LAB_REPORT_04_RAHMAN_40106588

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LABORATORY REPORT Electronics II Course: ELEC312 Lab Section: WL-X Experiment No.: 4 Date Performed: 2024 – 03– 20 Experiment Title: COMPARISON OF FREQUENCY RESPONSE OF COMMON-EMITTER (CE) AND COMMON-BASE (CB) AMPLIFIERS Name: Rahman, Md Wasique ID No.: 40106588 I certify that this submission is my original work and meets the Faculty’s Expectations of Originality Signature: Date: 2024 – 04 – 03
Abstract This Lab report studies how common emitter (CE) and common base (CB) amplifiers change their strength depending on the frequency. These amplifiers work best in the middle range of frequencies but get weaker as the frequencies get lower or higher. We use terms like lower 3dB point (f L ) and upper 3dB point (f H ) to talk about the frequencies where the strength drops to 71% of its highest level. CE and CB amplifiers have different ways of showing their strength in the middle frequency range, with CE having a negative sign because it flips the signal. Capacitors, like coupling and bypass ones, are important in affecting the lower 3dB frequency. Internal capacitance (Cµ and Cπ) in the transistor also impacts the upper 3dB frequency, making amplifier performance quite complicated. Also, considering junction capacitance in the transistor model makes it harder to understand how amplifiers behave. Therefore, the students will be using simulations and real measurements to study these changes and learn more about how CE and CB amplifiers work at different frequencies.
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Table of Contents
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Introduction Objectives The main objective of this experiment is to provide a comprehensive understanding on how to accurately check how common emitter (CE) and common base (CB) amplifiers react to different frequencies. We'll conduct experiments where we'll carefully test these amplifiers at different frequencies and record what happens. By doing these hands-on activities, students will get better at understanding how amplifiers work and how to read and understand the data they produce. This will help us not only in theory but also in real-world situations where we will need to work with amplifiers in the future. Theory The frequency response of common emitter (CE) and common base (CB) amplifiers illustrates how these devices operate across different frequencies. In CE amplifiers, the emitter terminal is grounded, the input signal goes to the base, and the output comes from the collector. They work in the active region to amplify signals, with their mid-band gain determined by the transistor's transconductance and load resistance, expressed as Av = -gmRC//RL, where RC and RL represent the collector and load resistance, respectively. The negative sign in the gain indicates that the input and output are out of phase. As the frequency rises, CE amplifier gain decreases due to impedance effects caused by coupling capacitors and internal transistor capacitances such as Cπ and Cµ. On the other hand, CB amplifiers ground the base, have the input at the emitter, and output at the collector, providing low input impedance and high current gain for various applications. Although their gain is like CE amplifiers, without inversion, they exhibit a flatter frequency response as they rely less on coupling capacitors. The mid-band gain for a CB amplifier is given by the expression Av = gmRC//RL, which is the same as the CE configuration except for the absence of the negative sign. This understanding helps engineers tailor amplifiers to specific frequency requirements, with experimental measurements of input signals and corresponding output amplitudes providing valuable insights into amplifier performance across frequency ranges.
Figure 1. The pin-out for this BJT P2N2222A Figure 2. A circuit diagram for the CE amplifier measurement Figure 3. circuit diagram for the CB amplifier measurement
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Procedure 1. Set up the CE amplifier circuit shown in Fig.2 using the BJT P2N2222A, referring to the pin-out diagram provided in Fig.1. 2. Replace Vin in Fig.2 with a Function Generator set to deliver a sine wave input, ensuring Vin (peak-to-peak) is around 40mV. 3. Use a DMM to measure VC initially, allowing for the calculation of IC and subsequently gm (gm = IC/25mV). 4. Measure VE and calculate VCE = VC – VE, ensuring it exceeds 0.3V to confirm active mode operation of the BJT for effective amplifier function. 5. Configure the function generator to f=1 kHz, with Vin (peak-to-peak) = 40 mV and VOFF = 0, and simultaneously observe Vin and V out on the oscilloscope, noting their phase relationship. 6. Conduct a "V out vs. f" measurement, starting at f=30 Hz and increasing by a factor of 3, covering the frequency range beyond f L and f H . 7. Plot Gain (G v ) vs. f on a semi-log graph to determine f L and f H . 8. For the CB amplifier measurements, assemble the circuit depicted in Fig.3, replacing Vin with the Function Generator configured similarly to the CE amplifier measurements, and repeat steps 1-5.
Results and Discussion Table 1. Data table for the circuit with CE amplifier The dB gain is calculated by the following equation: 20log ( V out V ¿ ) Figure 4. Plot of the circuit with CE amplifier Lower 3dB= 0.05 kHz Higher 3dB= 1 kHz
Lower 3dB= 0.05 kHz Higher 3dB= 1 kHz V C = 3.4514 V V E = -2.7295 V I C = 1.1977 mA Phase shift between Vin and Vout = 115.7ͦ Table 2. Data table for the circuit with CB amplifier
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Figure 5. Plot of the circuit with CB amplifier Phase shift between Vin and Vout ≈ 0ͦ For the Common-Emitter configuration, the lower -3dB cutoff frequency (f L ) falls within the range of 0.03 kHz to 0.3 kHz, while the upper -3dB cutoff frequency (f H ) ranges from 1 kHz to 300 kHz. This determination was made by computing 71% of the midband gain, resulting in a gain of 46.17 dB with a midband gain of 203.57 V/V. Regarding the Common-Base setup, the lower -3dB cutoff frequency (f L ) dips slightly below 0.3 kHz, while the upper -3dB cutoff frequency (f H ) extends beyond 1 kHz. This was achieved by computing 71% of the midband gain, which equates to 150 V/V and yields a gain of 43.52 dB.
