Differential Amplifier Basics

Oct 13, 2025 by thesweetdesigns.com 53 views

Introduction

Differential amplifiers are a crucial component in many analog and mixed-signal circuits, including amplifiers, filters, and data converters. They offer several advantages, including improved noise rejection, increased common-mode rejection ratio (CMRR), and enhanced linearity. However, the performance of differential amplifiers can be compromised by transistor mismatches, which can lead to reduced gain, increased distortion, and decreased CMRR. In this article, we will focus on the differential-mode gain of differential amplifiers with mismatched transconductance ($g_m$).

Differential Amplifier Basics

A differential amplifier is a type of amplifier that amplifies the difference between two input signals. It consists of two input transistors, two output transistors, and a biasing circuit. The input transistors are typically connected in a differential configuration, with the input signal applied to one transistor and the inverted input signal applied to the other transistor. The output transistors are connected in a common-source configuration, with the output signal taken from the drain of one transistor.

Transconductance ($g_m$) and its Importance

Transconductance ($g_m$) is a measure of the change in drain current ($I_D$) of a transistor in response to a change in gate voltage ($V_G$). It is a critical parameter in the design of differential amplifiers, as it determines the gain of the amplifier. In a differential amplifier, the transconductance of the input transistors ($g_m1$ and $g_m2$) determines the differential-mode gain.

Mismatched $g_m$ and its Impact

Mismatched $g_m$ occurs when the transconductance of the two input transistors is not equal. This can be due to various factors, including process variations, temperature changes, and aging. Mismatched $g_m$ can lead to reduced differential-mode gain, increased distortion, and decreased CMRR.

Derivation of Differential-mode Gain

To derive the differential-mode gain of a differential amplifier with mismatched $g_m$, we start with the small-signal equivalent circuit of the amplifier. The small-signal equivalent circuit consists of two input transistors, two output transistors, and a biasing circuit.

The differential-mode gain ($A_{DM}$) of the amplifier can be expressed as:

A_{DM} = \frac{V_{OUT}}{V_{IN}}

where $V_{OUT}$ is the output voltage and $V_{IN}$ is the input voltage.

Using the small-signal equivalent circuit, we can express the output voltage ($V_{OUT}$) as:

V_{OUT} = g_m1 \cdot r_o1 \cdot V_{IN1} + g_m2 \cdot r_o2 \cdot V_{IN2}

where $r_o1$ and $r_o2$ are the output resistances of the two output transistors.

Substituting the expression for $V_{OUT}$ into the equation for $A_{DM}$, we get:

A_{DM} = \frac{g_m1 \cdot r_o1 \cdot V_{IN1} + g_m2 \cdot r_o2 \cdot V_{IN2}}{V_{IN}}

Simplifying the expression, we get:

A_{DM} = g_m1 \cdot r_o1 + g_m2 \cdot r_o2

However, this expression assumes that the transconductance of the two input transistors is equal. In reality, the transconductance of the two input transistors can be mismatched, leading to a reduced differential-mode gain.

To account for the mismatched $g_m$, we can express the differential-mode gain as:

A_{DM} = g_m1 \cdot r_o1 + g_m2 \cdot r_o2 + \Delta g_m \cdot r_o

where $\Delta g_m$ is the mismatched transconductance and $r_o$ is the output resistance of the output transistors.

Red-Border Expression

The red-border expression is a simplified expression for the differential-mode gain, which is given by:

A_{DM} = \frac{g_m1 \cdot r_o1 + g_m2 \cdot r_o2}{1 + \frac{\Delta g_m \cdot r_o}{g_m1 \cdot r_o1 + g_m2 \cdot r_o2}}

This expression assumes that the mismatched transconductance ($g_m$) is small compared to the transconductance of the two input transistors.

Conclusion

In conclusion, the differential-mode gain of a differential amplifier with mismatched $g_m$ can be expressed as a function of the transconductance of the two input transistors, the output resistances of the output transistors, and the mismatched transconductance. The red-border expression provides a simplified expression for the differential-mode gain, which can be used to estimate the impact of mismatched $g_m$ on the performance of the amplifier.

References

Razavi, B. (2001). Design of Analog CMOS Integrated Circuits. McGraw-Hill.
Gray, P. R., & Meyer, R. G. (2001). Analysis and Design of Analog Integrated Circuits. John Wiley & Sons.

Appendix

The following appendix provides a detailed derivation of the differential-mode gain of a differential amplifier with mismatched $g_m$.

A.1 Small-Signal Equivalent Circuit

The small-signal equivalent circuit of the differential amplifier consists of two input transistors, two output transistors, and a biasing circuit.

