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This document covers key concepts in analogue electronics, focusing on advanced circuit analysis and applications. Topics include amplifiers, frequency response, feedback mechanisms, oscillators, and signal processing techniques. The material integrates theoretical explanations with circuit diagrams, mathematical derivations, and worked examples to support student understanding. It serves as a study resource for engineering students, reinforcing both fundamental principles and practical applications of analogue electronic systems.
Typology: Lecture notes
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types of diodes and their application Signal Diodes This term signal diode includes all devices that have been designed for use in circuits where large current and voltage ratings are not required. The usual requirements are for a large reverse resistance/forward resistance ratio and minimum junction capacitance. They are used as general purpose while others are suited to a particular circuit application e.g. as a detector. Power Diodes They are most often employed for the conversion of a.c into d.c as rectifiers. The important power diode parameters are PIV, maximum forward current and the resistance ratio. The PIV is in the range 50v – 100v with a maximum forward current of 30A. The forward resistance must be as low as possible to avoid considerable voltage drop across the diode when the large forward current flows. This resistance is usually not very much more than 1Ω or 2Ω. PIV- This is the maximum possible voltage across the diode when it is reverse biased. Zener Diodes The huge reverse current, which flows when the breakdown voltage of a diode is exceeded need not necessary result in damage of the source. A zener diode is fabricated in a way which allows it to be operated in breakdown region without damage, provided the current is restricted by the external resistance to a safer value. The large current at breakdown is brought about by 2 factors.
standardized reference voltages e.g. it is possible to obtain a zener diode with a reference (breakdown) voltage of 8.2V. An alternative name for the device is a voltage reference diode. Applications:
of triggering devices have been developed, which with suitable circuiting can control the point at which thyristor begin to conduct, thus a.c and d.c power variation is made possible allowing light, heat and motor speed control to be carried out economically and efficiently. UJT are ideally suited for firing thyristor, which connected in suitable relaxation oscillation circuit. Thyristor as a pnpn device
Fig1.3. Thyristor as PNPN device
In fig. 2b, positive voltage is applied to T 1 , then the middle junction has a reverse bias, positive at the N side of J 2 and negative at the P side. No forward current can flow between the end terminals because of the reverse bias across J 2. In fig 2c, negative voltage is applied to T 1. The J 1 and J 3 have reverse bias. Again, no current can flow between the end terminals because of the internal reverse bias. Now what the PNPN device needs is an added gate electrode to provide a trigger voltage that can start the current conduction.
1. Silicon controlled rectifier (scr) It is 4-layer PNPN device that consist of an alloy of N-type material into a silicon PNP pellet providing two main terminals namely anode and cathode. The gate contact is welded to P- region to form the third terminal usually referred to as gate. It is a rectifier with a control element. It consists of 3 diodes connected back to back with gate connection. It is widely used as a switching device in power control applications. It can control loads by
switching currents off and on up to many thousands times/sec. It can also switch on for a variable length of time, thereby delivering selected amount of power to the load. Basic operation of SCR
SCR symbol and its construction a) Biasing and operation When a voltage is applied across thyristor with polarity such that its anode is held negative with respect to its cathode PN Junction J 1 and J 3 are reverse biased and device only conducts a small leakage current. If the reverse voltage across the SCR is increased the leakage current will also increase slowly at first, and then at the point where an avalanche breakdown occurs it rapidly increases to a large value that may destroy the device. If a forward voltage is applied across the SCR so that the anode is held junction J 1 and J 3 will become forward biased but junction J 2 will be reverse biased. Also once again only small leakage current will be flowing and the SCR is said to be forward blocking state (OFF). This forward leakage current also increases slowly with increase in forward voltage until the breakdown voltage VBO is reached. At this point the current passed by SCR increases rapidly and the voltage across the device falls to a much lower value VT. SCR is made to conduct a large forward current when the forward anode-cathode voltage is smaller than breakdown voltage VBO by triggering or firing the device. Triggering is achieved by injecting a current pulse into the gate terminal. When a gate signal is applied, thyristor turns ON at a voltage below VB0 depending on the magnitude of gate current. The higher the gate current, the lower is the forward break sown voltage.
