Low-Voltage Variable Gain Amplifier With MOSFET Linearity Control
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Solution Overview
Problem
Conventional variable gain amplifiers (VGAs) have limited headroom due to the requirement of active current sources, which restricts the minimum input voltage and operating voltage, making it difficult to operate at lower voltages and reducing the headroom for upstream circuits.
Innovation Solution
A variable gain amplifier design that eliminates the need for active current sources between the input transistor and ground, using a MOSFET to control the transconductance gain and modulate the gate voltage, allowing for a lower minimum input voltage and operating voltage while improving linearity and reducing distortion.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If active current sources are used between input transistors and ground, then proper current supply is achieved, but minimum input voltage level increases and headroom is reduced
Solution Approach 1:
The patent removes the active current sources from the conventional VGA architecture. Instead of using active current sources I_DC1 and I_DC2 that require high minimum input voltages, the invention uses passive resistors R1 and R2 to ground, which significantly reduces the minimum input voltage requirement and increases headroom while still providing stable current supply through the transistor biasing network
Solution Approach 2:
The patent replaces the active current source mechanism with a passive resistor-based biasing network. The resistors R1 and R2 provide the necessary bias currents to the differential pair transistors Q1 and Q2 without requiring the high voltage headroom that active current sources demand, thus substituting an active electronic component with a passive component that has different electrical characteristics
2Adaptability or versatility
If conventional VGA architecture with current sources is used, then gain control is achieved, but operating voltage must be high and headroom for upstream circuits is reduced
Solution Approach 1:
The resistor R3 in the patent serves multiple functions: it provides bias current to the differential pair, sets the operating point for the transistors, and works in conjunction with the MOSFET M1 to enable gain control. This multi-functional design eliminates the need for separate active current sources, allowing the amplifier to operate at lower voltages while maintaining full gain control capability through the MOSFET's variable resistance
3Reliability
If minimum Vin level is increased to satisfy current source requirements, then current sources operate properly, but headroom for upstream circuits is reduced
Solution Approach 1:
The patent extracts and removes the problematic active current sources from the circuit. By replacing them with passive resistors R1 and R2, the minimum input voltage requirement drops from needing to satisfy Vce(sat) + Vbe requirements to only needing to overcome the voltage drops across the passive resistors, which are much smaller. This increases headroom for upstream DAC circuits that generate the input signals
Applied Scientific Principles
This section explains which scientific principles are used to turn an abstract innovation direction into a practical engineering solution.
Function Achieved in This Case
This design increases headroom, enables operation at lower voltages, reduces current consumption, and enhances overall linearity by minimizing distortion, making it suitable for RF transmitters and improving the performance of upstream circuits.
Implementation Method 1
The conductivity of the MOSFET controls the trans-conductance gain of the first transistor
Data Source
Figure 1~2
Figure 3A~3B
Figure 4~5
AI summary
In a variable gain amplifier, a base of a bipolar first transistor receives a first differential input signal. The emitter of the first transistor is connected in series between a first resistor and a MOSFET coupled to ground. An output of the amplifier is a current through the collector. The conductivity of the MOSFET controls a gain of the amplifier. A bipolar second transistor receives a second differential input signal, and the second transistor provides a modulated gate voltage to the MOSFET. The drain voltage of the MOSFET is modulated by the first differential input signal and thus undesirably generates distortion. To reduce the distortion, the modulated gate voltage causes the AC component for a certain DC voltage at the drain of the MOSFET to be lowered, improving linearity. Since no current source is used, the amplifier has a large headroom, allowing operation using a low operating voltage.