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AB64 RC-Coupled Amplifier with Feedback

Operating Manual Ver.1.1

An ISO 9001 : 2000 company

94-101, Electronic Complex Pardesipura, Indore- 452010, India Tel : 91-731- 2570301/02, 4211100 Fax: 91- 731- 2555643 e mail : [email protected] Website : www.scientech.bz Toll free : 1800-103-5050

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AB64

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AB64


RoHS Compliance

Scientech Products are RoHS Complied. RoHS Directive concerns with the restrictive use of Hazardous substances (Pb, Cd, Cr, Hg, Br compounds) in electric and electronic equipments. Scientech products are “Lead Free” and “Environment Friendly”. It is mandatory that service engineers use lead free solder wire and use the soldering irons upto (25 W) that reach a temperature of 450°C at the tip as the melting temperature of the unleaded solder is higher than the leaded solder.

AB64 RC-Coupled Amplifier with Feedback

Table of Contents

1. Introduction 4 2. Theory 6

3. Experiments • Experiment 1 9

Study of effect of negative feedback on gain of RC- Coupled Amplifier

• Experiment 2 11 Study of frequency response of RC–Coupled Amplifier with & without feedback

• Experiment 3 14 Measuring the gain and bandwidth of RC-Coupled Amplifier with different feedback factor

4. Data Sheet 16 5. Warranty 18

6. List of Accessories 18

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Introduction AB64 is a compact, ready to use RC-Coupled Amplifier with Feedback experiment board. This board is useful for students to understand the working and operation of RC-Coupled Amplifier with and without feedback. It can be used as stand alone unit with external DC power supply or can be used with Scientech Analog Lab ST2612, which has built in DC Power Supply, AC power supply, function generator, modulation generator, continuity tester, toggle switches, and potentiometer.

List of Boards : Model Name AB01 Diode characteristics (Si, Zener, LED) AB02 Transistor characteristics (CB NPN) AB03 Transistor characteristics (CB PNP) AB04 Transistor characteristics (CE NPN) AB05 Transistor characteristics (CE PNP) AB06 Transistor characteristics (CC NPN) AB07 Transistor characteristics (CC PNP) AB08 FET characteristics AB09 Rectifier Circuits AB10 Wheatstone bridge AB11 Maxwell’s Bridge AB12 De Sauty’s Bridge AB13 Schering Bridge AB14 Darlington Pair AB15 Common Emitter Amplifier AB16 Common Collector Amplifier AB17 Common Base Amplifier AB18 RC-Coupled Amplifier AB19 Cascode Amplifier AB20 Direct Coupled Amplifier AB21 Class A Amplifier AB22 Class B Amplifier (push pull emitter follower) AB23 Class C Tuned Amplifier AB24 Transformer Coupled Amplifier AB25 Phase Locked Loop (FM Demodulator & Frequency Divider / Multiplier) AB26 FET Amplifier AB27 Voltage Controlled Oscillator AB28 Multivibrator (Mono stable/Astable) AB29 F-V and V-F Converter AB30 V-I and I-V Converter AB31 Zener Voltage Regulator AB32 Transistor Series Voltage Regulator AB33 Transistor Shunt Voltage Regulator

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AB35 DC Ammeter AB37 DC Ammeter (0-2mA) AB39 Instrumentation Amplifier AB41 Differential Amplifier (Transistorized) AB42 Operational Amplifier (Inverting / Non-inverting / Differentiator) AB43 Operational Amplifier (Adder/Scalar) AB44 Operational Amplifier (Integrator/ Differentiator) AB45 Schmitt Trigger and Comparator AB49 K Derived Filter AB51 Active filters (Low Pass and High Pass) AB52 Active Band Pass Filter AB54 Tschebyscheff Filter AB56 Fiber Optic Analog Link AB57 Owen’s Bridge AB58 Anderson’s Bridge AB59 Maxwell’s Inductance Bridge AB66 Wien Bridge Oscillators AB67 Colpitt Oscillator AB68 Hartley Oscillator AB80 RLC Series and RLC Parallel Resonance AB82 Thevenin’s and Maximum Power Transfer Theorem AB83 Reciprocity and Superposition Theorem AB84 Tellegen’s Theorem AB85 Norton’s theorem AB88 Diode Clipper AB89 Diode Clampers AB90 Two port network parameter AB91 Optical Transducer (Photovoltaic cell) AB92 Optical Transducer (Photoconductive cell/LDR) AB93 Optical Transducer (Phototransistor) AB96 Temperature Transducer (RTD & IC335) AB97 Temperature Transducer (Thermocouple) AB101 DSB Modulator and Demodulator AB102 SSB Modulator and Demodulator AB106 FM Modulator and Demodulator