P 1
N 1
P 2 P (^1) N 2
N 1
P 2
N 2
P 1
N 1
P 2
N 2
P 1
N 1
P 2
N 2
Once the thyristor start conducting a forward current exceeding the minimum valve called the latching current , the gate signal is no longer required to maintain the device in its ON state. The thyristor returns to blocking state if the anode current falls below a level called holding current. Note hence that the IL is associated with turn ON process while IH to the turn OFF process. One the thyristor start conducting fully, gate loses control on it and the device can turn OFF i.e. become non-conducting only on reducing the forward current to a level below the holding current IH. This process of turning OFF is also referred to commutation.
b) Two transistor Analogy The principle of operation of an SCR can be explained by treating SCR to be constituted by two BJT transistors, one PNP and NPN. The gate current is amplified by the NPN transistor and then appears at the collector as hfe 2 Ig. This current then flows in the base of PNP transistor to appear at its collector as hfe 1. hfe 2 Ig. In turn this current appears at the base of NPN transistor where regenerative action takes place which result in device switching to its forward on characteristics.
SCR two-transistor model In the off-state of a transistor, collector current IC is related to emitter current IE as: IC = IE +ICBO Where is common-base current gain hfe and ICBO is common-base leakage current of collector-base junction. For transistor Q 1 : emitter current IE = anode current Ia and IC = collector current IC1. IC1 = 1 Ia + ICBO For Q2, IC2 = 2 Ik + ICBO The sum of the two collector currents is equal to external circuit current IA entering at anode terminal A. Ia = IC1+ IC Ia = 1 Ia + ICBO1 + 2 Ik + ICBO When gate current is applied, then Ik = Ia+ Ig. Substituting this value of Ik in the above Eq. gives: Ia = 1 Ia + ICBO1 + 2 (Ia + Ig) + ICBO Ia = 2 Ig +ICBO1+ICBO 1- ( 1 + 2 )
e) Application
Symbol and V-I characteristics of DIAC
3. Triac A triac is a semi-conductor device whose operation is similar to that of two SCR connected in reverse parallel. A triac is able to conduct a large conduct a large current in either direction or the other by a gate pulse of appropriate polarity. Operation The gate terminal is connected to both P 2 and N 3 so that the triac can be turned on by either the positive going or negative going pulse of gate current. The input and output of main terminal MT 1 , and MT 2 respectively which are connected to both an N-type and P-type region i.e. MT 1 is connected to N 1 and P 1 and hence it is possible for current to flow either: a) With MT 2 positive with respect to MT 1. The path will be P 1 , N 2 P 2 and N 4. b) With MT 1 positive with respect to MT 2. The path will be P 2 , N 2 , P 1 and N 1. When the triac is ON, a current flowing in MT 1 and MT 2 is known as the principal current. As with SCR, the triac can be turned on by; i) Apply a gate current ii) Exceeding the avalanche breakdown voltage iii) By allowing MT 1 – MT 2 applied voltage to increase at a rate in excess of maximum dv/dt value.
Static Characteristics of a TRIAC
(a) Circuit Symbol and (b) static V-I characteristics of a triac. When MT 2 is positive with respect to MTI, the triac operates in the 1st^ quadrant of its static characteristics, if it is not triggered, the small forward current increases slowly with increase in voltage until the breakdown voltage VB0 is reached and then the current increases rapidly. The device can be turned ON at a smaller forward current by injecting a suitable gate current and the characteristics show the effect of increasing gate current. The gate current must be maintained until the main current is at least equal to the latching current. When MT 1 is positive with respect to MT 2 , the triac operates in the 3rd^ quadrant and the current focus on the opposite direction. The triac can be triggered to operate in either quadrant by an application of either a positive or negative gate current pulse i.e.
