What is the role of a driver amplifier in a transmit chain and how do I select one?
Driver Amplifier Selection
The driver amplifier operates at the boundary between small-signal and large-signal behavior. Unlike an LNA (which operates far below compression) or a PA (which operates near or at compression), the driver must deliver its maximum output power while maintaining good linearity. This requires careful attention to the P1dB and OIP3 specifications relative to the PA's input requirements.
| Parameter | LNA | Driver | Power Amplifier |
|---|---|---|---|
| Noise Figure | 0.3-2.0 dB | 3-8 dB | 5-15 dB (not specified) |
| Gain | 10-25 dB | 10-20 dB | 8-15 dB |
| P1dB | -10 to +10 dBm | +15 to +25 dBm | +30 to +50 dBm |
| OIP3 | +5 to +25 dBm | +25 to +40 dBm | +40 to +55 dBm |
| DC Power | 10-100 mW | 0.5-5 W | 5-500 W |
Frequently Asked Questions
How much margin above the PA input do I need?
At least 3 dB between the driver's P1dB and the PA's required input power. More margin (6+ dB) provides better linearity at the cost of higher driver power consumption. The exact margin depends on the modulation quality requirements (EVM, ACPR).
Can I use a gain block as a driver?
Sometimes. Gain blocks with sufficient output P1dB can serve as drivers. Check that the gain block's OIP3 meets the system linearity requirement at the operating power level. Many gain blocks have OIP3 of +25 to +35 dBm, sufficient for driving PAs up to 1-2W.
What about variable gain?
Variable gain in the driver stage (using a digital step attenuator or VGA) allows the transmitter to adjust output power over a wide range. This is essential for cellular systems (power control) and phased arrays (amplitude taper). The VGA must maintain linearity across its gain range.