Frequency Output Circuit with Current Limiting for Noise Reduction
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Solution Overview
Problem
Frequency output circuits with open-collector output terminals emit unwanted electromagnetic waves due to rectangular waveforms, causing radio noise, and existing current mirror circuits fail to effectively limit current and smooth voltage changes, especially when voltage increases or decreases.
Innovation Solution
A frequency output circuit with an open-collector output terminal incorporating a current limiting circuit that includes a differential pair of transistors with resistors in the emitters and a current mirror circuit, along with optional low-pass filters and adjustable resistors, to smoothly switch between high and low potentials of the output waveform, effectively eliminating unwanted electromagnetic waves.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Object-generated harmful factors
If a current mirror circuit is provided to limit the current through the open collector, then the current can be limited, but the voltage change is not smooth and high-frequency electromagnetic waves are still emitted
Solution Approach 1:
A diode is introduced as an intermediary component between the current mirror circuit and the open-collector transistor. The diode's exponential current-voltage characteristic acts as a mediator that converts the limited current into smooth voltage transitions, effectively eliminating high-frequency electromagnetic waves while maintaining current limiting functionality.
Solution Approach 2:
The invention changes the operating parameters of the output stage by introducing a diode that operates in its exponential region. This parameter change transforms the linear current limiting into non-linear voltage control, achieving smooth voltage transitions that suppress electromagnetic radiation.
2Adaptability or versatility
If the AM frequency is lowered from 450 kHz to 150 kHz to expand the AM band, then the AM band can be expanded, but previously suppressed electromagnetic waves become radio noise
Solution Approach 1:
The diode is placed in advance in the signal path to pre-process the voltage transitions before they reach the output terminal. This preliminary action of smoothing voltage changes prevents the generation of high-frequency electromagnetic waves that would otherwise become radio noise at lower AM frequencies.
Data Source
AI summary
Unwanted electromagnetic waves are eliminated in a frequency output circuit comprising an open-collector output terminal whereby radio noise can be reduced. A frequency output circuit 10 comprises an open-collector transistor Q4 with an open-collector output terminal connected to a pull-up resistor Rf connected to an external power supply Vex. A current limiting circuit 16 limits a current through the output terminal such that a high potential and a low potential of the output waveform of a frequency signal outputted by an output circuit 12 are smoothly switched. The current limiting circuit 16 is formed by a differential pair circuit of transistors Q1 and Q2 each with a resistor provided in the emitter thereof, and a current mirror circuit comprising an open-collector transistor, for example.