LAB_REPORT_03_RAHMAN_40106588

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LABORATORY REPORT Electronics II Course: ELEC312 Lab Section: WL-X Experiment No.: 3 Date Performed: 2024 – 03– 06 Experiment Title: FREQUENCY RESPONSE OF MOS 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 – 03 – 20
Abstract This Lab report studies the frequency response characteristics of Common- Source (CS) and Common-Drain (CD) MOS amplifiers, providing analysis of MOS transistor behavior in different amplifier configurations. It discusses the role of bypass capacitors in facilitating AC short circuits and maintaining DC biasing stability. Furthermore, the paper thoroughly explores the implications of internal capacitances at high frequencies, elucidating their effects on amplifier gain and introducing negative feedback mechanisms. In addition to theoretical discussions, the paper offers practical insights into conducting AC analysis at high frequencies. It outlines the methodology for replacing MOS transistors with their high-frequency equivalent circuits and demonstrates the application of Miller's and Thevenin's theorems to simplify complex circuits. By providing a detailed examination of both theoretical concepts and experimental methodologies, this paper contributes to a deeper understanding of MOS amplifier design and analysis.
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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 of how well Common-Source (CS) and Common-Drain (CD) MOS amplifiers work with different frequencies. In Common- Source amplifiers, the input signal goes to the gate and the output comes from the drain, giving voltage gain. In Common-Drain amplifiers, the input signal also goes to the gate, but the output comes from the source, offering high input and low output resistance. By checking how these amplifiers handle different frequencies, the study wants to see their strengths and weaknesses. This includes understanding how MOS transistors behave, the importance of setting them up correctly, and how internal capacitances affect amplifier performance, especially at higher frequencies. This research helps us better understand how to design and improve MOS amplifiers for different uses. Theory The MOS transistor, equipped with four terminals, requires accurate setup for proper functionality. These terminals are known as Drain, Gate, Source, and Body, with the Body internally connected to the Source terminal in the circuits under discussion. Like Bipolar Junction Transistors (BJTs), MOS transistors can be configured in various arrangements like Common-Source (CS), Common-Drain (CD), or Common-Gate (CG) amplifiers, influencing the interaction between input and output signals. Figure 1 and Figure 2 provide illustrations of these amplifier configurations. Capacitors C1, C2, and C3 facilitate signal shortcuts at sufficiently high frequencies without disrupting the transistor's standard operation. However, at different frequencies, particularly high ones, certain internal components such as C gs and C gd become more significant, impacting the amplification of signals. For instance, C gd may introduce negative feedback, while C gs could attenuate the input signal. Analyzing the transistor's behavior at high frequencies involves substituting it with a high-frequency equivalent circuit, simplifying analysis and comprehension. In essence, this research contributes to an enhanced understanding of MOS amplifier design and optimization across diverse applications.
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Procedure Set up the circuit shown in Fig. 4, a common-source amplifier with a bypass capacitor (C-bypass) to improve gain. Investigate the impact of C-bypass and load resistor (RL) on mid-band gain. Measure mid-band gain while varying Vin frequency to find bandwidth (difference between upper and lower 3dB points). Plot gain vs. frequency using 50mV peak Vin. Remove RL and measure mid-band gain again, noting gain increase without RL. Use small signal analysis to explain, ignoring coupling capacitors C1 and C2. Reinsert RL, remove C-bypass, measure mid-band gain and bandwidth, and plot gain vs. frequency. Discuss significant gain decrease without C-bypass, using small signal analysis without C1 and C2. Note absence of C-bypass leads to higher bandwidth. Assemble the circuit in Figure 6, a common-drain amplifier or source follower. Measure and compare output (V out ) to input. Repeat V out measurement and comparison with input. Adjust input frequency to find lower and upper 3dB points. Change RL to 1K, measure V out , and explain observed decrease. Results and Discussion
Table 1. Data table for the circuit with RL The gain is calculated by the following equation: 20log ( V out V ¿ )
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Figure 7. Plot of the circuit with RL Lower 3dB= 0.05 kHz Higher 3dB= 1 kHz Lower 3dB= 0.05 kHz Higher 3dB= 1 kHz Lower 3dB= 14. 63dB at 300 kHz Higher 3dB= 25.32dB at 25 kHz
