RF power amplifiers
Narrowing the conduction angle to 90° raises the efficiency of an RF amplifier to 94 %, but takes away 2 dB of power and all the amplitude linearity.
01Power, efficiency and linearity
The power amplifier is the stage that consumes the most in a transmitter, and what it does not deliver to the antenna it turns into heat. Three figures describe it: the output power, the drain efficiency (or collector efficiency), \( \eta = P_{out}/P_{DC} \), and the linearity, which decides whether it can amplify a variable-envelope signal without distorting it. The same ideas as in low-frequency amplifiers apply here, with one advantage: the load is a resonant circuit, which lets only the fundamental through even if the transistor current is highly distorted.
02Classes A, AB, B and C: the conduction angle
In the linear classes the transistor works as a current source, and the classes are distinguished by the conduction angle α, the part of the cycle in which it conducts: 360° in class A, 180° in class B, in between in AB and less than 180° in C. The current is a segment of a sinusoid, and the resonant circuit takes its fundamental. With the same peak current and the output voltage swung up to the supply:
An amplifier with a resonant load and a fixed peak current. Above, the transistor current over one cycle and its fundamental; below, the efficiency (blue) and the power relative to class A (green) as a function of the angle.
03Switching classes: D, E and F
If the transistor works as a switch, in theory it does not dissipate: when it conducts there is no voltage, and when there is voltage it does not conduct. Class D uses two transistors that switch a square wave into a filter. Class E, by Sokal, uses a single transistor with a load network that makes the voltage reach zero just when the switch closes, eliminating the switching loss with the output capacitance. Class F controls the harmonics of the voltage to square it up. Practical efficiencies exceed 80 %, but these classes are not amplitude-linear: they serve for FM, FSK and other constant-envelope modulations, or with techniques that separate the envelope, such as envelope tracking and the Doherty amplifier.
04Linearity and back-off
Cellular and Wi-Fi modulations have a variable envelope and a peak-to-average power ratio of several decibels, as seen in complex envelope. So as not to clip the peaks, the amplifier operates with the average power several dB below compression, and in class AB the efficiency falls with amplitude. The transmitters unit quantifies that cost. The distortion that remains is corrected with digital predistortion: the processor applies to the signal the nonlinearity inverse to that of the amplifier.
05In the lab
Build a class C amplifier of a few watts in the HF band with a MOSFET, measure the supply current and the power in a 50 Ω load, and compute the efficiency for several bias points.
Measure the harmonics at the output with and without the low-pass filter, and compare them with the spurious emission limits.
Sweep the input power of a class AB amplifier and measure the compression point and the efficiency as a function of output power.
06Common mistakes
- Using class C with variable-envelope modulations. It destroys the amplitude information.
- Forgetting the harmonic filter. A class C or D amplifier emits strong harmonics that invade other bands.
- Measuring efficiency only at full power. In real operation the amplifier works backed off and is much less efficient.
- Mismatching the load without protection. With the antenna disconnected, the reflected wave can destroy the transistor.
07Self-assessment
What theoretical efficiency does an amplifier with a 180° conduction angle have?
78.5 %: it is class B.
How much does a 100 W amplifier with 60 % efficiency dissipate?
\( 100/0.6 - 100 \approx 67 \) W.
Why can class E have high efficiency even at high frequencies?
Because the load network brings the voltage to zero before closing the switch, and the energy of the output capacitance is not lost.
What modulations allow nonlinear amplifiers?
Constant-envelope ones, such as FM and FSK.
08Further reading
- Steve C. Cripps. RF Power Amplifiers for Wireless Communications. 2nd ed., Artech House, 2006. The analysis of conduction angle and the high-efficiency classes.
- Herbert L. Krauss, Charles W. Bostian and Frederick H. Raab. Solid State Radio Engineering. Wiley, 1980. A classic on solid-state power amplifiers.
- Nathan O. Sokal and Alan D. Sokal. Class E: A New Class of High-Efficiency Tuned Single-Ended Switching Power Amplifiers. IEEE Journal of Solid-State Circuits, 10(3), 1975. The original paper on class E.