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A short class with Sarah and her students. The other ways are under More.
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Today we are exploring differential amplifiers, particularly those made with BJTs. Can anyone tell me what the essential function of a differential amplifier is?
Is it to amplify the difference between two input signals?
Exactly! The primary function is to amplify the difference between two voltages. In our case, we are going to focus on using current mirrors in this context.
What exactly is a current mirror?
A current mirror is a circuit that copies the current flowing in one active device to another, keeping the current constant regardless of load changes. This is crucial in maintaining consistent biasing in our amplifiers.
So, it helps keep everything balanced?
Absolutely, balance is key. Disparities in current can lead to distortion in the output signal.
Got it! Thanks for clarifying.
Now, let's look at how we can replace our passive tail resistor with a current mirror. What do you think this accomplishes?
Does it improve the gain of the amplifier?
That's right! By using active devices instead of passive ones, we can improve our gain significantly. Have you heard of the term 'common mode gain'?
Yes, isn't that the gain for signals that appear at both inputs simultaneously?
Precisely! And with our current mirror in play, we can actually manage that gain effectively and minimize problems associated with signal distortion.
What happens if the components are not well matched?
Excellent question! Poor matching can lead to imbalances that could compromise the amplifier’s performance. We’ll be analyzing small signal models to ensure good matching.
Let’s get into small signal models. What’s the key reason for using small signal analysis?
To simplify complex circuits into manageable equations?
Correct! With these, we can calculate aspects such as the differential mode gain. Can anyone recall our formula for differential gain?
Is it based on the transconductance and impedance?
Yes, great memory! The differential mode gain can often be expressed as the transconductance multiplied by the load resistance.
Current mirrors do reflect, providing currents we can respect.
Imagine a seesaw representing a differential amplifier where the kids on each side are two signals, and the balance is maintained using current mirrors – the referees ensuring fair play for both kids.
C.A.B. - Current, Amplification, Balance - the three pillars of a well-functioning differential amplifier.
CMA = Current Mirror Advantage - reminding us of the benefits of using current mirrors.
Term
Function of a Current Mirror
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Term
Differential Amplifier Role
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A circuit designed to copy current from one branch of a circuit to another while maintaining the same current value.
An amplifier that amplifies the difference between two input signals.
A passive component in a differential amplifier that sets the tail current for the circuit.
The amplification factor for signals common to both inputs of the amplifier.
The amplification factor for the difference in signals applied to the two inputs of the amplifier.
This section discusses the usage of current mirrors in differential amplifiers using BJT.
The section explores how current mirrors are integrated into differential amplifiers, focusing on replacement of passive elements with active devices, and the impact on circuit performance. It emphasizes the importance of mirrored reference currents for consistent biasing and load conditions.
In this section, we delve into the implementation of current mirrors in BJT-based differential amplifiers, highlighting the replacement of tail resistors and passive loads with active components. The use of current mirrors not only sets the tail current accurately but also ensures better circuit performance in terms of common and differential mode gains. We analyze the small signal models that arise from these modifications and look at how differential mode gain can be optimized through careful design. The significance of matching components for maintaining balance in the circuit is also discussed thoroughly, essential for reliable amplification.