Exponential Current Converter Circuit for Wide VGA Gain Control

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Solution Overview

Problem

Existing current converter circuits struggle to maintain a uniformly varying output over several orders of magnitude, failing to provide linear control for variable gain amplifiers (VGAs) in RF transmitters.

Innovation Solution

A current converter circuit comprising first and second current converters with input and output branches, utilizing diode-connected transistors and adjustable tuning resistors, converts linear input current to exponentially varying output current, enhanced by a complementary differential amplifier for voltage conversion.

Engineering Contradictions & Design Principles

VSEngineering Contradiction Analysis

1Adaptability or versatility

If a single current converter circuit is used, then the circuit complexity is low, but the linear gain control range is limited and cannot maintain uniform output over several orders of magnitude

Engineering Contradiction:

Improvelinear gain control rangeVSAvoidcircuit complexity

Core Design Contradiction:

Adaptability or versatilityVSDevice complexity

Solution Approach 1:

The current converter circuit is divided into multiple independent current converter stages (first current converter, second current converter, etc.), each handling a specific segment of the overall gain control range. This segmentation allows each stage to operate within its optimal range while collectively covering several orders of magnitude, resolving the contradiction between extended range and circuit complexity.

Inventive Principle:

Principle #1Segmentation

Solution Approach 2:

Multiple current converter stages are combined through summing circuits to achieve an extended linear gain control range. The outputs of individual converters are summed to produce the final control signal, merging their individual ranges into a comprehensive control capability that spans several orders of magnitude without requiring a single overly complex converter.

Inventive Principle:

Principle #5Merging (Combining)

2Manufacturing precision

If multiple current converters are used to extend the linear gain control range, then the linearity is improved, but the device complexity increases

Engineering Contradiction:

Improvelinearity of gain controlVSAvoidnumber of current converters

Core Design Contradiction:

Manufacturing precisionVSDevice complexity

Solution Approach 1:

Each current converter stage is designed with specific local characteristics optimized for its operating range, using diode-connected transistors and tuning resistors to achieve uniform output variation within that range. This local optimization ensures high linearity in each segment while the overall system maintains good linearity across the extended range through proper design of individual stages.

Inventive Principle:

Principle #3Local quality

3Adaptability or versatility

If the output current range is extended over several orders of magnitude, then the gain control versatility is improved, but the output uniformity deteriorates

Engineering Contradiction:

Improvegain control rangeVSAvoiduniformity of output current

Core Design Contradiction:

Adaptability or versatilityVSStability of the object's composition

Solution Approach 1:

The circuit incorporates tuning voltage circuits and adjustable tuning resistors that allow dynamic adjustment of each current converter stage to optimize performance across different operating conditions. This dynamic tuning capability enables the system to maintain uniform output current characteristics across several orders of magnitude of gain control range, resolving the contradiction between extended range and output uniformity.

Inventive Principle:

Principle #15Dynamics

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

Extends the linear gain control range of VGAs, improving linearity and noise performance by maintaining consistent gain steps over a broader input current range.

Implementation Method 1

a converter is therefore required in such cases that converts a linearly varying input control signal to an exponentially varying output control signal

Methodology Applied

Scientific EffectExponential current-voltage relationship in diode-connected transistors:

Implementation Method 2

a current to voltage converter circuit connected to the output current branch and configured to convert an output current through the output current branch to a differential output voltage signal

Methodology Applied

Scientific EffectCurrent-to-voltage conversion:

Data Source

PatentEP4708682A1Current converter circuit

Publication Date: 2026.03.11 NXP BV

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AI summary

The disclosure relates to a current converter circuit (600) for converting a linear input current (lctrl_lin) to an exponential output current (lctrl_sum), the current converter circuit (600) comprising first and second current converters (601, 602), each of which comprises: an input current branch (6031, 6032) with an input current source (6041, 6042) connected in series with a tuning voltage circuit (6051, 6052) and a tuning resistor (6061, 6062) between a supply voltage line (607) and a common voltage line (608); and an output current branch (6091, 6092) with an output transistor (6101, 6102) having a collector connected to an output node (6111, 6112), a emitter connected to the common voltage line (608) and a base connected to the tuning voltage circuit (6051, 6052), wherein the output nodes (6111, 6112) of the first and second current converters (601, 602) are connected to a summing output node (612) of the current converter circuit (600).