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50.2.6. Voltage Gain with Finite Source Resistance
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Today, we'll start by discussing small signal analysis in common base amplifiers. Can anyone tell me why we would want to conduct small signal analysis?
It helps us analyze how the amplifier responds to small variations in input signals!
Exactly! When we analyze small signals, we simplify circuits by dropping DC components. For common base circuits, we focus on AC signals and use small signal parameters. Can anyone name a key parameter?
Is it the transconductance, gm?
Yes! Now, remember this acronym: GAIN, which stands for Gain, AC signal, Input impedance, and Network response. This will help you recall the key concepts in amplifier analysis.
What happens at the emitter in these circuits?
Good question! The voltage at the emitter due to the input signal becomes critical. It leads us to discuss the voltage gain next.
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Let’s derive the voltage gain for the common base configuration! Can anyone explain what we start with?
We apply KCL at the collector node, right?
Correct! We find the relationship between output voltage and input voltage. The equation we derive shows how gain is influenced by gm and the resistances. Does anyone remember that equation?
It’s Vout = Vin × (gm × ro)/(ro + R)!
Very good! This shows that increasing gm can enhance gain, but we must also consider the resistances which can attenuate the signal. For a quick memory aid, think of the phrase 'Gain is Compromised by Resistance.'
How does that relate to finite source resistance?
Excellent question! When source resistance is present, it can further decrease the voltage seen at the emitter, impacting the overall gain. Let’s discuss that next.
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Now, let’s talk about what happens when we include finite source resistance. Who can explain why this is significant?
It could weaken the input signal before it reaches the amplifier!
Exactly! When the source resistance is high relative to the amplifier's input resistance, a voltage drop occurs which reduces the signal strength. This is often summarized as 'Signal diminishes with resistance.' Can anyone derive the total gain considering this attenuation?
By using the ratio of the input voltage to the actual input voltage seen by the amplifier!
Exactly! This derived relationship will show that the overall performance may fall short of expectations in practical applications. It’s essential to take note of these effects in circuit design.
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Let’s pivot to the common gate configuration. How does it relate to our earlier discussions?
It also involves analyzing input and output voltages, similar to the common base!
Right! We can use many of the same principles. The key difference is where we apply the input. Can someone summarize?
We feed the signal to the source instead of the base!
Spot on! This can yield similar gains as the common base but with different implications. Let’s ensure we understand that while on paper, gains are similar, in practice we need to consider source resistance just as we did. What do we remember about input impedance from the common gate perspective?
It’s also low, which can affect amplification like in the common base configuration.
Exactly! Remember this phrase: 'Common Gate, Common Ground.' It’ll help you recall that these amplifiers share similar challenges regarding resistance and gain.
Overview
Short Summary
This section discusses how finite source resistance affects voltage gain in common base and common gate amplifiers.
Medium Summary
In this section, we analyze the small signal equivalent circuit for common base and common gate amplifiers, focusing on the impact of finite source resistance on voltage gain. It highlights the significance of input and output resistances and provides equations to understand the attenuation caused by source resistance.
Detailed Summary
In Analog Electronic Circuits, the voltage gain of amplifiers is significantly impacted by source resistance. This section details the small signal equivalent circuits for common base and common gate amplifiers. It outlines how the voltage gain is determined from the emitter voltage to the collector output and examines the implications of ignoring finite source resistance. When considering finite source resistance, the analysis reveals that the overall voltage gain is affected by the input and source resistances, leading to potential attenuation in the signal received by the amplifier. The section concludes by discussing the practical implications of these findings, noting that both configurations may not serve as effective voltage amplifiers due to high levels of attenuation under specific conditions.
Reference YouTube Videos
Audio Book
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Create a free accountSo, we have assumed that there is no source resistance. And as a result, we are saying that entire v it is coming here and then R is not having any role and then we are really not bothered about the input resistance and so on. But practically if you see that if we consider source resistance signal source resistance and if we look into the input resistance of this circuit, then we will be we will be getting a surprise.
Detailed Explanation
In this part, we start with the assumption that there is no source resistance in the circuit, meaning that the voltage input to the circuit is fully transferred without any loss or drop. However, in practical applications, we often encounter source resistance (the resistance of the signal source). This resistance affects the overall input resistance of the circuit, which can significantly reduce the voltage available at the input, leading to unexpected results.
