Question E7B03:
What circuit is required at the output of an RF switching amplifier?  

Choices:  
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 is  A: A filter to remove harmonic content  

To transmit a clean RF signal from a switching (Class D/E/F) amplifier, you must remove the strong harmonic energy created by the switching action.

Why A is correct:
- Switching amplifiers produce a pulse/square-like waveform rich in harmonics. A low-pass or band-pass filter at the output removes those harmonics so only the desired fundamental is sent to the antenna.
- The output network often also provides the 50 Ω match.
- Example harmonic content of a square-wave-like output:
  $$v(t) = \frac{4V_{\text{pk}}}{\pi}\sum_{k=1,3,5,\dots}^{\infty} \frac{1}{k}\sin(k\omega_0 t)$$
  This shows large odd-order harmonics that must be filtered.

Why the other choices are incorrect:
- B: A high-pass filter would attenuate the fundamental and pass more of the higher-frequency harmonics—the opposite of what you want. “Compensate for low gain at low frequencies” is not the goal at an RF PA output.
- C: A matched load resistor is not used on-air; it would waste power. Proper filtering and matching present the correct load and control switching transients. A dummy load is for testing, not a required output element.
- D: A temperature-compensating load resistor won’t “improve linearity” in a switching PA; these amplifiers are inherently non-linear, and linearity is restored by filtering, not by resistive loading.

Useful rules of thumb:
- Use a low-pass (HF) or band-pass (VHF/UHF) filter with cutoff near the operating band to strongly attenuate $2f_0, 3f_0,\dots$.
- Common designs use 5th–7th order low-pass filters to meet spurious limits.
- In Class E/F, the output network is designed to present a resistive load at $f_0$ and high impedance at harmonics.