Exp. No #6 FREQUENCY RESPONSE OF COMMON EMITTER AMPLIFIER OBJECTIVE The purpose of the experiment is to design a common emitter amplifier. To analyze and plot the frequency response of the amplifier with and without bypass capacitor. Also to compare the bandwidth of the amplifier with and without feedback. 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 RC coupled CE amplifier is widely used in audio frequency applications in radio and television receivers. Base current controls the collector current of a common emitter amplifier. A small increase in base current results in a relatively large increase in collector current. The resistors R 1 and R 2 are employed for the voltage divider bias of the transistor. Voltage divider bias provides high dc bias stabilization independent of variations in β. The input is coupled through coupling capacitor C C1 to the base. The purpose of the coupling capacitor is to couple the ac signal to the input of the amplifier and block dc. It isolates the input signal source and the voltage divider circuit. The output voltage is coupled from collector through the capacitor C C2. Emitter resistor R E provides current series feedback in the circuit. The emitter resistor R E stabilizes the operating point against temperature variations. The purpose of the bypass capacitor is to bypass signal currents to ground. 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. 47
CIRCUIT DIAGRAM PRELAB Figure1. Common Emitter Amplifier 1. Design a common emitter amplifier as per the design requirements given. Use SPICE tool to create the amplifier and observe the dc operating conditions. 48
2. Obtain a plot of the frequency response of the common emitter amplifier over the frequency range from 1 Hz through 10 khz. Observe the mid-band gain and bandwidth. 3. Obtain the frequency response with feedback and note down the mid-band gain and bandwidth. 4. Remove the emitter resistance and comment on the response of the amplifier. 49
Exp. No.: OBJECTIVE FREQUENCY RESPONSE OF COMMON EMITTER AMPLIFIER Date: DESIGN Output requirements: Mid-band voltage gain of the amplifier, A V = 50 V/V Select the general purpose transistor BC107. Specifications of BC107 Type: NPN Nominal ratings: V CE = 5 V, I C =2 ma, h FE =110 to 450 DC biasing conditions V CC =12 V, I C =5 ma V RC =40% of V CC = V RE =10% of V CC = V CEQ =50% of V CC = Design of Collector resistor R C V RC = I C x R C => V CEQ = V CC - V RC => V RC = V CC - V CEQ => R C = (V CC - V CEQ ) / I C = Design of Emitter resistor R E V RE = I E x R E => R E = Design of voltage divider R 1 and R 2 I B =I C /h FE Take 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 V R2 = 9I B R 2 = Then R 2 = V R1 = voltage across R 1 = V CC V R2 = Also V R1 =10I B R 1 = Then R 1 = Design of Load R L Take R L = 100 kω 50
Design of unbiased emitter resistor g m = I C / V T = (Take V T = 0.026 V) R E1 = R E = R E1 + R E2 => R E2 = R E - R E1 = 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 1/2 f x X C1 = X C2 Rout/10, where Rout = Rc. Then X C2 C C2 1/2 f x X C2 = Design of bypass capacitors C E To bypass the lowest frequency (say 100 Hz), X CE should be equal to one-tenth or less than the resistance R E. X CE R E /10 C E 1/2 x100 x X CE = OBSERVATION Circuit diagram 51
PRACTICE PROCEDURE DC Operating Point 1. Connect the voltage divider bias network from the circuit shown in Figure 1. 2. Apply DC bias voltage V CC and measure the operating point quantities: V B, V BE, V C, V CE, V RE, I C. Table1: Operating Point measurements Quantities Measured Value Designed Value Base Voltage, V B Base-emitter voltage, V BE Collector Voltage, V C Collector emitter voltage, V CE Collector Current, I C Emitter Voltage, V RE Inference 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. Table2: Amplifier gain measurements Small signal Input voltage, Vs Amplitude (V) Time Period (ms) Frequency (khz) Amplified output voltage, Vo Voltage gain, A V = 52
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. Repeat the above by removing the bypass capacitor. Table3: Frequency response with bypass capacitor Input voltage, Vs = mv Signal frequency (Hz) 10 Output voltage, Vo (Volts) Gain = Vo V S 20 log 10 (Gain) db 20 50 100 200 500 1k 2k 5k 10k 20k 50k 100k 200k 500k 1M 53
Inference Table4: Frequency response without bypass capacitor Input voltage, Vs = mv Signal frequency (Hz) 10 Output voltage, Vo (volts) Gain = Vo Vs 20 log 10 (Gain) db 20 50 100 200 500 1k 2k 5k 10k 20k 50k 100k 200k 500k 1M 54
Inference UNDERSTANDING & LEARNING 55
RESULTS AND CONCLUSION Prepared by: Name: Reg. No.: Experiment Date: Report Submission Date: Submission Delay:... ASSESSMENT Student Task Max. Marks Graded Marks Pre-lab Preparation 20 Inference 10 Signature Results & Conclusion 10 Post-lab / Viva-voce 10 Total 50 56