Rail-to-Rail Amplifier Input Stage With Switched Low Capacitance
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Solution Overview
Problem
Existing amplifiers face limitations in achieving a wide input range and low input capacitance, which restricts their performance and efficiency in signal processing.
Innovation Solution
The amplifier input stage incorporates a pair of n-type and p-type input transistors, along with isolation switches and a control circuit that selectively uses these transistors based on the common-mode voltage of the input terminals, decoupling unused transistors to reduce input capacitance and enhance the input range.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Adaptability or versatility
If multiple input transistors are used to extend input range, then input range is improved, but input capacitance increases
Solution Approach 1:
The input stage is segmented into multiple transistor pairs (first pair and second pair of input transistors) that can be independently activated. The control circuit selectively enables only the required transistor pair based on the input common-mode voltage level, thereby extending the overall input range while keeping the active capacitance low at any given moment.
Solution Approach 2:
The amplifier input stage dynamically switches between different transistor pairs based on the input common-mode voltage. The control circuit monitors the voltage level and activates the appropriate transistor pair (first pair for lower voltage range, second pair for higher voltage range), making the input stage adaptive and maintaining low input capacitance throughout the extended input range.
2Object-affected harmful factors
If isolation switches are added to decouple unused transistors, then input capacitance is reduced, but device complexity increases
Solution Approach 1:
The isolation switches extract and remove the capacitance contribution of unused input transistors from the input terminals. By placing switches between the input terminals and the inactive transistor pair, the harmful capacitance is effectively disconnected and taken out of the signal path, leaving only the necessary transistor pair connected to the inputs.
Solution Approach 2:
Isolation switches serve as intermediary elements between the input terminals and the input transistor pairs. These switches act as controllable mediators that can connect or disconnect specific transistor pairs based on the operating conditions, thereby managing the input capacitance dynamically without permanently increasing circuit complexity.
3Adaptability or versatility
If rail-to-rail input stage is implemented, then input range is extended, but power dissipation increases
Solution Approach 1:
Instead of keeping all input transistors continuously active to achieve rail-to-rail operation, the control circuit activates only the necessary transistor pair for the current input voltage range. This partial action approach extends the overall input range while avoiding the excessive power dissipation that would result from all transistors being constantly engaged.
Solution Approach 2:
The control circuit periodically evaluates the input common-mode voltage and switches between different transistor pairs as needed. This periodic monitoring and switching enables rail-to-rail input operation while minimizing power consumption by keeping only the required transistor pair active at any given time, rather than maintaining continuous operation of all transistors.
Data Source
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AI summary
Amplifiers with wide input range and low input capacitance are provided. In certain embodiments, an amplifier input stage includes a pair of input terminals, a pair of n-type input transistors, a first pair of isolation switches connected between the input terminals and the n-type input transistors, a pair of p-type input transistors, and a second pair of isolation switches connected between the input terminals and the p-type input transistors. The amplifier input stage further includes a control circuit that determines whether to use the n-type input transistors and/or the p-type input transistors for amplification based on a detected common-mode voltage of the input terminals. The control circuit opens the first pair of isolation switches to decouple the input terminals from the n-type input transistors when unused, and opens the second pair of isolation switches to decouple the input terminals from the p-type input transistors when unused.