Video AC Clamp Circuit With Leakage-Canceling Bias Isolation
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Solution Overview
Problem
Undesired leakage in the output of trans-conductor amplifiers in analog-video-clamp circuits causes artifacts in video signals, particularly during the blanking period, due to the inability of existing circuits to effectively manage leakage currents, leading to shifts in DC levels and signal distortions.
Innovation Solution
Introducing a reverse leakage current at the video-node and using a higher reference voltage for the trans-conductor amplifier, while incorporating a low-leakage process that allows for the use of leaky transistors without performance trade-offs, specifically by adding a transistor with a gate bias voltage to act as a resistor or diode to isolate and manage leakage currents.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Ease of manufacture
If a standard trans-conductor amplifier is used in the clamp circuit, then the circuit can be manufactured with standard processes, but leakage current appears at the output causing video signal artifacts
Solution Approach 1:
The patent converts the harmful leakage current into a beneficial effect by introducing a controlled reverse leakage current that cancels the unwanted forward leakage. The reverse leakage current flowing through the AC coupling capacitor during blanking periods compensates for the charge loss caused by forward leakage, thereby eliminating video artifacts while maintaining compatibility with standard CMOS processes
2Object-generated harmful factors
If reverse leakage current is introduced to cancel ordinary leakage current, then video signal artifacts are reduced, but the dynamic range of the trans-conductor amplifier decreases due to higher reference voltage requirement
Solution Approach 1:
The patent changes the reference voltage parameter to a higher value to enable the introduction of reverse leakage current. This parameter change allows the trans-conductor amplifier to operate in a regime where reverse leakage can be effectively utilized to cancel forward leakage, reducing video artifacts. The trade-off is an acceptance of reduced dynamic range as a consequence of this parameter modification
3Object-generated harmful factors
If a low-leakage process is used to reduce leakage current, then video signal quality improves, but manufacturing cost increases
Solution Approach 1:
The patent converts the harmful leakage current inherent in low-cost standard CMOS processes into a beneficial reverse leakage current. By carefully controlling the circuit operation to utilize reverse leakage, the invention achieves effective leakage cancellation without requiring expensive low-leakage specialized processes, thereby maintaining cost-effectiveness while improving video signal quality
Data Source
AI summary
This invention is a clamp circuit for a video input. The clamp circuit includes: a coupling capacitor; a differential amplifier comparing a video input to predetermined reference voltage; a clamp transistor having a gate connected to the output terminal of the differential amplifier and a source-drain path connected between a power supply voltage and a second terminal; a resistive element connecting the second terminal of the clamp transistor and the coupling capacitor; a first current sink carrying a first predetermined current from the coupling capacitor to ground; and a second current sink carrying a second predetermined current from the second terminal of the said clamp transistor to ground. The resistive element can be a transistor, a resistor, a diode or a switch.