Turning-ON process in a triac
Arrangements shown in (a) and (c) are most often employed since the use of the same polarity voltage for both the gate and the MT 2 terminal allows the power supplies to be derived from the same source, and this considerably simplified the circuiting of the trigger module. Advantages and Disadvantages of TRIAC over SCR Merits Triac can be triggered with positive or negative polarity voltage. Triac does not need a diode to protect against reverse voltage. Disadvantages Have low dv/dt voting compared to SCR. Reliability is less Triggering act for triac a careful consideration.
4. UJT (Unijuction Transistors) There are three types: i) Standard UJT ii) Complementary UJT (CUJT) iii) Programmable UJT (PUT) The UJT, CUJT and PUT are widely used for generation of trigger pulses for SCRs.
exceeds Vx, current flows through the diode so long as the applied voltage is less than Vx, current flows decreases the value of RB1, through the process of conductivity modulation.
UJT Relaxation Oscillator Triggering UJT is an extremely efficient switch mostly used as triggering device for SCRs and TRIACs. Below is a basic circuit of UJT as saw tooth voltage generator.
Basic circuit and voltage waveform of UJT oscillator
R 1 and R 2 are kept small compared with RB1 and RB2. When VBB is applied, C charges through R exponentially toward VBB with time constant 1 = RC. During this charging process, emitter circuit of UJT remains open circuit. At this point VC = VE. VC = VE = VBB (1- e-t/RC) When VC =VE reaches Vp= VBB + VD, the Pn junction between E and B, breaks down and UJT turns ON. Immediately C rapidly discharges through low value R 1 with very slow time constant 2 = RC. When VE drops to the VV, UJT turns off and the cycle is repeated.
Let T 1 , be the time taken by C to change from VV to VP through resistor R. Then VP = VBB + VD = VV + VBB (1-e-T1/RC) Assuming that VDVV the above equation gives
= 1– e-TI/RC Hence T 1 = RC In (1/1-) Total time period of one cycle =T = T 1 + T 2 But T 2 T 1 Hence T =T 1 Then 1/f = T= RC In 1 1- Uses:
Similarly, the overall current amplification is given by; Ai = Ai 1 × Ai 2 × Ai 3 ×...
.... The overall power gain is given by; Ap = Av. Ai and Gp = 10 log10 AP dB Suppose in a two-stage cascaded amplifier, first stage has a voltage amplification of 2000 (dB gain of the 20 log10 2000 = 66 dB) and second stage has corresponding values of 1000 (60 dB). If the ac output of first stage is fed into the second stage, the overall amplification would theoretically become = 1000 × 2000 = 2 × 106 which corresponds to a dB gain of (60 + 66) = 126 i.e. 20 log10 2 × 106 = 20 × 6.3 = 126. The above result would be true only when we neglect the loading effect of first stage by the second stage****. It would be approximately true so long as the impedance looking into the input of second stage is much greater than the output impedance of the first stage. Otherwise, the overall gain would be much less. Classification of Amplifiers The transistor amplifiers may be classified as to their usage , frequency capabilities, coupling methods and mode of operation. (i) According to use. The classifications of amplifiers as to usage are basically voltage amplifiers and power amplifiers. The former primarily increases the voltage level of the signal whereas the latter mainly increases the power level of the signal. (ii) According to frequency capabilities. According to frequency capabilities, amplifiers are classified as audio amplifiers , radio frequency amplifiers etc. The former are used to amplify the signals lying in the audio range i.e. 20 Hz to 20 kHz whereas the latter are used to amplify signals having very high frequency. (iii) According to coupling methods. The output from a single stage amplifier is usually insufficient to meet the practical requirements. Additional amplification is often necessary. To do this, the output of one stage is coupled to the next stage. Depending upon the coupling device used, the amplifiers are classified as R-C coupled amplifiers, transformer coupled amplifiers etc. (iv) According to mode of operation. The amplifiers are frequently classified according to their mode of operation as class A , class B and class C amplifiers. Amplifier Coupling
Circuit Operation The brief circuit operation is as under: (i) The input signal ni is amplified by Q 1. It is phase reversed (usual with CE connection); (ii) the amplified output of Q 1 appears across R 2; (iii) The output of the first stage across R 2 is coupled to the input at R 3 by coupling capacitor C 2. This capacitor is also sometimes referred to as blocking capacitor because it blocks the passage of dc voltages and currents; (iv) the signal at the base of Q 2 is further amplified and its phase is again reversed ; (v) the ac output of Q 2 appears across R 4; (vi) the output across R 4 is coupled by C 3 to load resistor R 5; (vii) The output signal v 0 is the twice-amplified replica of the input signal vi****. It is in phase with vi because it has been reversed twice.
