Exp. No #6 FREQUENCY RESPONSE OF COMMON COLLECTOR AMPLIFIER OBJECTIVE The purpose of the experiment is to analyze and plot the frequency response of a common collector amplifier. EQUIPMENT AND COMPONENTS USED 30 MHz Dual Channel Cathode Ray Oscilloscope 3 MHz Function Generator 0-30 V dc regulated power supply 4 ½ digit Digital Multimeter Transistor BC107 Resistors ¼W Electrolytic Capacitors Breadboard and Connecting wires BNC Cables and Probes THEORY In common collector amplifier, the external load is capacitor-coupled to the transistor emitter terminal. The important feature of common collector amplifier is that, its input resistance is very large and output resistance is small compared to other configurations. Its voltage gain is close to unity. There is no phase inversion between the input and output. Common collector amplifier is called as emitter follower as the phase of the output signal at emitter follows the phase of the input signal at base. CC amplifiers are used for impedance matching applications FURTHER READING 1. Robert Boylstad, Louis Nashelsky, Electronic Devices and Circuit Theory, PHI, 2008. 2. James Cox, Fundamentals of Linear Electronics: Integrated and Discrete, Delmar Thomson Learning, 2nd edition, 2001. 3. Theodore F.Bogart, Jeffrey S.Beasley, Electron Devices and Circuits, PHI. 4. Robert Diffenderfer, Electronic Devices, Delmar Cengage Learning, 2005. 57
CIRCUIT DIAGRAM Figure1. Common Collector Amplifier PRELAB 1. Design a common collector amplifier as per the design requirements given. Use SPICE tool to create the amplifier and observe the dc operating conditions. 2. Obtain a plot of the frequency response of the common collector amplifier over the frequency range from 1 Hz through 10 khz. Observe the mid-band gain and bandwidth. 58
Exp. No.: Date: FREQUENCY RESPONSE OF COMMON COLLECTOR AMPLIFIER OBJECTIVE DESIGN Select the general purpose transistor BC107. Specifications of BC107 Type: NPN Nominal ratings: V CB = 5 V, I C =2 ma, h FE =110 to 450 DC biasing conditions V CC =12 V, I C =2 ma V RC =40% of V CC = V RE =10% of V CC = V CE =50% of V CC = Design of Collector resistor R C V RC = I C x R C => R C = Design of Emitter resistor R E V RE = I E x R E, here I E = I C => R E = Design of voltage divider R 1 and R 2 I B =I C /h FE (choose h FE = 200) = Assume current through R 1 =10 I B and that through R 2 =9 I B to avoid loading the potential divider network R 1 and R 2 by the base current. V R2 = voltage across R 2 = V BE + V RE = Also VR 2 = 9I B R 2 = Then R 2 = V R1 = voltage across R 1 = V CC V R2 = Also VR 1 =10I B R 1 = Then R 1 = Design of Load R L Assume R L = 1 kω 59
Design of coupling capacitors C C1 and C C2 X C1 Rin/10. Here R in = R 1 ІІ R 2 Rin = Then X C1 So C C1 1/2πf x X C1 = X C2 Rout/10, where Rout = Rc. Then X C2 C C2 1/2πf x X C2 = OBSERVATION Circuit diagram PRACTICE PROCEDURE Amplifier Gain Measurements 1. Connect the circuit as shown in Figure 1. 2. Apply an input sine wave signal of 100mV, 1 khz from the function generator. 3. Observe the output in CRO. Calculate the corresponding gain and compare with the designed values. Table1: Amplifier gain measurements Small signal Input voltage, Vs Amplified output voltage, Vo Amplitude (V) Time Period (ms) Frequency (khz) Voltage gain, A V = 60
Inference Frequency Response 1. Keeping the magnitude of the input same, ie.,100mv, vary the frequency of the input signal and tabulate the output voltage for different frequencies. 2. Compute the gain and plot the Frequency versus Gain (db) using semi-log sheet. 3. From the plot, determine the values of (a) Mid band voltage gain, A V (mid), (b) Lower cut-off frequency, (c) Upper cut-off frequency and (d) Bandwidth. Table2: Frequency response of CC amplifier Input voltage, Vi = mv Signal frequency (Hz) 10 Output voltage, Vo (Volts) Gain = Vo V i 20 log 10 (Gain) db 20 50 100 200 500 1k 2k 5k 10k 20k 50k 100k 200k 500k 1M 61
Inference UNDERSTANDING & LEARNING 62
RESULTS AND CONCLUSION Prepared by: Name: Reg. No.: Date of Experiment : Date of Report Submission: Submission Delay (if any):... ASSESSMENT Student Task Max. Marks Graded Marks Pre-lab Preparation 20 Inference 10 Signature Results & Conclusion 10 Post-lab / Viva-voce 10 Total 50 63