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A short class with Sarah and her students. The other ways are under More.
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Good morning, class! Today we will look into differential amplifiers. Can anyone explain why we need to consider common mode voltage in these circuits?
Isn’t it because we want to ensure the transistors operate properly under varying input conditions?
Exactly! Common mode voltage affects how we operate the transistors. We need to make sure both are biased properly, typically above 0.6V for BJT operation. Let's remember this as 'Minimum Bias Voltage (MBV)'.
How do we find out what common mode voltage range is suitable?
That's great! We'll explore calculations that reveal upper and lower limits of common mode voltage later.
Now, let’s calculate the current through a resistor in the circuit. If you apply a 0.8V DC voltage across a 1 kΩ resistor, what current do you expect?
Oh! Using Ohm's law, I = V/R, we can find it!
So that would be 0.2 mA?
Correct! That current sets our voltage drops further down the circuit. Remember, this value is 'Critical Current (CC)'.
Could this affect output voltage?
Absolutely! That brings us to output voltage calculations when considering both differential and common mode gains.
What happens to output voltage when our input conditions change?
If the differential signal increases, the output should increase proportionally based on the gain!
Exactly! That’s why we compute differential mode gain, highlighted as 'Differential Gain (DGA)'. In our previous calculations, we found this gain to be 200 in one scenario.
And the common mode gain is lower, right? It affects how clean our output signal is?
Yes! We've seen how common mode components can distort our output. Balancing both gains is vital.
Can we adjust our design to improve the gain?
We certainly can! We will discuss modifications in upcoming sessions.
In amp design, don’t forget, common mode knows its debt.
Imagine two brothers, one represents V1 and the other V2, when they fight, their output is distorted, just like the amplifier!
DCs help Affirm: V biases are a must for normal amplify.
DGA: Differential Gain Amplifies (DGA).
Term
Differential Gain (DGA)
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Term
Critical Current (CC)
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This section discusses the analysis of differential amplifiers and numerical examples illustrating their operation.
It covers the concept of common mode voltage in differential amplifiers, explaining how to calculate suitable ranges while considering the operating points and gain effects. Several numerical examples elucidate the implications of these calculations on circuit design, ensuring both theoretical and practical understanding.
In this section, we delve into the workings of differential amplifiers, focusing on the impact of common mode voltage and associated numerical examples. The discussion begins with basic assumptions on DC voltage values required for operating the transistors in the active region. The students learn to calculate the suitable range of common mode voltages and the implications of varying these voltages on the device's performance, particularly within the context of a BJT-based differential amplifier. Multiple scenarios illustrate the changes in current, voltage drop, and overall gain. The complexities of distinguishing between differential and common mode gains are highlighted, alongside detailed calculations that demonstrate the operational limits imposed by the choice of voltage levels.