Variable Gain Amplifier Bias Control Without HF Response Peaking
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Solution Overview
Problem
Variable gain amplifiers with resistive degeneration suffer from parasitic capacitances that short out the degeneration resistance at high frequencies, leading to undesirable frequency response peaks and lack a power-saving mechanism at low gain settings due to constant bias current consumption.
Innovation Solution
A variable gain amplifier circuit incorporating a differential pair of transistors and a variable current source circuit, where the bias current is adjusted to vary the gain, reducing power consumption at lower gain settings and maintaining constant common mode voltage through a feedback amplifier, eliminating the need for adjustable degeneration resistance.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Adaptability or versatility
If resistive degeneration is used to implement variable gain amplifier, then gain control is achieved, but parasitic capacitances short out the degeneration resistance at high frequencies causing frequency response peaks
Solution Approach 1:
The patent extracts the bias current control function from the degeneration resistance network. Instead of using adjustable degeneration resistance that suffers from parasitic capacitance issues, the invention uses a variable current source to control gain, separating the gain control mechanism from the problematic resistive degeneration structure.
Solution Approach 2:
The patent changes the control parameter from resistance value adjustment to bias current variation. By varying the bias current through the differential pair transistors, gain is controlled without requiring adjustable degeneration resistance, thereby avoiding the high-frequency parasitic capacitance problem that causes frequency response peaks.
2Device complexity
If constant bias current is used in variable gain amplifier, then circuit operation is simplified, but power consumption cannot be reduced at low gain settings
Solution Approach 1:
The patent introduces dynamic bias current control that adapts to the required gain setting. The bias current is varied dynamically based on the desired gain level, allowing power consumption to be reduced at low gain settings while maintaining full performance at high gain settings, transforming the static constant current approach into a dynamic adaptive system.
Solution Approach 2:
The patent changes the bias current parameter from constant to variable based on gain requirements. By adjusting the bias current parameter in response to gain control signals, the circuit achieves both simplified operation and power-saving capability at low gain settings.
3Use of energy by moving object
If bias current is varied to control gain, then power saving is achieved at low gain, but common mode voltage of output signal varies
Solution Approach 1:
The patent introduces a feedback mechanism that monitors the common mode voltage of the output signal and adjusts circuit parameters to maintain it constant. This feedback loop compensates for the common mode voltage variations that occur when bias current is varied for gain control and power saving, ensuring stable operation.
Solution Approach 2:
The patent uses an intermediary circuit element or stage that decouples the bias current variation from the common mode voltage. This intermediary mechanism allows the bias current to be varied for power saving while preventing the common mode voltage from varying, acting as a buffer between the power control function and the voltage stability requirement.
Data Source
AI summary
A variable gain amplifier circuit includes a differential pair of transistors and a variable current source circuit. The differential pair of transistors generates an output signal based on an input signal. The variable current source circuit is coupled to the differential pair of transistors. A gain of the output signal relative to the input signal varies in response to variations in a bias current through the variable current source circuit. The variable gain amplifier circuit maintains a common mode voltage of the output signal substantially constant in response to the variations in the bias current through the variable current source circuit.