Lab 3: Amplifier Circuits LAB 3 AMPLIFIER CIRCUITS. (Small-signal Low-frequency voltage amplifier) EC0222 Electronic Circuits Lab Manual

Transcription

1 Lab 3: Amplifier ircuits 3. LAB 3 AMPLIFIR IRUITS xperiment 3.: xperiment 3.2: Transistor amplifier (Small-signal Low-frequency voltage amplifier) op-amp feedback amplifier 0222 lectronic ircuits Lab Manual

2 xperiment 3.: ommon mitter Amplifier 3.2 XPRIMNT 3. OMMON MITTR AMPLIFIR 3.. OBJTI a. To sketc a common emitter amplifier circuit and explain te operation of te circuit. b. To analyze te amplifier circuit to determine input resistance output resistance, voltage gain, current gain and power gain. c. To design a amplifier ircuit for te given specifications. d. To observe wit an oscilloscope, te transient signal voltages of te input and output of te amplifier. e. To measure te voltage gain of te amplifier over and range of frequencies and plot te frequency response curve. f. To determine te values of lower and upper 3-dB frequencies and 3-dB bandwidt. g. To trouble soot a non-operational amplifier. (i) To make a dynamic test wic will determine weter te ac amplifier is operating properly? (ii) To consider dc voltage and resistance norms at test points in te amplifier wic is operating properly, and to draw inferences as to te nature of te trouble from te voltage and resistance measurements in a defective amplifier HARDWAR RQUIRD a. Power supply : ariable regulated low voltage dc source b. quipments : AFO, RO, DMM c. Resistors : d. apacitors : e. Semiconductors : B 07 (or equivalent) f. Miscellaneous : Breadboard and wires lectronic ircuits Lab Manual

3 xperiment 3.: ommon mitter Amplifier PR LAB QUSTIONS. alculate te base bias voltage for te circuit sown below wen no signal source is present, and wen signal source is directly connected. cc2 33k 2.2k 2 rs k s 5k R 2.7k Fig. (a) 2. alculate te transistor collector voltage for te circuit of problem wit 2 present, and wit R L directly connected. 3. Te circuit sown in problem as te following transistor parameters: iekω, fe 85, oe 2µS. alculate Z i, Z O & A. 4. For te circuit in problem, recalculate Z i &A wen te bypass capacitor is removed from R. 5. For a circuit wit voltage divider bias, a bypass emitter resistor, a capacitor coupled signal source, and a capacitor-coupled load, (i) Sketc te dc equivalent circuit and write expressions for te dc voltages and currents. (ii) Sketc te ac equivalent circuit and approximate -parameter equivalent circuit and write expressions for Zi, Zo, Av, A i and A P. (iii) Draw ac and dc load lines on te output caracteristics of configuration and explain teir significances. 6. Te circuit as te following component values and parameter values: ie.4kω, fe 55, R L 47K, 2, r s 600Ω and Z O 3.9KΩ. Sketc te circuit and determine suitable resistor and capacitor values. Also, calculate Zi, Zo, A, A i and A P for te circuit designed lectronic ircuits Lab Manual

4 xperiment 3.: ommon mitter Amplifier THORY Amplifier is an electronic circuit tat is used to raise te strengt of a weak signal. Te process of raising te strengt of a weak signal is known as amplification. One importance requirement during amplification is tat only te magnitude of te signal sould increase and tere sould be no cange in signal sape. Te transistor is used for amplification. Wen a transistor is used as an amplifier, te first step is to coose a proper configuration in wic device is to be used. Ten te transistor is biased to get te desired Q-point. Te signal is applied to te amplifier input and gain is acieved amplifier operation onsider a amplifier circuit as sown in fig. 3-- cc 2 s R Fig. 3--(a) amplifier circuit (b) Waveforms for amplifier Wen te capacitors are regarded as ac sort circuits, it is seen tat te circuit input terminals are te transistor base and emitter, and te output terminals are te collector and te emitter. So, te emitter terminal is common to bot input and output, and te circuit configuration is termed ommon mitter (). From te voltage waveforms for te circuit sown in Fig. 3-- (a) it is seen tat tere is a 80 o pase sift between te input and output waveforms. Tis can be understood by considering te effect of a positive going input signal. Wen S increases in a positive direction, it increases te transistor B. Te increase in B raises te level of I, tereby increasing te drop across, and tus reducing te level of te. Te canging level of is capacitor-coupled to te circuit output to produce te ac output voltage, O. As S increases in a positive direction, O 0222 lectronic ircuits Lab Manual

