Question
What circuit is required at the output of an RF switching amplifier?
Answer Options
- A) A filter to remove harmonic content
- B) A high-pass filter to compensate for low gain at low frequencies
- C) A matched load resistor to prevent damage by switching transients
- D) A temperature compensating load resistor to improve linearity
Correct Answer: A
Explanation
A switching amplifier (like Class D or Class E) operates by using the active devices as rapid switches, generating a square wave output. While this provides extremely high efficiency, a square wave contains massive amounts of energy at the fundamental frequency’s harmonics (multiples). These harmonics must be removed to prevent out-of-band spurious emissions.
Therefore, a filter to remove harmonic content (specifically a sharp low-pass filter tuned to the fundamental frequency) is required at the output of an RF switching amplifier. This filter converts the square wave output back into a clean sine wave at the desired fundamental frequency while suppressing the unwanted harmonic energy well below FCC spurious emission limits.
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