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Today, we're going to explore differential amplifiers, particularly those using BJTs. Can anyone tell me what a differential amplifier does?
It amplifies the difference between two input signals.
Exactly! It amplifies the voltage difference between the two inputs. Now, why is this important?
It's important for noisy environments because it can reject common noise.
Great point! This rejection of common-mode signals is known as common-mode rejection. Remember the acronym CMRR for Common Mode Rejection Ratio.
Now, let's discuss how we set the tail current in these amplifiers. What component can we use?
Current mirrors!
Perfect! Current mirrors allow us to establish a stable tail current. Does anyone know what the reference current in a current mirror is usually derived from?
It comes from a voltage source, usually a V_CC.
Correct! We can approximate that current in terms of the reference current, which, in turn, allows us to analyze the common mode gain. Keep in mind the factor of nonideality due to Early voltage.
To analyze our circuit, we can draw a small signal equivalent model. Does anyone know what parameters vary in this model?
The transistor parameters like transconductance g_m and output resistances.
Exactly! These parameters will help us find the common mode and differential mode gains. Remember, g_m is crucial here. Can someone describe how it plays a role?
g_m contributes to how effectively our amplifier can convert input voltage to output current.
Well put! Now let's relate this to our common mode gain, which we represent with A_{cm}.
We just talked about common mode gain, but what benefits do active loads provide over passive ones?
Active loads improve gain and linearity in the circuit.
That's correct! Active loads can increase both differential and common mode gains. Remember the acronym for active load benefits: AGA - Amplified Gain Advantage.
So, using an active load helps achieve better performance overall?
Exactly! And it allows us to use the same design for simpler single-ended outputs as well.
Let's recap what we've learned about common mode gain and differential amplifiers. What are the primary components?
BJTs and current mirrors.
And the small signal model parameters like g_m and output impedance!
Excellent summary! Remember, the use of active loads enhances gain and offers better balance in differential amplifiers. Keep these concepts in mind as they are fundamental to analog electronics.
The gain applied to signals that are different between two inputs.
A measure of the ability of a differential amplifier to reject common-mode signals.
Active loads improve performance in terms of gain and linearity over passive loads.
Gain in the common mode is a trick, Active loads make signals thick.
Imagine a race where two cars start together; the differential amplifier only focuses on the speed difference, ignoring the noise of both engines revving up.
C-A-G: Current, Active, Gain - remember these key aspects of amplifiers!
C-M-R-R: Common Mode Rejection Ratio - it's key to how well amplifiers reject noise.
Term
Common Mode Rejection Ratio (CMRR)
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Term
Differential Amplifier Gain
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The amplification factor applied to the common mode signals in a differential amplifier.
An electronic amplifier that amplifies the difference between two input signals.
A circuit that provides a regulated output current that is proportional to an input reference current.
The measure of the output current change for a given change in input voltage in a transistor.
A load that uses active devices such as transistors to maintain high gain and linearity.
The parameter in BJTs that accounts for variations in current with changes in voltage.
This section discusses the analysis of common mode gain in differential amplifiers using BJTs and current mirrors.
The section provides an overview of how common mode gain is influenced by circuit configurations involving BJTs and current mirrors, emphasizing the advantages of using active load versus passive load in differential amplifiers.
In this section, we dive into the analysis of common mode gain in differential amplifiers constructed with bipolar junction transistors (BJTs). The focus is on how these amplifiers utilize current mirrors to replace traditional passive elements and enhance performance. We begin by examining the configuration of BJTs in differential amplifiers, particularly how tail current is established using current mirrors. The relationship between the reference current and the output currents of the transistors is explained, detailing how factors like Early voltage and β-loss can affect the output.
By establishing small signal models, we investigate how common mode gain can be approximated, and the significance of active load compared to passive load is highlighted. The section explains the implications of these configurations on overall amplifier performance, including improved common mode rejection ratio while maintaining high differential mode gain. We conclude by noting that modifications using active current mirrors not only simplify designs but also enhance signal fidelity in both differential and single-ended outputs.