AC Equivalent Circuit
(i) Input impedance r i.1= R 1 || β1. re. It is the input impedance of the first stage and not rin (base). (ii) Output impedance The output impedance of the first stage is r 0.1= R 2 || ri .2. It is so because the input of the second stage forms a part of the output of the first stage.
ri .2 = R 3 || β2. re 2 ≈ β2 re .2, Where re .1 and re .2 are ac junction resistances of the two transistors and are given by;
The output impedance of Q 2 is r 0.2 = R 4 || R 5 (iii) Voltage Gain
Example For the two-stage RC-coupled low-level audio amplifier shown in Fig, below compute the following: (i) ri (ii) Av1 (iii) Av2 and (iv) Av in dB. Neglect VBE and take re = 25 mV/IE.
Solution The input impedance of the cascaded amplifier is (i) ri = R 1 || βl. re .1 For finding re .1, we need IE .1 which approximately equals IC .1 Now, IC .1 = β. IB.
Also, IB1 = 12/ R 1= 12/0.6 M = 20 mA IC 1 = 100 × 20 = 2000 mA = 2 mA IE .1= 2 mA re .1 = 25/2 = 12.5 Ω ; β1 re .1= 100 × 12.5 = 1250 ri = R 1 || β1. re .1 = 0.6 M || 1250 Ω ≈ 1250 Ω (ii)
Example For the two-stage RC-coupled amplifier shown in Fig below compute the following: (i) ri , (ii) Av1 , (iii) Av2 , (iv) Av in decibels. Take β 1 = β 2=100. Neglect VBE and use re = 25 mV/IE.
Solution
As seen from Fig. above, amplifier gain falls off at very low as well as very high frequencies. At low frequencies, the fall in gain (called roll-off ) is due to capacitive reactance of the coupling capacitor between the two stages. The high- frequency roll-off is due to output capacitance of the first stage, input capacitance of the second stage and the stray capacitance.
Disadvantages of RC coupled amplifier. Due to large drop across collector load resistors, the collectors work at relatively small voltages unless higher supply voltage is used to overcome this large drop. Power Amplifiers Transistor power amplifiers handle large signals. Many of them are driven so hard by the input large signal that collector current is either cut-off or is in the saturation region during a large portion of the input cycle. Therefore, such amplifiers are generally classified according to their mode of operation i. e. the portion of the input cycle during which the collector current is expected to flow. On this basis, they are classified as: (i) Class A power amplifier ( ii ) class B power amplifier ( iii ) class C power amplifier (i) Class A power amplifier If the collector current flows at all times during the full cycle of the signal, the power amplifier is known as Class A power amplifier.