5 xperiment 3.: ommon mitter Amplifier 3.5 goes in a negative direction. Similarly, Wen S canges in a negative direction, te resultant decrease in B reduces te I level, tereby reducing R, and producing a positive going output. Te circuit in Fig. 3--(a) as input impedance (Z i ) and output impedance (Z O ). Tese can cause voltage division of te circuit input and output voltages. So, for most transistor circuits Z i and Z O are important parameters. Te circuit voltage amplification (A ), or voltage gain, depends on te transistor parameters and on resistor R and R L amplifier circuit elements and teir functions (i) (ii) Biasing circuit: Te resistances R, R 2 and R form te biasing and stabilization circuit. Te biasing circuit must establis a proper operating point, oterwise a part of te negative alfcycle of te signal may be cut-off in te output. Input capacitor, : An electrolyte capacitor is used to couple te signal to te base of te transistor. If it is not used, te signal source resistance, rs will come across R 2 and tus cange te bias. allows only ac signal to flow but isolates te signal source from R 2 (iii) mitter bypass capacitor, : An mitter bypass capacitor, is used parallel wit R to provide low reactance pat to te amplified ac signal. If it is not used, ten ac amplified ac signal following troug R will cause a voltage drop across it, tereby reducing te output voltage. (iv) oupling capacitor, 2 : Te coupling capacitor, 2 couples one stage of amplification to te next stage. If it is not used, te bias conditions of te next stage will be drastically canged due to te sunting effect of R. Tis is because R will come in parallel wit te upper resistance of te biasing network of te next stage, tereby altering te biasing conditions of te latter. In sort, te coupling capacitor 2 isolates te dc of one stage from te next stage, but allows te passage of ac signal amplifier circuit currents (i) Base current i B I B +i b were I B dc base current wen no signal is applied, i b ac base wen ac signal is applied and i B total base current 0222 lectronic ircuits Lab Manual

6 xperiment 3.: ommon mitter Amplifier 3.6 (ii) ollector current i I +i c were I zero signal collector current, i c ac collector current wen ac signal is applied and i total collector current (iii) mitter urrent i I + ie were I Zero signal emitter current, Ie ac emitter current wen ac signal is applied and i total emitter current It is useful to keep in mind tat I I B + I and i e i b +i c Also, I I and i e i c amplifier frequency response Te voltage gain of an amplifier varies wit signal frequency. It is because reactances of te capacitors in te circuit canges wit signal frequency and ence affects te output voltage. Te curve between voltage gain and signal frequency of an amplifier is known a frequency response. Figure 3--2 sows te frequency response of a typical amplifier. Fig Frequency response of amplifier It is clear tat te voltage gain drops off at low (< f L ) and ig (> f H ) frequencies wereas it is uniform over mid-frequency range (f L to f H ). (i) At low frequencies (< f L ), te reactance of coupling capacitor is quite ig and ence very small part of signal will pass from amplifier stage to te load. Moreover, cannot sunt te R effectively because of its large reactance at low frequencies. Tese two factors cause a falling of voltage gain at low frequencies. (ii) At ig frequencies (> f H ), te reactance of 2 is very small and it beaves as a sort circuit. Tis increases te loading effect of amplifier stage and serves to reduce te voltage gain lectronic ircuits Lab Manual

