave rectified load current is :

 

The rms value of output voltage is :

The rms value of load current is :

Ripple factor :

  • The output voltage of a rectifier is not a pure dc. It contains dc component as well as ac component.
  • The ac component present in the output are called ripples.
  • The ripple factor is a measure of purity of the dc output of a rectifier and is given by :

RIPPLE FACTOR OF A HALF WAVE RECTIFIER

The rms and average value of load current in a half wave rectifier are

Ripple factor of a full wave rectifier

EFFICIENCY OF A RECTIFIER

Defined as the ratio of dc power delivered to the load to the ac input power from the secondary winding of the transformer.

Efficiency of Half-wave Rectifier

The efficiency is maximum when RL >> rf . Thus the maximum efficiency of a half- wave rectifier is 40.6%

Efficiency of Full-wave Rectifier

Transformer Utilization Factor (TUF)

Comparison of various parameters of different rectifiers.

  1. (Vm/П) = 0.318
  2. (2Vm/П) = 0.636

FILTERS

  • Filters are circuits which smoothen ac variations in the rectifier output.
  • They remove the unwanted ac components of rectifier output.
  • Different types of filters are :
    Inductor Filter
    Capacitor Filter
    Inductor – Capacitor Filter
    Pi-Filter

CAPACITOR FILTER

  • When the rectifier output voltage is increasing to the maximum voltage Vm, the capacitor charges to Vm.
  • When the input voltage is less than that of capacitor voltage, capacitor starts to discharge through the load resistor RL.
  • Ripples can be reduced by allowing the capacitor to discharge slowly, So the value of load resistor can be taken of large value (Ʈ = RL C )

Pi – FILTER

Pi- Filter

INDUCTOR FILTER

INDUCTOR – CAPACITOR FILTER

VOLTAGE REGULATORS

  • Voltage regulator is a circuit whose function is to maintain a constant DC voltage in spite of the ac input fluctuations or changes in load resistance values.

ZENER DIODE VOLTAGE REGULATOR

  • A zener diode provides constant voltage when operated in the reverse breakdown region irrespective of current through it and variation in the applied voltage.
  • Thus a Zener diode can be used as a voltage regulator to provide constant dc output voltage.
  • Since the zener diode is connected in parallel with the load, the circuit is also known as shunt regulator.

Design of Zener Voltage Regulator

  • For acting as a voltage regulator, zener diode must be operated in the breakdown region.
  • Zener current should be maintained between IZmin and IZmax.
  • The current IZmin is the minimum current required to maintain the zener diode in the ON state.
  • The current IZmax is the maximum zener current that the zener diode can conduct without being damaged.
    • There should be minimum value of load resistance, RLmin to ensure that the zener diode will remain in breakdown region.
    • The zener diode should be selected in such way that, its maximum power rating is not exceeded.
  • The maximum power dissipation of a zener is,
    PZmax =VZ . Izmax
  • The minimum value of applied input voltage VSmin must be higher than Vz.

; IS = IZ + IL

LINE REGULATION OF ZENER DIODE REGULATOR – with varying input voltage

  • Here the load resistance RL is fixed and input voltage Vs is varied within limits.

  • As the input voltage Vs increases, the input current Is also increases.
  • This raises the current through the zener diode, with out 47 affecting the load current IL.
    • The increase in input current will also increase the voltage drop across the series resistance Rs, there by keeping the output voltage Vo across load resistor RL as constant.( Vo =Vs – Is.Rs)
    • If the input voltage is reduced, the input current also decreases.
    • Then the current through the zener diode will also decreases. The voltage drop across the series resistance will decrease.
    • Thus the load voltage and load current remains constant.
  • Line Regulation determines the amount the load voltage changes(Δvo) when the input source voltage changes (ΔVs). In ideal case, line regulation is zero.

LOAD REGULATION – WITH VARYING LOAD CURRENT

  • Here the input voltage Vs is fixed and load current IL is varied by varying the load resistance RL.

  • We know, Is = IZ + IL
  • When load current increases, current through zener diode will decrease as source current remains constant. And vice versa.
    • Therefore the load voltage remains constant .
    • Load Regulation determines the amount of change in output voltage, when load current changes


Switched Mode Power Supply(SMPS)

  • The zener shunt regulator has a major disadvantage that the zener diode is always ON and hence large amount of power is wasted.
  • When efficiency is a major concern, switching type regulator is often used.
  • SMPS is an electronic power supply that incorporates a control circuit to switch ON and OFF control element.
  • This results in low power consumption.
  • When the control element is ON more amount of time, the average value of output dc voltage will raise.