and many more…………

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Theory Single amplifier circuits, such as a common base, common emitter and common collector amplifiers are seldom found alone, as a single stage amplifier, in any system. Generally, at least two or more than two stages are connected in cascade combination. If the output of one amplifier is connected (coupled) to the input of another amplifier the stages are said to be connected in "cascade". The advantage of cascaded amplifiers is to develop an output voltage larger than either stage alone can develop. In fact, the overall gain of the cascaded amplifiers (called system gain) is the product of each individual stage gain. Because of this the gain of a single stage is not as important as the system gain. Designers usually set individual stage gains relatively low to reduce signal distortion. One of the very important requirements to cascade one stage of amplifier to another is the impedance matching. When the output impedance of previous stage matches with the input impedance of its next stage, maximum power is transferred. One of the coupling methods to couple the two stages is RC-Coupling. RC-Coupling has the advantages of wide frequency response and relatively small cost and size. RC coupled amplifier is simple & low cost circuit .In these circuit voltage divider biasing is used for drive the transistors BC547. Here in the circuit coupling capacitor is using before an input. Since the impedance of the capacitor is inversely proportional to frequency, the capacitor effectively blocks DC voltage and transmits AC voltage. When the frequency is high enough, the capacitive reactance is much smaller than the résistance. So capacitor used for this purpose is called a coupling capacitor.

RC Coupled Amplifier Stages

Figure 1 An RC-Coupled is the coupling network. C1 is the coupling capacitor which connects the output of Ql to the input of Q2. R2 will develop the signal to be applied to the base of Q2. Cl acts as a limiting factor at low frequencies because its reactance increases with a decrease in frequency and some point will be reached when a voltage drop will appear across it. This will reduce the size of the signal being applied to Q2. At medium frequencies the reactance of Cl is so small that it can be considered a short to the signal. C1 will also isolate any DC voltage developed at the collector of Q1 from the DC bias developed at the base of Q2.

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In the figure shown below RC feedback amplifier figure 2 we have used two-stage amplifier with the negative feedback. There are different feedback types for constant gain purpose. Feedback is of two types, positive feedback and negative feedback. Positive feedback used in Oscillators and Negative feedback used in amplifier circuits. Without any feedback, output changes with atmospheric conditions. That’s why feedback is used for constant output.

In the given circuit feedback from output is connected at emitter of the first stage through a feedback capacitor and resistance, capacitor is just a coupling capacitor but resistance is providing the amount of the feedback, means the voltage appeared at the emitter of first stage. Here the emitter voltage VE1 depends upon the output voltage variation because of the feedback resistance RF, voltage (VE1) is RE1.VOUT/ (RE1+ RF). Here gain is directly proportional to the feedback resistance. With using feedback gain is decrease. The feedback factor is,

β = RE1/ (RE1+RF) …..……(1) Feedback Factor range is 0.01 to 1 and Gain with feedback is

AVF = AV / (1+ β AV) ………..(2)

RC coupled amplifier with feedback

Figure 2

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Bandwidth of an RC-Coupled Amplifier : Bandwidth is a term used to describe the band of frequencies at which a particular amplifier amplifies the given input effectively.

Figure 3

The f1 and f2 points are known as half power points. The half power points are the points at which the signal amplitude dropped to 0.707 (3 dB) of the maximum signal amplitude. Any frequency below f 1 or above f 2 point is not considered a usable output from the amplifier. The bandwidth of the amplifier is the difference between the f 1 and f 2 points. It is generally accepted that in an RC-Coupled Amplifier the fl point is established by the coupling capacitor and by-pass capacitors and the f 2 point is set by the "shunt" or "stray wire" capacitance. Using Negative feedback bandwidth increases. The bandwidth of RC-Coupled Amplifier is given by,

Bandwidth (B) = fH – fL ..……….(3)

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Experiment 1 Objective : Study of effect of negative feedback on gain of RC–Coupled Amplifier Equipments : 1. Analog board AB64 2. DC power supply +12V from external source or ST2612 Analog Lab 3. Function Generator ST4064 4. Oscilloscope 5. 2mm patch cords Circuit diagram : Circuit used to study the operation of RC-Coupled Amplifier with and without feedback is shown below:

Figure 4

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Procedure :

• Connect +/-12V variable DC power supply at the indicated position from external source or ST2612 Analog Lab.

1. Switch ‘On’ the power supply. 2. Connect 2Vp-p, 2 KHz Sine wave signal at the input (between points Vin and

ground) of amplifier of AB64 board and observe the same on Oscilloscope CH I.

3. Observe the output waveform between points Vout and ground on Oscilloscope CH II and note output voltage (VOUT1) peak to peak.

4. Calculate the Voltage Gain without feedback (AV1). 5. Vary the potentiometer RF and adjust it to 60K and calculate feedback factor β

using equation (1). 6. Connect point ‘a’ to point ‘b’ to make a negative feedback.

7. Observe the output waveform between points Vout and ground on Oscilloscope CH II and note output voltage (VOUT2) peak to peak.

8. Calculate the Voltage Gain with feedback (AVF). 9. Compare the gain obtained by the feedback and without feedback.

Observation Table :

Without feedback With feedback S. N.

Input voltage VIN (Vpp) Output voltage

VOUT1 (Vpp) Gain AV1

β Output voltage VOUT2 (Vpp)

Gain AVF

Result : Voltage Gain of amplifier without feedback [VOUT1/ VIN] =………… Voltage Gain of amplifier with feedback [VOUT2/ VIN] =………… Feedback factor is…………… Conclusion : We can see that using feedback overall gain decreases.

