Logarithmic Circuit Temperature Compensation for Bandwidth Stability
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Solution Overview
Problem
Translinear logarithmic amplifiers (log amps) have limited bandwidth, especially at low input currents, due to high incremental emitter resistance and collector-junction capacitance, which restricts their dynamic range and accuracy.
Innovation Solution
The implementation of a dual-loop logarithmic circuit with a guard circuit maintaining constant collector-base voltage and a positioning circuit controlling the emitter voltage, along with adaptive compensation mechanisms that vary in response to input current, temperature, and frequency, to stabilize the circuit and enhance bandwidth.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If translinear logarithmic amplifiers are used to achieve logarithmic output, then the dynamic range is compressed for ease of processing, but the bandwidth is limited due to high incremental emitter resistance and collector-junction capacitance
Solution Approach 1:
The patent divides the feedback function into two separate loops: a guard circuit that maintains constant collector-base voltage and a positioning circuit that controls emitter voltage. This segmentation allows each circuit to optimize for its specific function, reducing the overall time constant and improving bandwidth while maintaining the logarithmic compression benefit.
Solution Approach 2:
The patent introduces an intermediary compensation circuit that actively counteracts the effects of collector-junction capacitance. This compensation mechanism serves as a mediator between the logging transistor and the feedback amplifier, eliminating the capacitance-induced bandwidth limitation while preserving the logarithmic output characteristic.
2Device complexity
If the collector-base voltage is allowed to vary, then the circuit operation is simpler, but the incremental emitter resistance increases limiting the bandwidth
Solution Approach 1:
The patent segments the voltage control functions into two independent circuits: the guard circuit specifically maintains constant collector-base voltage to reduce incremental emitter resistance, while the positioning circuit handles emitter voltage control. This segmentation enables the collector-base voltage to be held constant without overly complicating the overall circuit operation.
3Stability of the object's composition
If compensation circuits are added to stabilize the logarithmic output against temperature variations, then the temperature stability is improved, but the device complexity increases
Solution Approach 1:
The patent merges the temperature compensation function with the existing dual-loop feedback structure. The compensation circuits are integrated into the guard and positioning circuits, allowing temperature stabilization to be achieved without adding completely separate compensation pathways. This merging approach improves temperature stability while minimizing the increase in device complexity.
Solution Approach 2:
The guard circuit and positioning circuit serve multiple functions: they control voltage parameters for bandwidth improvement, provide logarithmic compression, and simultaneously offer temperature compensation. This multi-functionality reduces the need for separate dedicated compensation circuits, thereby limiting the increase in device complexity.
4Device complexity
If the emitter voltage is not controlled, then the circuit is simpler, but the bandwidth is reduced due to high incremental emitter resistance
Solution Approach 1:
The patent segments the voltage control into dedicated positioning circuitry that specifically manages emitter voltage. This segmentation allows the emitter voltage to be controlled to maintain low incremental emitter resistance without requiring complete redesign of the entire circuit, thus improving bandwidth with moderate complexity increase.
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
This approach significantly improves the bandwidth of translinear log amps by reducing the time constant and eliminating the effects of collector-junction capacitance, allowing for higher cutoff frequencies and expanded dynamic range, while maintaining stability across varying operating conditions.
Implementation Method 1
The operational amplifier (op amp) OA1 forces the collector current IC of transistor Q1 to equal the input current IX while maintaining the collector-base voltage very close to zero. The output signal VLOG is then equal to the base-emitter voltage of transistor Q1. Because the output has a logarithmic relation to the input
Implementation Method 2
Translinear logarithmic amplifiers (log amps) have limited bandwidth, especially at low input currents, due to high incremental emitter resistance and collector-junction capacitance
Implementation Method 3
a positioning circuit controlling the emitter voltage, along with adaptive compensation mechanisms that vary in response to input current, temperature, and frequency
Implementation Method 4
adaptive compensation mechanisms that vary in response to input current, temperature, and frequency, to stabilize the circuit and enhance bandwidth
Data Source
AI summary
An embodiment of a logarithmic circuit may include a logging transistor, and a multi-tanh circuit arranged to provide temperature compensation to the logging transistor, where the multi-tanh circuit comprises a multiplicity of multi-tanh cells. In another embodiment, a logarithmic circuit may include a logging transistor, and a multi-tanh circuit arranged to provide temperature compensation to the logging transistor, where the multi-tanh circuit includes a first set of outputs arranged to provide an output signal and a second set of one or more outputs that are diverted.