Block diagram explanation

  • The ac input supply is first rectified and then filtered.
  • This unregulated dc supply is given to the high frequency switching section which gives a chopped voltage or a square wave.
  • Fast switching semiconductor devices such as MOSFET or IGBT are used as control elements.
  • The switched(chopped) voltage is applied across the primary of the power transformer.
  • The switching pulses have fixed frequency(20 to 200kHz) and variable duty cycle.
  • Duty cycle: ratio of time during which waveform has high voltage to the time period of the waveform.

Block diagram for SMPS

  • Thus a voltage pulse train of suitable magnitude and duty cycle appears at the transformer secondaries.
  • This voltage pulse train is rectified and then smoothed by the output filter, which is either a capacitor or capacitor/inductor arrangement.
  • Regulation of the output is done by the control/feedback circuit.
  • If the output voltage decreases, control circuit switched ON the control element more amount of time.
  • Then the output voltage increases.
  • Similarly, if the output voltage increases, control circuit reduces the amount of ON time.
  • This makes the output voltage decrease.

SMPS

Advantages

  • Low audible noise
  • Small-transformer size

Disadvantages

  • High radio frequency interference
  • Low weight  Requires RF shielding and filtering
  • Complexity of the circuit

AMPLIFIER

  • Amplification is the process of increasing the strength of input signal by means of energy drawn from an external source.
  • The output wave form should same as the input waveform in the case of its shape.

CHARACTERISTICS OF AN AMPLIFIER

TRANSISTOR AS AN AMPLIFIER

  • Transistor can act as an amplifier, when it is operated in active region such that emitter base junction is forward biased & collector base junction is reverse biased.
  • Common emitter amplifier is the widely used configuration due to its high voltage gain and cascading capability.
  • Proper values of dc base current, collector current and collector-emitter voltages are chosen for ensuring transistor to be in active region, such that amplifier provides faithful amplification.

CE amplifier with dc bias

  • The process of applying DC voltages to operate the transistor in desired region is known as biasing.
  • After a transistor has been biased with proper dc conditions, ac source can be coupled to the base.
  • This produces fluctuations in the base current and hence in the collector current.
  • The output will be enlarged version of the input having same frequency of input but 180deg. phase shift.
  • The ac input is applied between base & emitter terminals 64 and output is available from collector & emitter terminals.

CE amplifier

  • The dc source VBB forward the emitter junction and VCC reverse biases the collector junction.
  • The resistor RB is used to limit the base current.
  • The resistor RC is connected to the collector terminal which is acting as a load.
  • The ac signal to be amplified is represented by the source VS.
  • The C1 capacitor blocks the dc components from the source and passes only ac components to the base.
  • The coupling capacitor C2 blocks the dc components and allow the ac to the output terminal.

The amplifier is called linear, if it does not change the shape of the signal.

Reason for 180° phase shift in output wave form

    • The expression for output voltage in CE amplifier is Vo =VCE =VCC – Ic.Rc
    • In the positive half cycle of the input, as input voltage increases, the base- emitter junction get more forward biased.
    • Hence base current increases, which increases the output collector current(since Ic = βIB).
    • when collector current(Ic) increases, the output voltage decreases(VCE).

      • In negative half cycle, as input voltage decreases, the forward biasing of base emitter junction decreases, hence base current decreases, which reduce the collector current.
      • When collector current (Ic) decreases, The output voltage (VCE) increases.
      • In this way CE amplifier introduce 180° phase shift. Output voltage will be an inverted version of input with amplification.

RC – coupled amplifier

  • CE amplifier is widely used in audio frequency applications in radio and TV receivers.
  • For the proper functioning of an amplifier, the transistor must be biased in the active region where the base current has complete over the collector current.
  • Thus a small increase in the base current results in a relatively large increase in the collector current and a small decease in the base current is followed by a large decrease in the collector current.
  • When large voltage gain is required, many stages of amplifiers are used.
  • The coupling stage in the RC- coupled amplifier forms an RC network.

71  Voltage divider bias technique is employed in the circuit.

Purpose of various components

  1. R1 and R2 :
    • Potential divider resistors
    • develop certain fixed voltage across them for proper biasing of transistors.
  2. RE :
    • stabilizes the operating point against temperature variation.
  3. CE :
    • act as bypass capacitor which bypass the ac signal developed across RE to ground.
    • Raises the gain of the amplifier but reduces its bandwidth.
  4. CC1 & CC2
    • Act as Coupling capacitors to couple the ac signal and block dc.
    • CC1 couples the input ac signal to Vin to the input of the amplifier.
    • CC2 is used to block the dc component of the output voltage from reaching the load resistor RL .

Single stage RC coupled common Emitter amplifier

Frequency response of amplifier

  • The frequency response of an amplifier is a graphical representation between the frequency ( x-axis) and gain (y-axis).
  • Here the gain is plotted in decibel versus frequency of input signal in log scale.
  • No real amplifier has equal gain at all frequencies.
  • The gain of the amplifier in the middle of its normal operating frequency range is mid-band gain.