The power amplifier must be biased in such a way that no part of the signal is cut off. Fig. ( i ) Shows circuit of class A power amplifier. Note that collector has a transformer as the load which is most common for all classes of power amplifiers. The use of transformer permits impedance matching, resulting in the transference of maximum power to the load e. g. loudspeaker. Fig. ( ii ) Shows the class A operation in terms of a. c. load line. The operating point Q is so selected that collector current flows at all times throughout the full cycle of the applied signal. As the output wave shape is exactly similar to the input wave shape, therefore, such amplifiers have least distortion. However, they have the disadvantage of low power output and low collector efficiency (about 35%). Class B power amplifier If the collector current flows only during the positive half-cycle of the input signal, it is called a class B power amplifier. In class B operation, the transistor bias is so adjusted that zero signal collector current is zero i. e. no biasing circuit is needed at all. During the positive half-cycle of the signal, the input circuit is forward biased and hence collector current flows. However, during the negative half-cycle of the signal, the input circuit is reverse biased and no collector current flows. Biasing condition for class-B operation has been shown both on current transfer characteristic and output characteristic for CE configuration in Fig. below
The operating point Q shall be located at collector cut off voltage. It is easy to see that output from a class B amplifier is amplified half-wave rectification. In a class B amplifier, the negative half-cycle of the signal is cut off and hence a severe distortion occurs. However, class B amplifiers provide higher power output and collector efficiency (50 - 60%). Such amplifiers are mostly used for power amplification in push-pull arrangement. In such an arrangement, 2 transistors are used in class B operation. One transistor amplifies the positive half cycle of the signal while the other amplifies the negative half-cycle. Power Relations for Class-B Operation (i) Input DC Power: Pin(dc) = VCC Idc where Idc is the average or dc current drawn from the supply. If IC(max) is the maximum or peak value of collector or output current, then
(ii) DC Power Loss in Load:
(iii) AC Power Output in Load :
Where I = rms value of output ac current V = rms value of output ac voltage
V m = maximum value of output ac voltage (iv) DC Power Loss in Collector Region or Transistor
Class-B Push-Pull Amplifier It employs two identical transistors operating as a single-stage of amplification****. The base of the two CE -connected transistors have been connected to the opposite ends of the secondary of the input transformer T 1 and collectors to the opposite ends of the primary of the output transformer T 2. For getting a balanced circuit, the two emitters have been returned to the centre tap of T 1 secondary and V CC connected to the centre tap on the primary of T 2. Since zero bias is required for cut-off, the two bases have been earthed. A push-pull amplifier is sometimes referred to as balanced amplifier****.
Principle of Operation During the positive half-cycle of the signal, A is turned ON because its base is driven positive. It draws collector current I C -1 in the upward direction from V CC. Meanwhile, transistor B remains OFF because its base has negative voltage. Hence, IC -2 = 0. Obviously, one positive half-cycle of the output signal appears across secondary load RL of T 2. During negative half-cycle of the input signal, B conducts whereas A remains OFF. Hence, IC -2 is taken by B but IC -1 = 0. Now, negative output half-cycle is produced across R L because IC -2 is pulled down through secondary of T 2. It is obvious that in the absence of input signal, neither A nor B draws any collector current. Hence, there is no drain on V CC battery.
In simple words, crossover distortion occurs as a result of one transistor cutting off before the other begins conducting. Class-C Amplifier In such amplifiers, the active device i.e. the transistor is biased much beyond cut- off. Hence,
1. Output current flows only during a part of the possible half-cycle of the input signal, 2. There is no output current flow during any part of the negative half-cycle of the input signal, 3. Output signal has hardly any resemblance with the input signal. It consists of short pulses only, 4. class-C amplifiers have high circuit efficiency of about 85 to 90%. Because of his distortion, class-C amplifiers are not used for audio-frequency work. They are used for high-power output at radio frequencies ( i.e. RF amplifiers) where harmonic distortion can be removed by simple circuits. In reality, they are used as high-frequency power switchers in radio transmitters rather than as amplifiers. Tuned Amplifier The gain of a transistor amplifier depends directly on the value of its load impedance. Such high impedance can be obtained by using a high- Q tuned or
resonant LC circuit as load. The frequency response curve of the amplifier assumes the same shape as the resonance curve of the tuned circuit. Obviously, only a narrow band of frequencies around the resonant frequency fo would be amplified well whereas other frequencies would be discriminated against. Non-linear distortion is eliminated because of high selectivity of the load impedance. Hence, output is nearly sinusodial. With the removal of distortion, high amplifier efficiency can be achieved by operating the transistor in its nonlinear region.