7 xperiment 3.: ommon mitter Amplifier 3.7 Moreover, at ig frequency, capacitive reactance of base-emitters junction is low wic increases te base current. Tese reduce te current amplification factorβ. Due to tese two reasons, te voltage gain drops off at ig frequency. (iii) At mid frequencies (f L to f H ), te voltage gain of te amplifier is constant. Te effect of coupling capacitor 2 in tis frequency range is suc as to maintain a uniform voltage gain. Tus, as te frequency increases in tis range, reactance of decreases wic tend to increase te gain. However, at te same time, lower reactance means iger almost cancel eac oter, resulting in a uniform fain at mid-frequency amplifier analysis Te first step in ac analysis of amplifier circuit is to draw ac equivalent circuit by reducing all dc sources to zero and sorting all te capacitors. Fig sows te ac equivalent circuit. rs s Fig A equivalent circuit for amplifier Te next step in te ac analysis is to draw -parameter circuit by replacing te transistor in te ac equivalent circuit wit its -parameter model. Fig sows te -parameter equivalent circuit for circuit. rs s + i Zi. Zb. B R3 feib /oe Zc. Zo. - o - + Fig parameter equivalent circuit for amplifier Te typical circuit performance is summarized below: Device input impedance, Z b ie (3--) ircuit input impedance, Z i Z (3--2) b 0222 lectronic ircuits Lab Manual

8 xperiment 3.: ommon mitter Amplifier 3.8 Device output impedance, ircuit output impedance, Z Z O (3--3) oe R Z R (3--4) fe ircuit voltage gain, A ( R ) (3--5) ircuit current gain, ie fer RB A i R + R )( R + ) (3--6) ( L ie ircuit power gain, A P A x A i (3--7) amplifier circuit design Design of circuit normally commences wit a specification of supply voltage, minimum voltage gain, frequency response, source impedance, load impedance, stability factor and Q-point. cc 2 s R Fig amplifier circuit (redrawn from fig. 3--(a)) Selection of I, R and R fe From eq. 3--5, A ( R ) ie For satisfactory transistor operation, Ic sould not be less tan 500µA. A good minimum Ic to aim for is ma. Te sould typically be around 3 to ensure tat te transistor operates linearly and to allow a collector voltage swing of ± wic is usually adequate for small-signal amplifier Note: R sould normally be very muc larger tan R L, so tat R L as little effect on voltage gain. Select 5 for good bias stability in most circumstances lectronic ircuits Lab Manual

9 xperiment 3.: ommon mitter Amplifier 3.9 Note: Wen >> B, will be only sligtly affected by any variation in B (due to temperature cange or oter effects) Once, and Ic are selected, R is determined as R Ten, R and R are calculated as Selection of bias resistors R R and I R I As discussed in lab-, experiment-., section-., selection of voltage divider current (I 2 ) as I /0 gives good bias stability and reasonably ig input resistance. Te bias resistors are calculated as R 2 B and R I 2 I 2 B Selecting R 2 0R gives I 2 I /0 te precise level of I 2 can be calculated as I 2 B /R 2 and tis can be used in te equation for R. Selection of bypass capacitor, Basically te capacitor values are calculated at te lowest signal frequency tat te circuit is required to amplify. Tis frequency is te lower cut-off frequency, f L. oose X ie at f L for calculation to give te smallest value for te bypass capacitor. + fe Selection of coupling capacitors, and 2 Te coupling capacitors and 2 sould ave a negligible effect on te frequency response of te circuit. To minimize te effects of and 2, te reactance of eac coupling capacitor is selected to be approximately equal to one-tent of te impedance in series wit it at te lowest operating frequency of te circuit (f L ). X X 3 Z i + rs 0 Z O + R 0 L Usually, R L >> Z O and often Z i >> r S, so tat Z O and r S can be omitted in te above equations lectronic ircuits Lab Manual

