MOSFET Active Resistance for Temperature-Stable RF Power Detection
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Solution Overview
Problem
Logarithmic power detectors in RF circuits experience significant output voltage variations due to temperature changes, as the gain of the difference amplifier remains constant, leading to a 60% variation over a temperature range of −40 C to 100 C, necessitating a resistance that varies with thermal voltage to mitigate this effect.
Innovation Solution
A VT-dependent resistor is implemented in an integrated circuit, comprising a MOSFET and an operational amplifier, with two current sources: one proportional to absolute temperature (IPTAT) and another constant current source (IREF), which together create an effective series resistance (RSUM) that varies with thermal voltage, thereby compensating for temperature-induced changes.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Stability of the object's composition
If a constant gain difference amplifier is used in a logarithmic power detector, then the amplifier provides stable amplification, but the output voltage varies by about 60% over a temperature range of -40 C to 100 C due to thermal voltage dependence
Solution Approach 1:
The patent changes the resistance parameter of the input resistor from a constant value to a temperature-dependent value. Specifically, the resistor value is made to vary with thermal voltage (VT) such that it increases with temperature, which compensates for the constant gain amplifier's output variation. This parameter change allows the system to maintain stable output voltage across temperature ranges while preserving the amplifier's stable gain characteristic.
2Device complexity
If the gain of the difference amplifier is kept constant, then the amplifier design is simple, but the output voltage varies directly with absolute temperature due to thermal voltage in the input stage
Solution Approach 1:
The patent modifies the resistor parameter to be temperature-dependent, specifically making it proportional to thermal voltage. This allows the simple constant gain amplifier design to be retained while improving measurement precision. The temperature-varying resistor value compensates for thermal voltage effects, thereby maintaining accurate output voltage readings across different temperatures without complicating the amplifier design.
3Temperature
If a temperature compensation mechanism is added to the difference amplifier, then output voltage stability improves, but the circuit complexity increases
Solution Approach 1:
Instead of adding a complex active temperature compensation mechanism, the patent simply changes the parameter of an existing resistor to be temperature-dependent. By making the input resistor value vary with thermal voltage, the circuit achieves temperature compensation passively. This approach improves output voltage stability while avoiding the need for additional active compensation components or complex control circuits.
Applied Scientific Principles
This section explains which scientific principles are used to turn an abstract innovation direction into a practical engineering solution.
Function Achieved in This Case
The VT-dependent resistor effectively stabilizes the output voltage of the difference amplifier by ensuring the resistance increases with temperature, minimizing variations over the temperature range, and allowing for a unity gain factor at any temperature, reducing output voltage fluctuations by up to 60%.
Implementation Method 1
The feedback loop forces an increasing voltage (VPTAT) across a constant current IREF, thus producing an effective resistance that is also increasing
Implementation Method 2
The VT-dependent resistor has a resistor R2 in series with a MOSFET, and an operational amplifier (OpAmp) driving the gate of the MOSFET
Implementation Method 3
Thermal voltage is a voltage produced within a p-n junction due to the action of temperature
Data Source
AI summary
A controlled active resistance. The active resistance is implemented on an integrated circuit. In some embodiments, the active resistance includes a MOSFET. In alternate embodiments, the active resistance includes a MOSFET and a resistor. The control for the active resistance includes a reference resistor and an operational amplifier. The control for the active resistance further includes two current sources: i) a current source producing a current that is proportional to absolute temperature, and ii) another current source that is produced by a bandgap voltage reference. In one aspect, the active resistance generates an effective resistance that is proportional to thermal voltage. In another aspect, the active resistance generates an effective resistance that is proportional to inverse of the thermal voltage. In an alternate aspect, the current sources have various dependencies, and the active resistance generates an effective resistance that is proportional to those dependencies.