Distortion in Amplifiers Amplifiers are supposed to produce an output which does not differ from the input in any respect except amplitude i.e. the output is expected to be larger than the input. In actual practice, it is impossible to construct such an ideal amplifier whose output is an exact duplication or replica of the input. The output is always found to differ from the input either in its waveform or frequency content. This difference between the output and input of an amplifier is called distortion. The amplifier distortions may be divided into two broad categories depending on the region of the characteristic used by the transistor and the associated circuit and device reactances. (a) non-linear distortion This occurs when transistor operates in the non-linear region of its characteristic. (i) When we visualize the signal in time domain, it is called amplitude distortion or waveform distortion;
(ii) When we think of the signal in the frequency domain, it is called harmonic distortion. This distortion occurs when input signal is of one frequency, say, a pure sine wave. (iii) Intermodulation (IM) distortion—when input signal has more than one frequency (like speech). Non-linear distortion occurs in the case of large-signal inputs when the active device is driven into the non-linear regions of its characteristic. (b) linear distortion It occurs even when the active device is working on linear part of its characteristic with small-signal inputs****. It is primarily due to frequency- dependent reactances associated with the circuit or active device itself and occurs when input signal is composite i.e. has signals of different frequencies (say, fundamental and its harmonics). However, output contains no frequencies other than those at the input. It may be further subdivided into: (i) frequency distortion —due to unequal amplification of different frequencies present in the input signal; (ii) Phase or delay distortion —due to unequal phase shift of various signal components. Hybrid parameters Every linear circuit having input and output terminals can be analyzed by four parameters (one measured in ohm, one in mho and two dimensionless) called hybrid or h Parameters. Hybrid means “mixed”. Since these parameters have mixed dimensions, they are called hybrid parameters.
This circuit has input voltage and current labeled v 1 and i 1. This circuit also has output voltage and current labelled v 2 and i 2. Note that both input and output currents ( i 1 and i 2) are assumed to flow into the box ; input and output voltages ( v 1 and v 2) are assumed positive from the upper to the lower terminals.
When we analyse circuits in which the voltages are of opposite polarity or where the currents flow out of the box, we simply treat these voltages and currents as negative quantities. It can be proved by advanced circuit theory that voltages and currents in Fig. above can be related by the following sets of equations: v 1 = h 11 i 1 + h 12 v 2………………………………….. (i) i 2 = h 21 i 1 + h 22 v 2 ………………………………….. (ii) In these equations, the hs are fixed constants for a given circuit and are called h parameters. h 11 has the dimension of ohm, h 12 is dimensionless, h 21 is dimensionless, h 22 has the dimension of mho. Following points may be noted about h parameters: (i) Every linear circuit has four h parameters ; one having dimension of ohm, one having dimension of mho and two dimensionless (ii) The h parameters of a given circuit are constant. If we change the circuit, h parameters would also change. (iii) Suppose that in a particular linear circuit, voltages and currents are related as under: v 1 = 10 i 1 + 6 v 2 i 2 = 4 i 1 + 3 v 2 Here we can say that the circuit has h parameters given by h 11 = 10 Ω ; h 12 = 6 ; h 21 = 4 and h 22 = 3 Ʊ Determination of h Parameters The major reason for the use of h parameters is the relative ease with which they can be measured. The h parameters of a circuit can be found out as under: (i) If we short-circuit the output terminals, we can say that output voltage v 2 = 0. Putting v 2 = 0 in equations ( i ) and ( ii ), we get, v 1 = h 11 i 1 + h 12 x 0 i 2 = h 21 i 1 + h 22 x 0 h 11 = i for v 2 = 0 i.e. output shorted And h 21 = for v 2 = 0 i.e. output shorted Let us now turn to the physical meaning of h 11 and h 21. Since h 11 is a ratio of voltage and current ( i.e. v 1/ i 1), it is an impedance and is called “ input impedance
