Stacked Power Amplifier Bias Circuit for Variable VCC Tracking
Find Innovative SolutionsGenerate Solutions
Solution Overview
Problem
Existing bias circuits for stacked amplifier architectures are not tolerant of supply voltage variations, leading to unreliable performance, slow response to voltage changes, and significant gain variations, particularly in applications where the DC supply voltage varies significantly.
Innovation Solution
Implementing controlled source follower circuits coupled to both a variable and a constant voltage source, which maintain a constant bias voltage above a reference level and vary with the supply voltage below it, ensuring reliable and fast adjustments to voltage changes.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Speed
If a bias circuit is connected directly to a variable voltage source VCC, then the amplifier responds quickly to voltage changes, but the gate-to-drain voltage VGD exceeds design capability at low VCC values, causing device failure
Solution Approach 1:
A source follower circuit is introduced as an intermediary between the variable voltage source VCC and the FET gate. The source follower's gate is connected to VCC through a controller, and its source outputs the bias voltage to the FET gate. This intermediary buffers the direct connection, allowing the FET gate voltage to track VCC variations while preventing excessive voltage differences that would cause device failure.
2Stability of the object's composition
If a constant voltage source VCON is used for the bias circuit, then gain variation is minimized, but the amplifier cannot respond quickly to VCC changes
Solution Approach 1:
The bias circuit transitions from a static constant voltage source to a dynamic source follower circuit. The source follower dynamically adjusts its output voltage based on the controller's gate voltage, which is derived from VCC. This dynamic behavior allows the circuit to maintain stability (minimal gain variation) while simultaneously responding quickly to VCC changes, resolving the contradiction between stability and speed.
3Device complexity
If the bias circuit uses a resistive ladder connected to VCC, then the circuit is simple, but large gain variations occur when VCC changes
Solution Approach 1:
The source follower circuit acts as an intermediary voltage buffer between the simple resistive ladder (or constant voltage source) and the FET gate. This intermediary decouples the gain stability from the supply voltage variations, allowing the use of simple biasing components while achieving stable gain across varying VCC conditions.
4Stability of the object's composition
If the gate voltage is kept constant, then gain variation is eliminated, but the gate-to-drain voltage VGD becomes excessively large at low VCC, leading to device failure
Solution Approach 1:
The gate voltage transitions from a completely constant value to a dynamically adjusted voltage through the source follower circuit. The source follower's output voltage dynamically tracks the controller's reference voltage (derived from VCC), ensuring the FET operates in the saturation region with appropriate VGD margins across all VCC conditions, while maintaining stable gain.
Data Source
AI summary
Stacked amplifier architectures that have bias circuits that are tolerant of supply voltage variations, which provide good reliability, are fast to follow VCC changes, and exhibit small variations in gain as VCC varies. One embodiment encompasses a bias circuit for an amplifier stack, including a source follower circuit configured to output a bias voltage to a transistor within the amplifier stack and configured to be coupled to a variable voltage, and a controller coupled to the source follower circuit, configured to be coupled to a constant voltage, and configured to apply a constant voltage to the source follower circuit. The source follower circuit generates a reference voltage and (1) outputs a constant bias voltage when the variable voltage is above the reference voltage, and (2) outputs a bias voltage proportional to the variable voltage when the variable voltage is below the reference voltage.