A.2 Output Voltage

The output voltage ($V_{OUT}$) can be expressed as:

V_{OUT} = g_m1 \cdot r_o1 \cdot V_{IN1} + g_m2 \cdot r_o2 \cdot V_{IN2}

A.3 Differential-Mode Gain

The differential-mode gain ($A_{DM}$) can be expressed as:

A_{DM} = \frac{V_{OUT}}{V_{IN}}

Substituting the expression for $V_{OUT}$, we get:

A_{DM} = \frac{g_m1 \cdot r_o1 \cdot V_{IN1} + g_m2 \cdot r_o2 \cdot V_{IN2}}{V_{IN}}

Simplifying the expression, we get:

A_{DM} = g_m1 \cdot r_o1 + g_m2 \cdot r_o2

To account for the mismatched $g_m$, we can express the differential-mode gain as:

A_{DM} = g_m1 \cdot r_o1 + g_m2 \cdot r_o2 + \Delta g_m \cdot r_o

where $\Delta g_m$ is the mismatched transconductance and $r_o$ is the output resistance of the output transistors.

A.4 Red-Border Expression

The red-border expression is a simplified expression for the differential-mode gain, which is given by:

A_{DM} = \frac{g_m1 \cdot r_o1 + g_m2 \cdot r_o2}{1 + \frac{\Delta g_m \cdot r_o}{g_m1 \cdot r_o1 + g_m2 \cdot r_o2}}

Q: What is the impact of mismatched $g_m$ on the differential-mode gain of a differential amplifier?

A: Mismatched $g_m$ can lead to reduced differential-mode gain, increased distortion, and decreased common-mode rejection ratio (CMRR). The differential-mode gain of a differential amplifier with mismatched $g_m$ can be expressed as a function of the transconductance of the two input transistors, the output resistances of the output transistors, and the mismatched transconductance.

Q: How can I estimate the impact of mismatched $g_m$ on the differential-mode gain of a differential amplifier?

A: You can use the red-border expression to estimate the impact of mismatched $g_m$ on the differential-mode gain of a differential amplifier. The red-border expression is a simplified expression for the differential-mode gain, which is given by:

A_{DM} = \frac{g_m1 \cdot r_o1 + g_m2 \cdot r_o2}{1 + \frac{\Delta g_m \cdot r_o}{g_m1 \cdot r_o1 + g_m2 \cdot r_o2}}

This expression assumes that the mismatched transconductance ($g_m$) is small compared to the transconductance of the two input transistors.

Q: What are the factors that contribute to mismatched $g_m$ in a differential amplifier?

A: The factors that contribute to mismatched $g_m$ in a differential amplifier include process variations, temperature changes, and aging. These factors can cause the transconductance of the two input transistors to be mismatched, leading to reduced differential-mode gain, increased distortion, and decreased CMRR.

Q: How can I minimize the impact of mismatched $g_m$ on the differential-mode gain of a differential amplifier?

A: You can minimize the impact of mismatched $g_m$ on the differential-mode gain of a differential amplifier by using techniques such as:

Using transistors with matched $g_m$
Using a biasing circuit that minimizes the impact of mismatched $g_m$
Using a differential amplifier with a high CMRR
Using a differential amplifier with a high gain

Q: What are the applications of differential amplifiers with mismatched $g_m$?

A: Differential amplifiers with mismatched $g_m$ are used in a variety of applications, including:

Audio amplifiers
Instrumentation amplifiers
Medical devices
Automotive systems

Q: How can I design a differential amplifier with mismatched $g_m$?

A: You can design a differential amplifier with mismatched $g_m$ by using a combination of transistors with matched $g_m$ and a biasing circuit that minimizes the impact of mismatched $g_m$. You can also use a differential amplifier with a high CMRR and a high gain to minimize the impact of mismatched $g_m$.

Q: What are the limitations of differential amplifiers with mismatched $g_m$?

A: The limitations of differential amplifiers with mismatched $g_m$ include:

Reduced differential-mode gain
Increased distortion
Decreased CMRR
Increased sensitivity to process variations, temperature changes, and aging

Q: How can I measure the differential-mode gain of a differential amplifier with mismatched $g_m$?

A: You can measure the differential-mode gain of a differential amplifier with mismatched $g_m$ using a variety of techniques, including:

Using a signal generator to generate a differential input signal
Using an oscilloscope to measure the output voltage
Using a spectrum analyzer to measure the frequency response of the amplifier

Q: What are the future directions for research on differential amplifiers with mismatched $g_m$?

A: The future directions for research on differential amplifiers with mismatched $g_m$ include:

Developing new techniques for minimizing the impact of mismatched $g_m$
Developing new architectures for differential amplifiers with mismatched $g_m$
Developing new applications for differential amplifiers with mismatched $g_m$