Cascode Variable Gain Amplifier With Constant-Current Gain Switching
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Solution Overview
Problem
Conventional variable gain amplifiers suffer from linearity degradation at low voltage operation and increased parasitic capacitance, leading to ringing and degraded frequency characteristics, especially when integrating multiple cascode amplifiers for high-frequency signal amplification in wireless communication devices.
Innovation Solution
A variable gain amplifier design featuring cascode amplification units and current sources with a control circuit to manage transistor switching, maintaining constant current through the load circuit, and incorporating feedback and attenuation circuits to optimize gain and impedance matching, reducing parasitic capacitance and noise factors across a wide frequency band.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Power
If multiple cascode amplifiers are connected in parallel to increase gain, then the gain is improved, but parasitic capacitance increases leading to degraded frequency characteristics
Solution Approach 1:
The amplifier is divided into multiple cascode amplification units, each with a small number of transistors (typically 2-3), rather than using a single large amplifier. This segmentation allows the total gain to be achieved through parallel connection of multiple small units, thereby reducing the parasitic capacitance at any single node while maintaining the required overall gain.
2Adaptability or versatility
If cascode amplifiers are switched to change gain values, then the gain is adjusted, but input impedance changes from desired value causing signal loss
Solution Approach 1:
The amplifier employs dynamic switching of cascode amplification units based on the required gain level. The control circuit selectively activates or deactivates specific units to achieve different gain values (e.g., 0 dB, 10 dB, 20 dB, 30 dB). This dynamic configuration maintains a relatively stable input impedance across different gain settings, preventing signal loss while providing adaptable gain control.
3Adaptability or versatility
If current steering method is used to vary gain, then the gain is changed, but DC current through load resistor changes causing linearity degradation at low voltage
Solution Approach 1:
Instead of using a single load resistor with varying current, the amplifier divides the amplification function into multiple parallel cascode units. Each unit has its own current path and contributes equally to the total output current. This segmentation ensures that the total DC current through the load remains constant regardless of which units are active, thereby maintaining linearity at low voltage operation while still achieving gain variation through selective unit activation.
Data Source
AI summary
A cascode amplification unit includes a plurality of cascode amplifiers connected in parallel. Each of the cascode amplifiers has two transistors. A cascode current source unit includes a plurality of cascode current sources connected in parallel. Each of the cascode current sources has two transistors. Both the output end of the cascode amplification unit and the output end of the cascode current source unit are connected to a load circuit. The control circuit turns on and off each cascode transistor of the cascode amplifier and each cascode transistor of the cascode current source so that an amount of current passing through the load circuit is constant.