PA Load-Line (Cripps)
The Cripps load-line method sizes the load a transistor must see to deliver its maximum power. From the supply voltage, the knee (saturation) voltage, and the peak current, it gives the optimum load resistance, the maximum output power, and — for the chosen conduction class — the theoretical drain efficiency and DC draw. This is the starting point for output-match and PA back-off design.
Equations & Parameters ▸
\(R_{opt}=\dfrac{2\,(V_{DD}-V_{knee})}{I_{max}} \qquad P_{out}=\dfrac{(V_{DD}-V_{knee})\,I_{max}}{4}\)
\(\eta_{max}:\ \text{A}=50\%,\ \text{AB}\approx60\%,\ \text{B}=78.5\%,\ \text{F}\approx90\% \qquad P_{DC}=\dfrac{P_{out}}{\eta}\)
\(\eta_{max}:\ \text{A}=50\%,\ \text{AB}\approx60\%,\ \text{B}=78.5\%,\ \text{F}\approx90\% \qquad P_{DC}=\dfrac{P_{out}}{\eta}\)
| VDD | Supply (drain/collector) voltage (V). |
| Vknee | Knee / saturation voltage (V) — the on-state voltage drop. |
| Imax | Peak drain current (A). |
| Class | Conduction class, setting the theoretical maximum drain efficiency. |
| Ropt | Load line the device must see for maximum voltage/current swing. |
Reference: S. C. Cripps, RF Power Amplifiers for Wireless Communications, 2nd ed., Artech House, 2006.
Inputs
V
RailV
On-state dropA
Max swingMax efficiency
Results
Load line
Optimum load Ropt—
Max output power—
Output power (dBm)—
Efficiency
Max drain efficiency—
DC power at Pmax—
Dissipation at Pmax—
Diagram