Class AB Cascode Amplifier Filtering for High-Frequency Linearity
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Solution Overview
Problem
Static class AB amplifiers suffer from high-frequency distortion due to variations in transconductance and output resistance, which cannot be suppressed by loop gain, affecting their linearity and failing to meet strict FCC requirements for third-order intermodulation distortion.
Innovation Solution
Incorporating cascode stages with high-pass filters that input AC components to the gate electrodes of transistors, increasing gain and reducing voltage swings, thereby improving the amplifier's linearity by enhancing the AC components of the input signals and reducing distortion.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Ease of operation
If a stage is added before the output stage to set DC levels, then the amplifier can drive the output stage appropriately, but the variations in transconductance and output resistance generate distortion
Solution Approach 1:
The amplifier is divided into multiple stages: input stage, cascode stage (with filter), and output stage. Each stage has a specific function - the input stage provides voltage amplification, the cascode stage with filter sets DC levels and suppresses distortion, and the output stage drives the load. This segmentation allows the distortion-generating stage to be isolated and compensated.
Solution Approach 2:
A feedback network is implemented that feeds back a portion of the output signal to the input stage. This feedback mechanism compensates for the distortion generated by the cascode stage by adjusting the input signal accordingly, thereby reducing the overall distortion of the amplifier.
2Object-generated harmful factors
If loop gain is used to suppress distortion, then distortion can be reduced at low frequencies, but it cannot suppress distortion at high frequencies
Solution Approach 1:
A filter circuit is introduced as an intermediary between the input stage and output stage. This filter selectively processes the signal to reduce distortion components while preserving the useful signal, particularly effective at high frequencies where conventional feedback fails.
Solution Approach 2:
The amplifier utilizes parameter changes in the cascode stage transistors, controlled by the filter output, to dynamically adjust the operating point and reduce distortion. The filter modifies the AC component of the control signal to compensate for nonlinearity across the frequency range.
3Power
If the amplifier operates with large signal swing, then it can drive the load effectively, but the variations in transconductance and output resistance increase distortion
Solution Approach 1:
The feedback network compensates for the nonlinearity caused by large signal swings by continuously adjusting the input signal based on the output signal. This maintains the desired output power while correcting for the distortion introduced by varying transconductance and output resistance.
Solution Approach 2:
The cascode stage utilizes parameter changes in response to the filtered control signal to maintain linear operation even during large signal swings. The dynamic adjustment of transistor operating parameters compensates for the inherent nonlinearity during high-power operation.
Data Source
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AI summary
The present invention provides a class AB amplifier (100), wherein the class AB amplifier (100) includes a cascode stage with a filter (120_1) and an output stage (130_1). The cascode stage with the filter (120_1) is arranged for receiving an input signal to generate a first driving signal and a second driving signal, wherein the filter (222) filters the input signal to generate an filtered input signal, and at least one of the first driving signal and the second driving signal is generated according to the filtered input signal. The output stage (130_1) is coupled to the cascode stage (120_1), and is arranged for generating an output signal according to the first driving signal and the second driving signal.