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Let's begin with an overview of the common base amplifier. Can anyone remind us of the primary components involved?
The input and output ports are at the emitter and collector, respectively.
Exactly! And we also consider the DC biasing as well as Thevenin equivalent resistance. Knowing this helps us define our AC ground. Next, why do we connect a coupling capacitor at the input?
To ensure AC signals can pass while blocking DC.
Right! This brings us to the performance metric we will analyze: voltage gain. The expression for voltage gain is based on various parameters. Can anyone tell me what influences voltage gain?
The small signal parameters, including transconductance and load resistance, play a critical role.
Good point! Remember the acronym 'GIR' for Gain, Impedance, and Resistance relationship. Voltage gain incorporates these factors.
In summary, we've discussed the setup components and initial parameters for our voltage gain calculation.
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Now, let's calculate small signal parameters. Can anyone state what parameters we need?
We need to find transconductance, collector current, and output resistance.
Great! The transconductance, g_m, is given by the equation based on thermal voltage. What's the typical value?
It's usually 26 mV at room temperature.
That's correct! Following this, we determine the output resistance considering the Early voltage. Can someone summarize how we might find that value?
By using the early voltage with the collector current.
Exactly! Remember the phrase 'DC is primary' as it reassures us that all calculations hinge on our DC bias setups.
To recap, today we calculated important parameters like g_m and output resistance that will directly reflect in our voltage gain calculations.
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Moving on, letβs explore the impact of input source resistance on voltage gain. How does it affect the gain?
Higher source resistance will likely decrease the voltage gain due to voltage division.
Precisely! Letβs consider an example. If R_s is set to 10kOhm, and our original gain is 108.85, what do we expect?
The actual gain would decrease significantly due to attenuation from the high source resistance.
Exactly! We realize that with real-world components, particularly high R_s values, we could see attenuation resulting in gains below 1. This highlights practical design considerations where we must account for component variability.
To summarize, source resistance has a pronounced effect on voltage gain, showcasing the complexity in real-world applications.
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Now that weβve calculated voltage gain and also discussed source resistance, let's look at the design side. What factors should guide amplifier design?
We should consider the trade-off between voltage gain and bandwidth.
Correct! The low input impedance of common base amplifiers can lead us to choose designs that yield higher bandwidth. Remember the acronym 'GIVE' for Gain, Input, Voltage, and Efficiency.
How can we ensure we do not compromise on performance?
Good question! It's about finding the best component values and slowly iterating based on simulations and practical setups. Always validate theoretical results against real-world measurements.
To conclude, today's exploration of voltage gain and operational configurations has set a solid foundation for effective amplifier design.
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This section emphasizes the methodology for calculating voltage gain in common base and common gate amplifier configurations. It reviews key performance metrics such as input and output impedance, and how varying source resistance influences overall circuit performance.
In this section, we will explore the voltage gain calculation for common base and common gate amplifier configurations through practical numerical examples. The key focus is on understanding the relationship between various parameters involved in amplifier design and performance.
We first revisit the setup of our common base amplifier, pointing to specific components such as collector biasing, Thevenin resistance, and significant capacitors affecting circuit behavior. By providing detailed small signal analysis and defining the necessary parameters (such as the transistor's current gain, Early voltage, and thermal voltage), we can proceed to derive the performance metricsβchiefly the voltage gain.
The voltage gain is expressed as a function of the transistor's small signal parameters and circuit resistances. Numerical examples will breakdown calculations to find voltage gain, input/output impedance, capacitance, and upper cutoff frequency. Insights into design guidelines based on observed results will aid in understanding circuit performance improvements.
Through this focused approach, students will develop a solid grasp of voltage gain dependencies on circuitry, enabling them to tackle practical amplifier design challenges.
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So, let us go to the numerical example of common base amplifier. What we do have here it is the basic circuit given here and you can see that we do have ideal bias as well as more practical bias of the collector terminal. And also at the base node of this circuit we do have a DC voltage along with Thevenin equivalent resistance of the DC source. And since it is common base configuration and we want this base node should be AC ground to ensure that we are also connecting C large capacitor there.
This chunk introduces the setup of a common base amplifier, mentioning the ideal and practical biases in the circuit. The base node is kept at AC ground through a large capacitor, which allows AC signals to pass while blocking DC. This capacitor ensures that the base does not influence the AC performance.