Figure 11. Simulation for the circuit with RL. TheoreticalGain ∈ 3 dBfor f = 1 kHz = 20log V out V ¿ = 20log ( 1.134 V 0.066 V ) = 24.70 dB % Error Gain = | Experimentalvalue − Theoreticalvalue Theoreticalvalue | ∗ 100 = | 23.728 dB − 24.70 dB 24.79 dB | ∗ 100 = 3.92%
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Figure 12. Simulation for the circuit without RL. TheoreticalGain ∈ 3 dBfor f = 1 kHz = 20log V out V ¿ = 20log ( 1.199 0.0576 V ) = 26.37 dB % Error Gain = | Experimentalvalue − Theoreticalvalue Theoreticalvalue | ∗ 100 = | 28.19 dB − 26.37 dB 26.37 dB | ∗ 100 = 6.90%
Figure 13. Simulation for the circuit with RL & without C By_pass. TheoreticalGain ∈ 3 dBfor f = 1 kHz = 20log V out V ¿ = 20log ( 0.0744 0.0552 V ) = 2.59 dB % Error Gain = | Experimentalvalue − Theoreticalvalue Theoreticalvalue | ∗ 100 = | 3.22 dB − 2.59 dB 2.59 dB | ∗ 100 = 24.32%
Figure 14. Simulation for the circuit with common-drain amplifier TheoreticalGain ∈ 3 dBfor f = 1 kHz = 20log V out V ¿ = 20log ( 0.038392 0.0544 V ) =− 3.027 dB % Error Gain = | Experimentalvalue − Theoreticalvalue Theoreticalvalue | ∗ 100 = | − 3.30 dB + 3.027 dB − 3.027 dB | ∗ 100 = 9.02%
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1. Comment on the mid-band gains obtained for the CE and CB amplifiers obtained by simulation. If they are very different (more than 10%) explain why? We observed that the midband gain of the common-base setup is more than 10% less than the common-emitter one. This big gap happens because the input resistance is hooked up differently in each circuit. The way the input signal works with the base-emitter junction and the emitter-ground connection also adds to why these two setups perform differently. 2. Provide the mid-band gains obtained for the CE and CB amplifiers obtained by lab bench work. If they are very different (more than 10%) explain why? Common-Emitter: I C = V C R C = 3.4514 V 10 KΩ = 0.345 mA gm = I C V T = 1. 25 mV = ¿ A v = − gmR C R L = ¿ Common-Base: I C = V C R C = ¿ gm = I C V T = ¿ A v = − gmR C R L
3. Comment on the phase relation of the signals at the output and input for the CE and CB amplifiers (use the lab bench results). Based on the experimental data, it's evident that the phase shift observed in the Common-Emitter (CE) configuration was substantially greater compared to that in the Common-Base (CB) configuration. Specifically, the phase shift measured for the CE setup was 115.7°, whereas for the CB configuration, it was minimal, approximately 0°. This significant difference suggests distinct characteristics in the phase behavior of these two circuit configurations, likely stemming from their respective design and operational principles. 4. Do the f H values of the CE and CB amplifiers differ in the lab bench work? If yes, why? The results from both amplifiers don't match up. In particular, the common-base (CB) amplifier has a higher high-frequency cutoff (f H ) than the common-emitter (CE) one. This difference shows that the two amplifiers behave differently at higher frequencies. Looking more closely at how they respond to different frequencies could help explain this gap. 5. What is the role of the transistor ‘beta’ (hfe) on the values of (i) fL, (ii) fH? Justify by proper analysis. Consider only the CE amplifier. The beta transistor controls how much current flows forward. Even a small alteration in this transistor can lead to a big change in the collector current. And when the collector current shifts, it affects both low and high frequencies. Also, there's a formula that shows how beta impacts the gain: Av = β * (Rc / Rb). This formula explains how changes in beta affect the amplifier's gain, highlighting the need for keeping beta stable to maintain consistent performance across different frequencies. 6. What is the role of the transistor ‘gm’ on the values of (i) fL, (ii) fH? Justify by proper analysis. Consider only the CE amplifier. The transconductance (gm), which represents how the amplifier responds to changes in input voltage, relies on the beta ( 𝛽 ) value of the transistor. Therefore, alterations in beta not only affect gm but also influence the midband gain of the amplifier. This midband gain, in turn, plays a significant role in determining the amplifier's performance across various frequency ranges. By understanding the
relationship between beta, gm, and midband gain, we gain insights into how the amplifier behaves at different frequencies and can optimize its performance accordingly. Conclusion In summary, this lab experiment explored how CE and CB amplifiers behave at different frequencies. They work best in the middle frequency range but weaken at extremes. Concepts like lower and upper 3dB points pinpoint frequencies where their strength diminishes. CE amplifiers flip signals, while CB amplifiers show a more consistent response. Capacitors and internal transistor capacitances complicate amplifier performance. Moreover, answering the Lab questions allowed the students for a thorough exploration of the practical applications and comparative assessment of these amplifiers enriching our understanding of their operational nuances and gave us a deeper insight into how versatile and useful these amplifiers can be in different situations. Overall, the successful completion of this experiment was confirmed during the demonstration to the lab instructor, effectively achieving the lab's objectives as the experimental values determined were very close to the theoretical values calculated (most analysis having close to 10% error).
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References 1) Sedra, A.S., Smith, K.C., Carusone, T.C., Gaudet, V., Microelectronic Circuits, Oxford University Press: 8th edition, © 2020 2) Alexander, C. K., & Sadiku, M. N. O. (2007). Fundamentals of electric circuits. Boston: McGraw-Hill Higher Education. 3) Shiyu Q., Laboratory Manual, ECE Electronics II, ELEC 312, Winter 2024.
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