Table 2. Data table for the circuit without RL
Figure 8. Plot of the circuit without RL Lower 3dB= 15.18 dB at 350 kHz Higher 3dB= 28.41 dB at 10 kHz
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Table 3. Data table for the circuit with RL & without C By_pass Figure 9. Plot of the circuit with RL & without C By_pass Lower 3dB= -3.39 dB at 1000 kHz Higher 3dB= 4 dB at 0.02 kHz
Table 4. Data table for the circuit with common-drain amplifier Figure 10. Plot of the circuit with common-drain amplifier Lower 3dB= -8.78 dB at 1000 kHz Higher 3dB= -2.76 dB at 0.01 kHz Mid band: 1 KHz Mid band gain: -1.41 dB
In the first part of the experiment, we looked at the common-source amplifier. We saw how the load resistor and bypass capacitor affect the midband gain. When we plot the gain against frequency, we saw that both the gain and bandwidth increase as the frequency goes up. This is different from what happens when all the components are in the circuit. We can understand this change by looking at the circuit. When we took out the bypass, the signal going to the source gate gets smaller, so the gain through the resistor also decreases. Then, we moved on to the next part of the experiment, where we studied the common-drain amplifier. First, we verified the V out of the MOS amplifier with everything connected. Then, we changed the load resistance to 1kΩ and saw how the output voltage changes. The data in the fourth table shows that when we switch the load to 1kΩ, the V out values change. This tells us that the output voltage drops when the load resistance decreases. For step 3, whether there's a bypass capacitor when looking at a circuit's small signal behavior really matters for how much the signal gets stronger and how wide the range of frequencies it can handle is. If there's a bypass capacitor, the electrical signal kind of skips past a certain resistor (let's call it R), making it seem like it's directly connected to the power supply. This setup stops the signal from losing power as it goes through R, making the signal stronger. But if there's no bypass capacitor, the signal must go through R, which slows it down and makes it weaker. As for the bandwidth, without a bypass capacitor, it can handle a wider range of frequencies because it doesn't get slowed down by other parts of the circuit. But if there's a bypass capacitor, it acts like a filter, only letting through higher frequencies, which narrows down the range of frequencies the circuit can handle. So basically, having a bypass capacitor makes the signal stronger but limits the frequencies it can handle, while not having one does the opposite.
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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%
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%
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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. Considering the current IC technology, which one is smaller, MOSFET or a BJT? MOSFET is considered smaller due to the characteristics of its components. 2. How does the frequency response (gain vs. frequency) curves of the CS and CD amplifiers compare? The frequency response curves show that the common-source reaches a higher voltage compared to the common-drain. Additionally, in the common-source configuration, the circuit without the bypass capacitor exhibits greater midband gain as the frequency increases. In contrast, in the common-drain setup, the gain decreases as the load resistance decreases. 3. Is it possible to design a CD amplifier with a voltage gain > 1? The frequency response curve for the common drain indicates that the voltage gain remains less than one. This holds true even when reducing the load resistance to 1kΩ, with only minor observed changes. 4. If you are asked to design a voltage buffer amplifier, which CS and CD stage(s) will you consider? The common-drain amplifier, also known as the source follower, would be considered for designing a voltage buffer, as it features low output resistance. 5. In the circuits that you worked with, what would be the maximum input signal level for small signal ac equivalent circuit to be applicable? Is 50mV amplitude for the input signal satisfactory for this purpose? The results indicate that the input signal has negligible effects on the circuit and remains consistent throughout the experiment. Therefore, a 50mV amplitude is sufficient for manipulating both MOS amplifiers.
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Conclusion In summary, the experiment produced favorable results that validated the theoretical concepts governing MOS amplifiers. By exploring different circuit configurations and frequencies, we were able to closely examine how loads and capacitors influence the performance of the circuit, with a specific focus on common-source and common-drain arrangements. 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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