10 xperiment 3.: ommon mitter Amplifier 3.0 Design problem (i) Design a single stage transistor amplifier using B07 transistor wit cc 5, Q 5, 3, R L 47KΩ and f L 00Hz. (ii) Determine Z i, Z O, A, A i and A P for te circuit designed in problem (i). Procedure Given 5, 5, 3, R L 47kΩ and f L 00Hz. Te data seet of B07 transistor sows: ie 3kΩ and F 90 Selection of R R << R L so tat R L will ave little effect on te circuit voltage gain. Select R Selection of R R I Were I 47K 4. KΩ (Standard value) I R (5 5 3) R. 4 R 3 R 2. 4KΩ (use a standard 2.2 kω).4ma Selection of R and R 2 Selection of voltage divider current I 2 as I /0 gives good bias stability and reasonably ig input resistance Selecting R 2 0 R gives I 2 I /0 i.e., 0 2KΩ 22KΩ (standard value) I.4mA and I 2 40µ A 0 0 R B 5 ( B + ) 5 ( ) 66.43KΩ (use standard 68kΩ) I 40µ A 40µ A 2 Selection of and 2 Te coupling capacitors and 2 sould ave negligible effect on te frequency response of te circuit. So, te reactance of eac coupling capacitor is selected to be approximately equal to /0 t of te impedance in series wit it at te lowest operating frequency for te circuit. ma 0222 lectronic ircuits Lab Manual

11 xperiment 3.: ommon mitter Amplifier 3. X Z R i 0 R 0 68K 22K 3K 2 ie 2πf X 0 L 2 π µ F 47K X KΩ µ F (use a standard 0.33µF) 2πf X 2 π 00 2 L 2 Selection of ie X + fe 3KΩ F (use a standard 00µF) 2πf X 2 π µ L alculation of Z i, Z O, A, A i and A P Input impedance, Z i ie 68K 22K 3K 2. 54KΩ Output impedance, Z O R 4.7kΩ fe 90 oltage gain, A ( R ) ( 47K) K ie fer RB 90 (68K 22K) urrent gain, Ai ( R + R )( R + ) ( + 47K)( + 3K) Power gain, A P A x A i X K L ie cc5 68K uF s 6uF 22K R 2.2K 47K Fig amplifier circuit wit design values of components 0222 lectronic ircuits Lab Manual

12 xperiment 3.: ommon mitter Amplifier TROUBL SHOOTING A AMPLIFIR Wen you are faced wit aving to trouble soot a circuit, te first ting you need is a scematic wit te proper dc and signal voltages labeled. You must know wat te correct voltages in te circuit sould be before you can identify an incorrect voltage. cc5 0dc 6uF s 68K 5.7dc B 22K 2 5dc 0.33uF 3dc R 2.2K 0dc 47K Fig amplifier wit correct voltages indicated Instability After te circuit as been constructed, te power supply sould be set to te appropriate voltage and ten connected and switced on. An oscilloscope sould be connected to monitor te output of te amplifier to ceck tat te circuit is not oscillating. If te circuit is oscilloscope te oscillations must be stopped before proceeding furter. Amplifier instability can be te result of incorrect design or poor circuit layout. It can also be caused by feedback along te conductors from te power supply to te circuit. To stabilize an unstable amplifier, commence by connecting a 0.0µf decoupling capacitor from te positive supply line to ground. Were a plus minus supply is used, connect capacitor from eac supply line to ground. If te circuit is still unstable, small sunt capacitors sould be connected from te transistor collect terminal to ground, or between collector and base lectronic ircuits Lab Manual

13 xperiment 3.: ommon mitter Amplifier 3.3 cc decoupling capacitor sunt capacitor 2 Fig amplifier circuit wit decoupling capacitors and small sunt capacitors to eliminate oscillations in an unstable D oltage measurements Once it is establised tat te circuit is stable te next step is to measure te dc voltage levels at all transistor terminals. A digital multimeter (DMM) sould be used IF te dc vulgates are not satisfactory; tey must be corrected before proceeding furter. cc5 68K 5dc 5.7dc B 3dc 22K R 2.2K Fig quivalent dc voltages for te amplifier designed Te measured dc voltage levels sould at least sow tat (i) Te B junction of te transistor sould be forward biased. Te B may vary from 0.65 to 0.75 (for silicon transistor). (ii) Te B may range from approximately alf te supply voltage to almost te full battery voltage lectronic ircuits Lab Manual