Examples & Analogies
Imagine you are trying to water your garden using a hose. If the hose is too narrow (representing high source resistance), less water (voltage) reaches your plants (the amplifier input), even if you have a full bucket (ideal voltage) waiting at the start.
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Create a free accountIf I stimulate the circuit by say v and then if we observe the corresponding current here in say i then if I take the ratio of that will be giving us the input resistance... So, if I take the ratio of this v and i to get the input impedance of the circuit, what we are getting here it is r coming in parallel with this impedance namely ( ) right and then of course, we do have this R .
Detailed Explanation
Here, we are measuring the input impedance of the circuit by observing the current that flows through it when we apply a voltage. The input impedance is defined as the ratio of the voltage applied (v) to the current that flows (i). The important point is that this impedance is not just a single value; it includes various resistance components from the circuit, which may occur in parallel or series with the input resistance.
Examples & Analogies
Think of a traffic intersection where cars (current) are entering based on the road (voltage) available. If there are alternative routes (parallel resistances), the flow of traffic changes based on the ease of passage. Similarly, the input impedance can change based on how many paths current can take in the circuit.
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Create a free accountSo, if I see the signal here if the value of this R , it is much higher than R the signal here s in it will be very small. So, as a result we have to consider another attenuating factor.
Detailed Explanation
In this section, we focus on the implications of having a significantly high source resistance compared to the input resistance of the amplifier circuit. When the source resistance is much larger, it causes a substantial voltage drop before the signal even reaches the amplifier, leading to very weak signals at the input. This weakening of the signal is known as attenuation.
Examples & Analogies
Imagine trying to hear someone speaking quietly from a distance (high source resistance). You may hear muffled sounds and have difficulty understanding their words. Similarly, if the source resistance is too high, the intended signal to the amplifier gets weakened or lost.
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Create a free accountSo, basically we like to say that this voltage gain it will not remain so, nice. In fact, voltage gain... we need to derive the expression of the corresponding voltage gain when source resistance is taken into account.
Detailed Explanation
Finally, we discuss the major conclusion: with the influence of source resistance, the voltage gain that was ideally expected from the amplifier will not be achieved. It will be reduced due to the interactions between the input resistance of the amplifier and the source resistance, leading to a diminished overall voltage gain.
Examples & Analogies
Think about a water pump that usually raises a bucket very high (ideal voltage gain). If there are leaks in the hose (source resistance), the amount of water arriving at the bucket is significantly less than expected, and you can't raise the full bucket as high as before (decreased voltage gain).
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Key concepts
Core takeaways and short definitions to help you quickly recall the key ideas from this section.
- Finite Source Resistance:
Source resistance can significantly lower the voltage seen at the amplifier input.
- Common Base vs. Common Gate:
Differences in signal application between configurations affect voltage gain.
- Voltage Gain Derivation:
Clear equations show how gain relates to input and output resistances.
Examples
Step-by-step examples to apply the section's ideas and test your understanding.
When a source resistance of 1k ohm is connected to a common base amplifier with a 1k ohm input resistance, attenuation can become significant, leading to a reduced output voltage.
In a common gate amplifier, applying a signal to the source might yield a similar gain as a common base, but input resistance needs careful consideration to avoid signal loss.
Memory aids
Imagine a quiet speaker at a concert. The louder it is, the clearer the sound; amplifiers must ensure the volume stays right amidst resistances, or the message can get lost.
Remember 'GICE' for Gain, Input Resistance, Current gain, Emitter voltage to connect amplifier behaviors.
Flash Cards
Glossary
Voltage Gain
The ratio of output voltage to input voltage in an amplifier.
Source Resistance
The resistance presented by the source which provides the input signal to the amplifier.
Input Impedance
The impedance looking into the amplifier at the input terminals.
Transconductance (gm)
The ratio of the change in output current to the change in input voltage.
Common Base Configuration
An amplifier configuration where the base terminal is common to both input and output.
Common Gate Configuration
An amplifier configuration where the gate terminal is common to both input and output in MOSFETs.