    • The frequencies at which the gain falls by 3dB from the mid-band gain are termed as lower cut-off frequency and upper cut-off frequency.
    • The presence of coupling capacitors reduces the gain at low frequencies.
    • At low frequencies, capacitive impedance increases as it is inversely proportional to frequency of signal.
    • In addition the bypass capacitor CE can’t effectively bypass the AC signals to ground results in negative feedback which reduces gain.
      • The gain is reduced at high frequencies due to the presence of internal capacitance such as stray capacitance.
      • These capacitances are parasitic capacitances internally formed at transistor junctions.
      • All coupling and bypass capacitors are considered short- circuit at mid-band frequencies while all internal capacitive effects are considered open-circuit.

Feedback

  • Process of returning a part of the output of a system to the input.
  • Depending upon the whether the feedback energy aids or opposes the input signal, there are two types of feedback.
  1. Negative Feedback: opposes the input signal
  2. Positive Feedback : aids the input signal

For positive feedback: Xi = Xs+Xf

For negative feedback: Xi = Xs-Xf

  1. Negative or Degenerative feedback

Concept of negative feedback

  • Advantages:
    • Negative Feedback
    • Stabilizes gain
    • Reduction in distortion and noise
    • Increase in input impedance and decrease in output impedance
    • Extends bandwidth
    • Improves the frequency response
  • Disadvantages:
    • Reduces gain

Positive or Regenerative feedback

Positive Feedback

  • Advantages:
    • Increases the overall gain.
  • Disadvantages:
    • Distortion and instability increases.
    • Poor stability
    • Reduction in bandwidth

Application: Employed in oscillator circuits

Oscillators

  • Any circuit which is used to generate AC voltage without input signal is called oscillator.
  • To generate AC voltage, the circuit is supplied energy from the DC source.
  • A transistor amplifier with proper positive feedback can act as an oscillator.
  • Oscillators are classified in terms of their output waveform, frequency range, components or circuit configuration.
  • If the output waveform is sinusoidal, it is called sinusoidal oscillator otherwise it is called relaxation oscillator, which include square, triangular and sawtooth waveforms.

Barkhausen Criterion of Oscillation

  • Oscillators employ both active and passive components.
  • The active components(transistors) provide energy conversion mechanism.
  • Passive components determine the frequency of oscillation.
  • The random motion of free electrons in the resistor generates a voltage called noise voltage across the resistance act as input to the amplifier.

RC Oscillators

  • For an oscillator to produce sustained oscillations, positive feedback must be provided with the amplifier.
  • The output of a single stage common emitter amplifier, which is 180deg. Shifted will pass through a phase-shift network to make the total phase shift of 360/zero degree.
  • Used for generating frequency in the range of 100Hz to 100KHz.
  • The RC-phase shift oscillator is an oscillator which employs a transistor amplifier and an R-C network as the frequency selective circuit.

  • It produces a sine wave output using positive feedback from the resistor-capacitor combination.
  • The values of resistors R and capacitors C used in feedback network have been chosen so that at the required frequency, the output voltage is phase shifted by 180 degree.
  • The transistor amplifier provides another 180degree phase shift thus satisfying the Barkhausen criteria of the total phase shift of 360 degree.
  • If a common emitter amplifier is used, with a resistive collector load, there is 180degree phase shift between the voltages at the base and the collector.

Public address system/ PA system

  • PA system is an electronic sound amplification and distribution system with a microphone, amplifiers and loudspeakers used in many applications such as addressing a large public, announcements in offices and institutions etc.

  • Microphone:- transducer which senses sound signals and converts them in to corresponding electrical signals that can be processed by the rest of the system.
  • Preamplifier:- increase the amplitude of signal coming from microphone enabling for further processing.
  • Power amplifier:- it takes the amplified signal from preamplifier and boosts the current so that it is strong enough to drive the loud speaker.
  • Loud speaker:- converts the electrical signal back into a sound wave, which will be an amplified version of original sound.

  • Mixer:- the output of microphones is fed to a mixer stage. The function of mixer is to effectively isolate different channels from each other before feeding to the main amplifier
  • Equalization is the process of altering the frequency response of an audio system using filters.
  • It adjusts the amplitude of audio signals at particular frequencies.
  • Equalization may also be used to eliminate unwanted signals, make certain instruments or voices more prominent.
    • Graphic equalizer:- allows the user to see graphically and control individually a number of different frequency bands.
    • Low frequency (popularly called bass) of the signal is amplified and converted into audio using low frequency speakers(popularly called woofers).
  • Similarly high frequency audio signals are amplified and fed to high frequency loud speakers.