RF Squarer Circuit with Variable Gain TIA for PVT Stability
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Solution Overview
Problem
RF squarer circuits face challenges in achieving high gain while maintaining bandwidth and stability over process, voltage, and temperature (PVT) variations, leading to significant gain fluctuations.
Innovation Solution
The RF squarer circuit incorporates a variable gain transimpedance amplifier (TIA) and a current-mode RF multiplier with a replica path for gain control, using a cascade of transconductance amplifiers and degeneration transistors to regulate gain and compensate for PVT variations, ensuring relatively constant gain and high bandwidth.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Power
If a cascade of amplification stages is used to achieve high gain, then the gain is improved, but the bandwidth is reduced and noise increases
Solution Approach 1:
The patent replaces the conventional cascade of voltage amplification stages with a current-mode architecture. The RF squarer uses a Gilbert cell multiplier that directly converts RF input to output current without intermediate voltage amplification stages, eliminating the bandwidth-limiting effect of cascaded amplifier poles while maintaining high gain through current amplification.
Solution Approach 2:
The patent introduces a transimpedance amplifier (TIA) as an intermediary stage that converts the output current of the RF multiplier to output voltage. This TIA is designed with a single pole at a higher frequency than traditional cascaded amplifiers, preserving bandwidth while providing the necessary gain. The TIA uses a large feedback resistor to achieve high gain without creating multiple bandwidth-limiting poles.
2Power
If the resistance of feedback resistors in TIA is increased to achieve high gain, then the gain is improved, but the bandwidth is reduced due to pole created with parasitic capacitances
Solution Approach 1:
The patent moves the pole creation from the voltage domain to the current domain by using a current-mode architecture. The RF multiplier operates in current mode, and the TIA converts current to voltage only at the final output stage. This dimensional change allows the use of large feedback resistors for high gain without the same bandwidth penalty, as the critical RF signal path remains in the current domain where parasitic capacitances have less impact.
3Device complexity
If conventional RF squarer circuits are used, then the circuit is simple, but the gain varies significantly over process, voltage and temperature (PVT)
Solution Approach 1:
The patent implements automatic gain control (AGC) feedback to stabilize the output gain over PVT variations. The AGC circuit monitors the output signal level and adjusts the bias currents of the Gilbert cell multiplier and TIA to maintain constant gain. This feedback mechanism compensates for PVT drift without significantly increasing circuit complexity, as it uses standard analog control techniques.
Solution Approach 2:
The patent uses bias current as a controllable parameter to stabilize gain over PVT. By adjusting the bias currents of the RF multiplier and TIA based on temperature and process conditions, the circuit maintains optimal operating points. This is achieved through temperature-compensated bias circuits that dynamically adjust current levels to counteract PVT effects on transistor characteristics.
Data Source
AI summary
An RF squarer circuit comprises a first RF multiplier and a first variable gain transimpedance amplifier (TIA). The first RF multiplier receives an RF input signal RFIN and provides a first output current. The first TIA receives the first output current as an input. The first TIA provides an output voltage VOUT.