Reference Bias Circuit for Predictable Amplifier Transconductance
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Solution Overview
Problem
Existing reference sources for integrated circuits do not adequately control the predictability of small-signal gain due to factors affecting current and gain differently, leading to unpredictable variations.
Innovation Solution
A circuit with one or more reference transistors and a feedback loop that regulates control voltage to achieve a predetermined differential transconductance, using a small voltage offset to monitor and control the differential transconductance, and employing matched or mismatched reference transistors to assess and adjust the control voltage.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If a reference source controls the bias setting of transistors, then the dependence on uncontrollable factors such as temperature is minimized, but the predictability of small-signal gain still varies due to factors affecting current and gain differently
Solution Approach 1:
A feedback loop is introduced that includes a test current source applying a small test current to the reference transistor, a test voltage measurement circuit measuring the resulting voltage change, and a control mechanism that adjusts the bias to maintain constant differential transconductance. This feedback mechanism directly addresses the predictability issue by continuously monitoring and correcting gain variations.
Solution Approach 2:
The invention changes the control parameter from simple bias voltage to differential transconductance (gm), which is the ratio of small-signal current change to gate-source voltage change. By regulating gm directly through the feedback loop, the system ensures predictable small-signal gain while maintaining stable bias conditions.
2Manufacturing precision
If the sizes of reference transistors differ by ratio N:1, then the drain current can be adjusted to compensate for gate-source voltage offset, but the small-signal gain remains unpredictable due to differential transconductance variations
Solution Approach 1:
The feedback loop measures the actual differential transconductance by applying a small test voltage offset and measuring the resulting current difference, then adjusts the bias to maintain the desired transconductance value. This compensates for manufacturing variations in transistor size ratios and ensures predictable small-signal gain.
Solution Approach 2:
The invention replaces reliance on precise mechanical/manufacturing control of transistor size ratios with an electrical measurement and feedback system. Instead of depending solely on fabrication precision, the system electrically characterizes the transistors and actively adjusts bias to achieve the desired differential transconductance.
Data Source
Figure 1
Figure 2~3
AI summary
A circuit has a reference source (12) for supplying a bias signal to set a small signal transconductance of an amplifier transistor in an amplifier (10) to a predetermined value. The reference source has at least one reference transistor (120a-b, 30). A feedback circuit (128, 129, 38) has an input coupled to the main current channel of the reference transistor or reference transistors (120a-b, 30) and an output coupled to the control electrode of the reference transistor or reference transistors (120a-b, 30). The feedback circuit controls a control voltage at the control electrode, so as to equalize an offset current and a difference between main currents flowing through the current channel of the reference transistor or reference transistors (120a-b, 30), obtained with and without a small voltage offset added to the control voltage. The main currents flowing with and without a small voltage offset may be obtained by using a first and second reference transistor (122a,b), matching each other and an offset voltage source (126) coupled between the control electrodes of the first and second reference transistor (122a,b), to apply the small voltage offset between their control electrodes.