Differential Op-Amp Input Stage With Extended Linear Loading Range
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Solution Overview
Problem
Operational amplifier circuits with differential input stages face nonlinear relationships between loading currents and input voltage differences, limiting their linear range, especially in LCD driver applications, where large input voltage differences lead to decreased transconductance and improper operation.
Innovation Solution
Incorporating a linearity enhancement circuit, such as a bias control circuit or voltage maintaining circuit, into the differential input stage of the operational amplifier circuit to extend the linear range by maintaining the input transistors in the triode and saturation regions, thereby stabilizing transconductance and preventing current accumulation.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Manufacturing precision
If larger bias current is provided for the differential pair to increase the linear range, then the linear range is improved, but power consumption increases
Solution Approach 1:
The patent changes the operating region parameter of the input transistors from saturation region to triode region by adjusting the voltage relationship between drain and gate terminals. Specifically, it ensures that the drain voltage of each input transistor is higher than its gate voltage by a certain threshold, forcing the transistor into triode region operation. This parameter change allows the differential pair to maintain linear transconductance characteristics without requiring increased bias current, thus resolving the contradiction between linearity and power consumption
Solution Approach 2:
The patent introduces dynamic control of the drain voltages of the input transistors through additional control circuits. The drain voltages are dynamically adjusted to maintain the triode region operation condition (Vdrain > Vgate + Vth) across varying input conditions. This dynamic voltage control ensures that the transistors remain in the optimal operating region, maintaining linear relationship between input voltage difference and loading current without increasing power consumption
2Adaptability or versatility
If the input voltage difference is too large, then the differential pair operates nonlinearly, but increasing the input voltage difference is needed for certain applications
Solution Approach 1:
The patent fundamentally changes the operating region parameter from saturation to triode region. In triode region, the drain current has a linear relationship with the gate-source voltage difference, which maintains linearity even for large input voltage differences. The patent ensures this by controlling the drain voltage to be sufficiently higher than the gate voltage, keeping the transistor in triode region throughout the extended input voltage range, thus achieving both adaptability and linearity
Solution Approach 2:
The patent implements dynamic voltage control to adapt the operating point of the differential pair to large input voltage differences. By dynamically adjusting the drain voltages and maintaining the triode region condition, the circuit adapts to various input conditions while preserving linear transconductance characteristics, enabling the differential pair to handle large input voltage ranges without losing linearity
Data Source
AI summary
An operational amplifier circuit is provided. The operational amplifier circuit includes a differential input stage circuit and a loading stage circuit. The differential input stage circuit includes an input circuit, a voltage maintaining circuit, and a current source. The input circuit includes a first input transistor and a second input transistor, for receiving a first and a second input signals, respectively. The voltage maintaining circuit includes a first branch circuit and a second branch circuit. The first branch circuit is coupled to the first input transistor for receiving the first input signal, and the second branch circuit is coupled to the second input transistor for receiving the second input signal. The current source is coupled to the first input transistor and the second input transistor. The loading stage circuit is coupled to the voltage maintaining circuit.