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Experiment 2 Objective : Study of Frequency Response of RC–Coupled Amplifier with and without feedback Equipments Needed : 1. Analog board AB64 2. DC power supply +12V from external source or ST2612 Analog Lab 3. Function Generator ST4064 4. Oscilloscope 5. 2 mm patch cords Circuit diagram : Circuit used to study the Frequency Response of RC-Coupled Amplifier is shown below:

Figure 5

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Procedure :

• Connect +12V variable DC power supply at their indicated position from external source or ST2612 Analog Lab.

1. Switch ‘On’ the power supply. 2. Connect 2Vp-p, 20Hz Sine wave signal at the input (between points Vin and

ground) of amplifier of AB64 board and observe the same on oscilloscope CH 1.

3. Observe the output waveform between points Vout and ground on Oscilloscope CH 2.

4. Increase the input frequency from lowest value and observe the output waveform amplitude on Oscilloscope.

5. Calculate gain in dB and plot a semi log graph between AV (dB) and Frequency. 6. Measure frequency range for which the output wave amplitude is 3dB down the

maximum amplitude on graph (this will give two values of frequency fL and fH, the lower 3dB frequency and higher 3dB frequency respectively) as shown in figure 5.

7. Calculate Bandwidth of RC-Coupled Amplifier without feedback using Equation 3.

8. Keep the potentiometer RF at 60K and calculate feedback factor β using Equation 1.

9. Connect the patch chord between ‘a’ and ‘b’.

10. Follow procedure from step 3 to 7. This will give a plot between AVF (dB) and Frequency.

11. Calculate Bandwidth of RC-Coupled Amplifier with feedback using Equation 3.

12. Compare the frequency response and Bandwidth of RC-Coupled Amplifier with & without feedback.

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Observation Table :

Without feedback With feedback β =……. S

N Freq

(Hz)

Amp. VOUT (Vpp)

AV(dB) 20log10

( VOUT /VIN)

Freq (Hz)

Amp VOUT (Vpp)

AVF(dB) 20log10

( VOUT / VIN)

Result : Without feedback, fL (lower 3dB frequency) = ……………………

fH (higher 3dB frequency) = …………………… Bandwidth (fH – fL) = ……………………

With feedback, Feedback factor = …………………….

fL (lower 3dB frequency) = …………………… fH (higher 3dB frequency) = ……………………

Bandwidth (fH – fL) = …………………….

Conclusion : With negative feedback gain decreases and bandwidth increases.

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Experiment 3 Objective : Measuring voltage gain and bandwidth of RC–Coupled Amplifier with different feedback factor Equipments Needed : 1. Analog board AB64 2. DC power supply +12V from external source or ST2612 Analog Lab 3. Function Generator ST4064 4. Oscilloscope 5. 2 mm patch cords Circuit diagram : Circuit used to study the Frequency Response of RC-Coupled Amplifier is shown below:

Figure 6

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Procedure :

• Connect +12V variable DC Power Supply at their indicated position from external source or ST2612 Analog Lab.

1. Switch ‘On’ the Power Supply. 2. Connect 2Vp-p, 2 KHz Sine wave signal at the input (between points Vin and

ground) of amplifier of AB64 board and observe the same on oscilloscope CH 1.

3. Set potentiometer RF at different value 5K, 15K, 30K, 50K and 60K respectively and correspondingly observe the output waveform on Oscilloscope CH 2 and calculate feedback factor (β), gain (AVF) and bandwidth (BW) similarly as done in last experiments.

4. Compare gain and bandwidth of RC-Coupled Amplifier with different feedback factor.

Observation Table : SN

Input voltage (Vpp)

Feedback resistance

(RF)

Feedback factor(β)

RE1/(RE1+RF)

Output voltage (Vpp)

Gain (AVF)

Bandwidth (BW)

Result : Gain AVF = …………….. Bandwidth BW = ……………..

Conclusion : With increase the feedback factor gain is decreases and bandwidth is increases.

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Data Sheet

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AB64


Warranty 1. We guarantee the product against all manufacturing defects for 24 months from

the date of sale by us or through our dealers. Consumables like dry cell etc. are not covered under warranty.

2. The guarantee will become void, if

a) The product is not operated as per the instruction given in the operating manual.

b) The agreed payment terms and other conditions of sale are not followed.

c) The customer resells the instrument to another party. d) Any attempt is made to service and modify the instrument.

3. The non-working of the product is to be communicated to us immediately giving full details of the complaints and defects noticed specifically mentioning the type, serial number of the product and date of purchase etc.

4. The repair work will be carried out, provided the product is dispatched securely packed and insured. The transportation charges shall be borne by the customer.

For any Technical Problem Please Contact us at [email protected]

List of Accessories

1. 2 mm Patch Cords (Red) ...................................................................... 3 Nos. 2. 2 mm Patch Cord (Blue) .........................................................................1 No. 3. 2 mm Patch Cord (Black) ..................................................................... 3 Nos. 4. e-Manual.................................................................................................1 No.

Updated 26-03-2009

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