Cascode Power Amplifier Biasing for AM-PM and Temperature Drift
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Solution Overview
Problem
Power amplifier nonlinearities, particularly gain variation with temperature and AM-PM non-ideal transfer characteristics, pose challenges for modern digital communication systems, requiring effective minimization in the analogue domain to ensure efficient operation across wide frequency ranges.
Innovation Solution
A power amplifier circuit with a cascode device configuration, utilizing a current source providing a biasing current proportional to absolute temperature and another current source providing a complementary current to absolute temperature, which helps in reducing gain variation and controlling the DC level at the input and output terminals respectively.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Stability of the object's composition
If conventional power amplifier circuits are used, then amplification function is provided, but gain variation with temperature exceeds acceptable limits (over 2 dB)
Solution Approach 1:
The patent applies parameter changes by utilizing the temperature-dependent characteristics of bipolar transistors (Vbe decreases with temperature, Ic increases with temperature) to generate compensating currents. The first current source generates a current that increases with temperature to compensate for gain decrease, while the second current source generates a current that decreases with temperature to control cascode device operation, thereby stabilizing the overall amplifier gain across temperature variations.
Solution Approach 2:
The patent implements feedback mechanisms where the temperature-dependent currents from the first and second current sources continuously adjust the biasing conditions of the amplifier stages. The feedback loop senses temperature effects through transistor characteristics and automatically compensates by adjusting the operating points of the common-emitter and cascode devices, maintaining stable gain without external temperature sensing circuits.
2Reliability
If complex baseband digital pre-distortion schemes are used to counteract AM-PM non-ideal transfer characteristic, then AM-PM distortion is reduced, but device complexity and cost increase
Solution Approach 1:
The patent applies preliminary action by pre-compensating for AM-PM distortion through careful biasing design before the signal enters the amplifier stages. The specific biasing currents applied to the common-emitter and cascode devices are predetermined to optimize the transfer characteristic and minimize AM-PM conversion across the operating range, eliminating the need for complex digital pre-distortion processing.
Solution Approach 2:
The patent converts the inherently non-linear behavior of power amplifiers into a benefit by designing the biasing scheme to exploit the natural compression characteristics. The biasing currents are configured to ensure that the amplifier operates in a region where non-linearities produce desirable compression effects rather than harmful distortion, improving signal fidelity through the amplifier's natural characteristics rather than fighting against them.
3Adaptability or versatility
If power amplifier operates over very wide frequency range, then versatility is improved, but AM-PM spread increases and requires more complex compensation
Solution Approach 1:
The patent applies universality by designing a biasing scheme that functions effectively across a very wide frequency range (DC to several GHz). The current sources and biasing network are configured to provide stable operating conditions that remain effective regardless of frequency variations, allowing the same circuit topology to serve multiple frequency bands without requiring frequency-specific compensation circuits.
Data Source
AI summary
A power amplifier circuit, comprising: an input for receiving an input signal to be amplified; a power supply; an amplifier, coupled to the input and the power supply; and a cascode device coupled between the power supply and the amplifier. The circuit is characterized by: a first current source coupled between the input and the amplifier, configured to provide a biasing current which is proportional to absolute temperature; and a second current source for controlling the cascode device, configured to provide a current which is complementary to absolute temperature (CTAT).