Logarithmic Current-to-Voltage Converter With Emitter Resistance Compensation
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Solution Overview
Problem
Existing logarithmic converters suffer from poor log-linear dynamic range and high log error due to the collector-to-base voltage of NPN bipolar transistors not being close to zero, especially for small input currents, and errors introduced by emitter resistance in logarithmic current-to-voltage conversion.
Innovation Solution
Incorporating an emitter resistance compensation circuit with a replica of the logarithmic bipolar transistor to process a copy of the input current and generate a compensation signal that adjusts the logarithmic voltage to correct for emitter resistance errors, thereby achieving high accuracy and low log error in logarithmic current-to-voltage conversion.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If a trans-linear logarithmic detector uses the trans-linear properties of a bipolar transistor to provide logarithmic conversion, then the circuit can provide logarithmic detection, but the collector-to-base voltage of the NPN bipolar transistor is not very close to zero, leading to poor log-linear dynamic range for small input currents
Solution Approach 1:
An operational amplifier is introduced as an intermediary component to control the collector-to-base voltage of the bipolar transistor. The op-amp maintains this voltage close to zero through feedback control, thereby eliminating the harmful voltage error while preserving the logarithmic detection function
Solution Approach 2:
The circuit changes the operating parameters of the bipolar transistor by using the op-amp to regulate the collector-to-base voltage. This parameter control transforms the transistor's operation to achieve both accurate logarithmic conversion and extended dynamic range
2Measurement precision
If an operational amplifier is used to control the collector-to-base voltage close to zero, then the log-linear dynamic range improves, but the device complexity increases
Solution Approach 1:
The operational amplifier serves multiple functions: it controls the collector-to-base voltage, provides feedback regulation, and enables accurate logarithmic conversion across a wide dynamic range. This multi-functionality justifies the added complexity by delivering superior performance
3Measurement precision
If a second copy of a logarithmic detector is included and driven by a reference current to cancel the saturation current, then the measurement accuracy improves, but the device complexity increases
Solution Approach 1:
A second copy of the logarithmic detector is created and driven by a reference current. This copy is used to cancel the saturation current effect through differential measurement, improving accuracy despite the increased structural complexity
4Measurement precision
If emitter resistance compensation is implemented using a replica of the logarithmic bipolar transistor, then the log error decreases and accuracy improves, but the device complexity increases
Solution Approach 1:
A replica of the logarithmic bipolar transistor is created to model and compensate for emitter resistance effects. This copy processes a copy of the input current to generate a compensation signal that corrects log errors, achieving high precision despite increased circuit complexity
Solution Approach 2:
The compensation circuit uses feedback mechanisms to adjust the logarithmic voltage output based on the replica transistor's response. This feedback loop continuously corrects for emitter resistance errors, maintaining high accuracy across the dynamic range
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 solution provides high accuracy and low log error in logarithmic current-to-voltage conversion by effectively compensating for emitter resistance errors, improving dynamic range and noise performance, especially at low input current levels.
Implementation Method 1
a trans-linear logarithmic detector that uses the trans-linear properties of a bipolar transistor to provide logarithmic conversion
Implementation Method 2
an emitter resistance compensation circuit that includes a replica of the logarithmic bipolar transistor. The emitter resistance compensation circuit processes a copy of the input current to generate an emitter resistance compensation signal that adjusts the logarithmic voltage to correct for an error introduced by an emitter resistance of the logarithmic bipolar transistor
Data Source
Figure 1A
Figure 1B
Figure 2
AI summary
Logarithmic current-to-voltage converters with emitter resistance compensation are disclosed herein. In certain embodiments, a logarithmic current-to-voltage converter includes a logarithmic bipolar transistor that converts an input current to a logarithmic voltage, and an emitter resistance compensation circuit that includes a replica of the logarithmic bipolar transistor. The emitter resistance compensation circuit processes a copy of the input current to generate an emitter resistance compensation signal that adjusts the logarithmic voltage to correct for an error introduced by an emitter resistance of the logarithmic bipolar transistor. By providing emitter resistance compensation in this matter, logarithmic current-to-voltage conversion with high accuracy and low log error is achieved.