CMOS Analog Switch Circuit Substrate Biasing
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Solution Overview
Problem
Existing CMOS analog switch designs suffer from substrate effects, leading to increased threshold voltage and conduction resistance, which hinder normal switching operations and introduce harmonic distortion, and are prone to latch-up and leakage when supply voltage is not applied, causing the substrate voltage to float.
Innovation Solution
A CMOS analog switch circuit design that includes voltage extractors to bias the substrate voltage to ground, using a combination of MOSFETs to equalize source and substrate voltages and a charge pump to maintain voltage levels, ensuring stable operation regardless of supply voltage presence.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Device complexity
If supply voltage is directly applied to the sources of NMOS and PMOS, then the switch can operate with simple circuit structure, but substrate effect increases threshold voltage and conduction resistance
Solution Approach 1:
The patent introduces a substrate voltage node as an intermediary between the power supply and the MOS transistor substrates. This substrate voltage node is controlled by control signals to dynamically adjust substrate potentials, thereby eliminating substrate effects without requiring complex additional circuitry. The intermediary substrate voltage node mediates between the simple power supply connection and the need for substrate effect compensation.
Solution Approach 2:
The patent dynamically changes the substrate voltage parameter based on the control signal state. When the switch is on, the substrate voltage is adjusted to match the source voltage to eliminate substrate effects. When the switch is off, the substrate voltage is maintained at a different level. This dynamic parameter change allows the circuit to maintain simple structure while achieving reliable switching operation free from substrate effects.
2Device complexity
If substrate voltage is not biased to ground, then circuit operation is simplified, but substrate voltage floats causing latch-up and leakage when supply voltage is not applied
Solution Approach 1:
The patent makes the substrate voltage biasing dynamic rather than static. The substrate voltage node is connected to ground through a path that is selectively enabled by control signals. When the switch is off or when supply voltage is not applied, the substrate voltage is biased to ground to prevent latch-up and leakage. When the switch is on, the substrate voltage is dynamically adjusted to match the source voltage. This dynamic approach maintains simple circuit operation while ensuring reliability across different operating conditions.
3Quantity of substance
If substrate effect is not compensated, then device count is minimized, but threshold voltage and conduction resistance increase
Solution Approach 1:
The patent makes the existing control signals serve multiple functions. The same control signals that drive the NMOS and PMOS gates are also used to control the substrate voltage node. This multi-functionality allows substrate effect compensation without adding dedicated control circuits or increasing device count. The control signals universally manage both the switching operation and the substrate voltage adjustment.
Data Source
AI summary
A Complementary Metal-Oxide-Semiconductor (CMOS) analog switch has a circuit structure such that when a supply voltage is applied, the CMOS analog switch biases voltages at both ends of a Metal-Oxide-Semiconductor Field Effect Transistor (MOS) device, which switches on upon application of supply voltage, to a substrate node of MOS, or biases the substrate voltage of MOS device to a ground voltage state during a switching-off operation. The substrate voltage of MOS device in floating state is still biased to the ground voltage state even when abnormal, high voltages are applied to both ends of the MOS device. As a result, threshold voltage and conduction resistance decrease compared to related analog switches, and frequency bandwidth increases.