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A short class with Sarah and her students. The other ways are under More.
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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.
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.
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.
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.
Source resistance can significantly lower the voltage seen at the amplifier input.
Differences in signal application between configurations affect voltage gain.
Clear equations show how gain relates to input and output resistances.
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.
In amplifiers, don't be steered, by resistances that strike your yield.
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.
GAINS: Gain, AC response, Input impedance, Non-inverting configuration, Source resistance.
Term
Voltage Gain
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Term
Finite Source Resistance
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Term
Common Base Configuration
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Term
Common Gate Configuration
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The ratio of output voltage to input voltage in an amplifier.
The resistance presented by the source which provides the input signal to the amplifier.
The impedance looking into the amplifier at the input terminals.
The ratio of the change in output current to the change in input voltage.
An amplifier configuration where the base terminal is common to both input and output.
An amplifier configuration where the gate terminal is common to both input and output in MOSFETs.
This section discusses how finite source resistance affects voltage gain in common base and common gate amplifiers.
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.
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.