14 xperiment 3.: ommon mitter Amplifier 3.4 Inferences from D voltage measurements D voltage readings are used to draw inferences of proper or improper junctioning in transistor circuits. To sown tis, we will assume certain abnormal voltages in te circuit of Figure 3--9 and analyze te possible causes of tese voltages Possible troubles could be (a) an open in te emitter circuit, (b) an open in te base circuit, (c) base-emitter sort circuit, (d) base sort-circuited to ground Possible troubles include (a) open collector circuit, and (b) collector sort-circuited to ground. 3. collector to emitter sort-circuited Possible troubles include (a) tere is no current following in te emitter, or (b) te emitter is sort-circuited to ground. Dynamic test Wen satisfactory dc levels are establised trougout te circuit, dynamic test may proceed. Suppose tat tere is no signal at te output terminals of te amplifier for a specified signal input. Te dynamic signal-tracing metod can be used to determine wat is wrong wit te circuit. Te procedure is as follows. A sine-wave signal no larger tan te amplifier can andle is injected into te input terminals of te amplifier and observed at tese terminals wit an oscilloscope. If te observed signal is normal, te oscilloscope probe is moved to point B (base) of te amplifier. (Refer Fig.3--7) Te sine-wave signal at tis point sould be approximately te same as at te input terminals if te amplifier input is normal. If tere is no signal at te base, two possible reasons exist. Te first is tat te capacitor is open. Te second is tat te base terminal is sort circuited to ground. An open capacitor may readily be found by connecting a 0-µf capacitor across and observing wit oscilloscope te output signal. If an output signal appears, tis indicates tat capacitor is open. If no signal appears at output terminals, te oscilloscope probe is connected to te base. No signal indicates a sort circuit in te base circuit. We may also determine if capacitor 2 is open by signal tracing. Assume tat te input circuit, including, is found to be operating correctly, but tat tere is no signal at te output terminals of te amplifier. Te oscilloscope probe is ten connected directly to te collector of transistor. If normal signal appears at te collector, but non exists at te output terminals, we know tat 2 is open lectronic ircuits Lab Manual

15 xperiment 3.: ommon mitter Amplifier 3.5 Resistance measurements Resistance measurements in transistor circuits, always made wit power turned off, are elpful in determining defective components. Te resistance measured at te base B to G sould be 22kΩ. Te resistance measured at te emitter to G sould be 2.2kΩ. Te resistance from collector to G would be te sum of R, R and R 2 ; in tis case 94.7kΩ. Tese value, ten, are te standard or norm for te circuit of Fig cc0 68K B DMM 22K R 2.2K Fig Resistance measurements in transistor circuit Te most obvious resistor defects are opens, wic can be spotted very easily. For example, if te resistance measured from to G is infinite ( ), ten eiter R is open, or te connective wiring is open. So, te resistance cecks and continuity cecks will reveal were te defect is XPRIMNT. D voltage measurements. Assemble te dc equivalent of te amplifier circuit you ave designed, as sown in fig Use te B 07 transistor, or equivalent..2 Measure Q point and oter transistor terminal voltages and currents as per te procedure given in experiment.. Tabulate te readings in table 3--. ompare te measured values wit te calculated values. oltage and urrent quantities Measured values alculated values Table 3-- D oltage measurements 0222 lectronic ircuits Lab Manual

16 xperiment 3.: ommon mitter Amplifier Transient voltage measurements 2. Feed 00m (peak-to-peak) sinusoidal signal at KHz frequency as te input signal s to te circuit sown in Fig Observe te input and output voltages simultaneously on a RO. Note down te amplitude, frequency and pase difference between te two voltages in te table ompute te gain of te amplifier circuit and compare it wit te calculated value. 2.3 Plot on a linear grap te transient voltage of te input & output of te amplifier circuit Particulars Amplitude (volts) Time period (msec) Frequency (Hz) Input oltage Output oltage Table 3--2 Transient oltage measurements 3. Frequency response curve measurements 3. In te above assembled circuit, keep te magnitude of te source same, ie.,00m and decrease te frequency from KHz and measure voltage gain of te amplifier at eac frequency. Now increase te frequency from KHz to MHz and measure te voltage gain of te amplifier at eac frequency. Take at least 5 readings on eiter side of te KHz frequency. Tabulate te reading in table Plot on a semi log grap seet te frequency response (voltage gain vs frequency) curve using te above measurements. 3.3 From te plot, determine te values of (a) Mid band voltage gain, A (mid), (b) Lower cut-off frequency, (c) Upper cut-off frequency and (d) Bandwidt. Input oltage, S m Signal Frequency Output oltage (Hz) (volts) oltage Gain, O oltage Gain, db ( O S ) 20log0 ( O S ) Table 3--3 Frequency response curve measurements 0222 lectronic ircuits Lab Manual

