Power Amplifier Bias Circuit for Parasitic Diode Suppression
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Solution Overview
Problem
In power amplifier modules, parasitic diodes between a transistor's base and collector can turn on when the power source voltage is low and RF signal power is high, causing signal distortion and oscillation due to a base potential exceeding the collector potential.
Innovation Solution
A bias circuit with a first transistor, a capacitor connected to ground, and a second transistor with a bypass circuit that feeds bias current to a node when its voltage is below a threshold, preventing parasitic diode activation by reducing bias control current and limiting output power.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Power
If power source voltage is low and RF signal power is high, then amplifier output power is high, but base potential exceeds collector potential causing parasitic diode turn-on
Solution Approach 1:
The bypass circuit proactively detects when the node voltage approaches the threshold that would cause parasitic diode turn-on, and preemptively feeds back bias current to limit the output power before the harmful condition occurs. This prevents the base potential from exceeding the collector potential in the first place.
Solution Approach 2:
The bypass circuit continuously monitors the voltage at the node and dynamically adjusts the bias current supplied to the amplifier's base/gate. When the node voltage exceeds the threshold, the feedback mechanism reduces the bias current to limit output power and prevent parasitic diode activation, creating a closed-loop control system.
2Productivity
If bias current is increased to improve amplifier performance, then amplification capability is improved, but parasitic diode turn-on risk increases under low voltage conditions
Solution Approach 1:
The bias current supplied to the amplifier is made dynamic rather than fixed. The bypass circuit continuously adjusts the bias current based on real-time voltage conditions at the node, allowing maximum bias current for optimal amplification under normal conditions, and automatically reducing it when voltage conditions threaten to cause parasitic diode turn-on.
Solution Approach 2:
The system changes the bias current parameter dynamically based on operating conditions. Under normal high-voltage conditions, high bias current provides strong amplification. When voltage drops and the node voltage approaches the threshold, the bias current parameter is reduced to prevent parasitic diode activation, optimizing performance across varying conditions.
Data Source
AI summary
A bias circuit includes: a first transistor having an emitter or a source which supplies a bias to an amplifier operating by a power source voltage through a first resistive element and a base or a gate; a first capacitor having a first end electrically connected to the base or the gate of the first transistor and a second end connected to a ground; and a second transistor having a collector or a drain electrically connected to the base or the gate of the first transistor, a base or a gate electrically connected to the base or the gate of the first transistor, and an emitter or a source connected to a node which is supplied with a signal with experience of being amplified by the amplifier and the power source voltage.