Think of the capacitor as a door that allows people (AC signals) to come in and out freely while keeping the room (the circuit) at a stable temperature (DC voltage).
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If we see that this I it is given as 1 mA and Ξ² is 100. So, we may approximate that collector current I = I and which is same as 1 mA, so this is the DC current. And then the voltage at the base it is given as 6 V and the emitter voltage here it can be obtained by considering 6 V here and V all of 0.6 V. So, we can say at the emitter node we do have 6 β 0.6 that is 5.4 V.
To find the operating point, you need the collector current (1 mA) and the DC voltage at the base (6 V). The emitter voltage is then calculated by subtracting the base-emitter voltage drop (0.6 V from a BJT) from the base voltage. This yields an emitter voltage of 5.4 V, which is critical for determining how the transistor operates within the desired region.
Imagine a water tank where the base voltage is like the height of the water level, and the BJT drop (0.6 V) is akin to a pipe that reduces the height. The effective water height (emitter voltage) is what determines how high you can pump the water (current) down the line.
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So, using those parameters so we can find the corresponding voltage gain. So, let us see how we do get the voltage gain. So, the voltage gain starting from the emitter terminal to the collector terminal and if I call that voltage gain it is A , you may recall from our previous days discussion it was ( ). In fact, if you do if you compare and 1 definitely, we can approximate this by only.
The voltage gain (A_v) of the amplifier is determined by the ratio of the output voltage change to the input voltage change. This relationship simplifies to the output resistance and the transconductance when calculated correctly. In this amplifier, as noted, the gain is approximately 108.85 due to favorable parameters.
Consider a loudspeaker. If you shout (input signal) and the speaker amplifies your voice to ten times louder (output signal), the loudness ratio is like the voltage gain β showing how effectively your input produces a larger output.
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Now, next thing is that we can try to find the output impedance. The output resistance it is resistance of this R this path and then the other resistance coming from the active device in parallel. So, that is in parallel with whatever the impedance will be seen.
The output impedance is significant in understanding how much the amplifier will load the previous stage. It's calculated based on the resistances in the circuit and the characteristics of the transistor. The aim is to keep the output impedance low to enable better voltage transfer to the load.
Think of output impedance like the width of a pipe. A narrower pipe (higher impedance) restricts water flow, while a wider pipe (lower impedance) allows for better flow to the garden (load).
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if we see the capacitance at this node for this signal, of course this coupling signal coupling capacitor it is quite large. So, whenever we are talking about input capacitance it is coming the emitter to whatever the ground node AC ground node will be considering that is the net capacitance.
In examining the input capacitance, the capacitors define how the amplifier will respond to high-frequency signals. It's important because it sets an upper cutoff frequency, essentially determining the range of frequencies the amplifier can handle effectively.
Imagine a sponge that holds water (capacitors). If the sponge is too porous (too much capacitance), it can soak up frequencies that you might not want, like unwanted noise. The right amount of porosity allows you to keep only the useful 'signals'.
Learn essential terms and foundational ideas that form the basis of the topic.
Key Concepts
Voltage Gain: The ratio of the output voltage to the input voltage.
Input and Output Impedance: Critical parameters affecting circuit loading and signal integrity.
Small Signal Parameters: Essential in analyzing transistor performance in amplifiers.
See how the concepts apply in real-world scenarios to understand their practical implications.
A common base amplifier with a measured voltage gain of 108.85 under ideal conditions.
How introducing a source resistance of 10k Ohms reduces the voltage gain to 0.27 due to attenuation.
Use mnemonics, acronyms, or visual cues to help remember key information more easily.
When gain is high, signals fly, but low input can make them die.
Imagine a racecar needing a strong road (high gain) but if the road is bumpy (low impedance), it can't go fast.
Use 'GIR' for Gain, Impedance, and Resistance to remember their influence on performance.
Review key concepts with flashcards.
Review the Definitions for terms.
Term: Voltage Gain
Definition:
The ratio of output voltage to input voltage in an amplifier.
Term: Input Impedance
Definition:
Resistance seen by the source when connected to the amplifier input.
Term: Output Impedance
Definition:
Resistance provided by the amplifier at its output terminals.
Term: Small Signal Parameters
Definition:
Parameters that describe the behavior of transistors under small input signals, important for AC analysis.
Term: Thevenin Equivalent Resistance
Definition:
A simplification of a complex circuit into a simple equivalent circuit with a voltage source and a resistant.