DC input random offset voltage for diff. amplifier with current-mirror load

  • #1
eyeweyew
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TL;DR Summary
how the expression for DC input offset voltage for diff. amplifier with current mirror load come about
I am trying to figure out how the derivation of equation 6.69 come about on page 426 in the book Analysis and Design of Analog Integrated Circuits, 5th Edition by Gray and Meyer. They defined on page 424 under section 6.3.3 the input offset voltage (VOS) of op amps with differential inputs and single-ended outputs as the differential input voltage for which the dc output voltage is midway between the supplies. But I don’t see VSS and VDD in the equation so I am not sure if they are using this definition since they did mention another definition for VOS under 6.3.3 which is the differential input voltage for which the op-amp output voltage is zero but I try to follow the similar derivation on pg. 234-236 under 3.5.6.6 and 3.5.6.7 for diff. amp with resistors pair as loads but I still don’t get how do they come about equation 6.69?Analysis and Design of Analog Integrated Circuits, 5th Edition
http://fa.ee.sut.ac.ir/Downloads/AcademicStaff/24/Courses/73/[Gray___Meyer]_Analysis_and_Design_of_Analog_Integrated_Circuits_5th_ed.pdf
 
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  • #2
eyeweyew said:
But I don’t see VSS and VDD in the equation so I am not sure if they are using this definition ...
The supply voltages can be assumed typical, equal and opposite, and sufficient for the circuit to operate.

Supply voltage is independent, since the circuit is symmetrical. The supplies are isolated from the differential input pair, by the collector or drain voltage of the other transistors employed.

Part of the elegance of employing a current mirror as the load, is supply voltage independence, while the gain of the differential input pair is effectively squared.
 
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  • #3
eyeweyew said:
TL;DR Summary: how the expression for DC input offset voltage for diff. amplifier with current mirror load come about

I am trying to figure out how the derivation of equation 6.69 come about on page 426 in the book Analysis and Design of Analog Integrated Circuits, 5th Edition by Gray and Meyer. They defined on page 424 under section 6.3.3 the input offset voltage (VOS) of op amps with differential inputs and single-ended outputs as the differential input voltage for which the dc output voltage is midway between the supplies. But I don’t see VSS and VDD in the equation so I am not sure if they are using this definition since they did mention another definition for VOS under 6.3.3 which is the differential input voltage for which the op-amp output voltage is zero but I try to follow the similar derivation on pg. 234-236 under 3.5.6.6 and 3.5.6.7 for diff. amp with resistors pair as loads but I still don’t get how do they come about equation 6.69?Analysis and Design of Analog Integrated Circuits, 5th Edition
http://fa.ee.sut.ac.ir/Downloads/AcademicStaff/24/Courses/73/[Gray___Meyer]_Analysis_and_Design_of_Analog_Integrated_Circuits_5th_ed.pdf
Ok, Never mind. The derivation is actually on page 332-333 in the same book.
 

What is DC input random offset voltage in a differential amplifier?

DC input random offset voltage in a differential amplifier refers to the inherent voltage difference that exists between the two input terminals of the amplifier when the output is zero, despite equal input voltages. This offset can arise due to mismatches in transistor properties, variations in component values, and other asymmetries in the circuit design.

How does a current-mirror load affect the offset voltage in a differential amplifier?

A current-mirror load in a differential amplifier can influence the offset voltage primarily through mismatches in the current mirror transistors. These mismatches can lead to unequal currents in the branches of the differential pair, thereby affecting the balance and increasing the offset voltage. The precision of the current mirror plays a crucial role in minimizing this effect.

What are the typical causes of random offset voltage in differential amplifiers?

The typical causes of random offset voltage in differential amplifiers include variations in transistor threshold voltages, differences in transistor sizes, variations in passive component values like resistors and capacitors, thermal gradients, and supply voltage differences. These factors can all contribute to the asymmetry in the electronic characteristics of the differential pair and the current mirrors.

How can the random offset voltage be minimized in differential amplifiers?

To minimize random offset voltage in differential amplifiers, designers can employ techniques such as carefully matching the transistors used in the differential pairs and current mirrors, using components with tighter tolerance levels, implementing thermal management strategies to reduce gradients across the circuit, and designing with enhanced feedback mechanisms. Additionally, calibration and trimming techniques post-manufacturing can also be used to adjust and compensate for offset voltage.

What is the impact of random offset voltage on the performance of differential amplifiers?

The impact of random offset voltage on the performance of differential amplifiers includes reduced accuracy and precision in signal amplification, potential issues with signal integrity, and increased noise levels. This can particularly affect applications requiring high precision and low noise, such as instrumentation amplifiers in measurement systems and audio amplifiers. Managing and minimizing offset voltage is crucial for maintaining the overall performance and reliability of these systems.

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