17 xperiment 3.: ommon mitter Amplifier Resistance measurements 4. In te above assembled circuit, keep te magnitude and frequency of te source same, ie., 00m pp at KHz frequency. 4.2 onnect a potentiometer R in (variable resistance) in series wit te circuit input terminal and te signal source, as sown in fig onnect a two-cannel RO to simultaneously monitor te input and output signal voltage waveforms. cc5 68K 2 R4 0.33uF 6uF 0k R5 s 22K R 2.2K 50k Fig. 3-- xperimental circuit to measure input and output impedances 4.4 Adjust te POT until a new output signal O, equal to one-alf te original measured value of O is obtained. Now, remove R in from te circuit and measure its resistance using DMM. Te measured value in oms equals te input impedance, Z i. 4.5 To measure te output impedance Z O of te amplifier, connect a potentiometer R out to te output circuit. 4.6 Adjust te POT until a new output signal O, equal to one-alf te original measured value of O is obtained. Now, remove R out from te circuit and measure its resistance using DMM. Te measured value in oms equals te output impedance, Zo. Tabulate te readings in table Particulars Measured alue alculated alue Input Impedance, Z i Output Impdance, Z O Table 3--4 Resistance Measurements 0222 lectronic ircuits Lab Manual

18 xperiment 3.: ommon mitter Amplifier Post Lab questions. eck your understanding by answering tese questions. (a) Te voltage gain of te amplifier in Fig. 3-- is 50. Wen is opened te gain of te amplifier sould (increase, decrease, remain te same) (b) Te circuit of fig. 3-- must amplify sine-wave signals in te frequency range 20 to 20,000 Hz. Te design value of R is 2200Ω. Te igest value X of wic will act a good bypass for tis amplifier is Ω at Hz. Te value of is µf. (c) In an audio amplifier te collector-to-base must be (forward, reverse) biased. (d) Te ac signal voltage measured at te base of a amplifier is 50m. Te output signal voltage measured at te collector is 2.5. Te voltage gain of te amplifier is (e) Te rms voltage measured at te collector of te amplifier in Fig. 3-- is 4.6 witout load. Wen a 250Ω load is connected across te output, te rms voltage measured at te collector load is 2.3. Te output impedance of te circuit is Ω. (f) A amplifier as a gain of 50, an input impedance of 000Ω, and an output impedance of 200Ω. Te power gain of tis amplifier is. (g) Te decibel power gain of te amplifier in question (f) is db. () A sine wave injected into te base of te transistor Fig results in a normal output. Wen te generator leads are moved to te input, no signal appears in te output. Te most probable cause of trouble is a. (i) In te circuit sown in Fig. 3--9, a tecnician measures 0 at collector. 2. I is. (j) If R in Fig is open, a resistance ceck from to G will measure Ω. (k) If te emitter is sort circuited to base in Fig. 3--9, a resistance ceck from te base to ground will sow approximately Ω. (l) For te conditions in question (k), te voltage at te collector will measure (m) If R in Fig is open, te voltage measured from to G will be 2. How do coupling capacitors and 2 affect te frequency response? Wy? 3. Wat is te effect on te amplifier performance of omitting R? 4. Wat is te effect on input impedance of removing bypass capacitor? 5. (a) Wat is te pase relationsip between te input and output signals of a amplifier? (b) Was tis relationsip confirmed by te results of your experiments? xplain ow. 6. Is te output impedance of a amplifier a fixed quantity? onfirm your answer by referring specifically to any substantiating data in tis experiment lectronic ircuits Lab Manual