Depletion-Mode Switch Circuit for Negative Signal Isolation
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Solution Overview
Problem
Depletion-mode field-effect transistors (DMFETs) used as switches in analog circuits face challenges in maintaining an OFF-state when signals swing negative, as existing techniques with fixed negative voltages can cause the switch to turn ON due to positive overdrive voltage, leading to undesired conduction.
Innovation Solution
A negative discriminator determines the most negative voltage in the system and a charge pump converts it to a lower voltage, typically by 2.5 V, ensuring the DMFET remains in an OFF-state by applying this voltage to the gate, maintaining a constant negative gate-source voltage below the threshold voltage.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If a fixed negative voltage is applied to the gate terminal of the DMFET, then the DMFET can be maintained in an OFF-state under normal conditions, but the DMFET may turn ON undesirably when the source terminal voltage swings negative and approaches the gate voltage
Solution Approach 1:
The gate voltage is changed from a fixed value to a dynamic value that tracks the source voltage. The discriminator continuously monitors the source voltage and adjusts the gate voltage accordingly, making the gate voltage dynamic rather than static. This dynamic adjustment ensures the DMFET remains in the desired OFF-state even as signal conditions change.
Solution Approach 2:
A feedback mechanism is introduced through the discriminator and charge pump circuitry. The discriminator monitors the source terminal voltage and provides feedback control to adjust the gate voltage. This closed-loop feedback ensures that the gate-source voltage difference is maintained below the threshold voltage, preventing undesired conduction during negative signal swings.
2Reliability
If the gate voltage is adjusted to prevent undesired conduction during negative signal swings, then switching reliability is improved, but the circuit complexity increases due to additional components
Solution Approach 1:
An intermediary circuit consisting of a discriminator and charge pump is introduced between the source terminal and the gate terminal. The discriminator acts as a mediator that processes the source voltage information and generates the appropriate gate voltage. This intermediary approach achieves reliable switching control while keeping the DMFET itself simple and maintaining its inherent advantages.
Data Source
AI summary
This document discloses, among other things, a switch circuit that includes a depletion-mode field-effect transistor (DMFET) having an ON-state and an OFF-state, wherein the DMFET is configured to couple a first node to a second node in the ON-state, and wherein the DMFET is configured to isolate the first node from the second node in the OFF-state, a negative charge pump that is coupled to a gate terminal of the DMFET, the charge pump configured to supply a negative charge pump voltage to the gate terminal of the DMFET, and a negative discriminator coupled to the charge pump, the discriminator configured to compare a first voltage at the first node and a second voltage at the second node and determine the negative charge pump voltage based on the comparison.