What is a distributed amplifier and when would I use it instead of a single stage amplifier?
Distributed Amplifier Topology
The distributed amplifier (also called a traveling-wave amplifier) is the only topology that achieves gain from near-DC to the transistor's fT in a single stage. It works by absorbing the parasitic capacitances of the transistors into the artificial transmission line structure, rather than resonating them out (which limits bandwidth in conventional amplifiers).
| 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 |
Bias and Operating Point
The gain of an n-transistor distributed amplifier is approximately: G = (n·gm·Z0/2)², where gm is each transistor's transconductance and Z0 is the line impedance (50 Ω). With 4 transistors and gm = 40 mS per device: G = (4×0.04×50/2)² = 16 = 12 dB. Doubling the transistor count doubles the voltage gain (6 dB increase). The gain is frequency-independent up to the artificial line's cutoff frequency.
- Performance verification: confirm specifications against the application requirements before finalizing the design
- Environmental factors: temperature range, humidity, and vibration affect long-term reliability and parameter drift
- Cost vs. performance: evaluate whether the application demands premium components or standard commercial grades
- Interface compatibility: verify impedance, connector type, and mechanical form factor match the system architecture
- Margin allocation: include sufficient design margin to account for manufacturing tolerances and aging effects
Stability Considerations
The noise figure of a distributed amplifier is higher than a single-stage LNA (typically 3-6 dB) because subsequent transistors contribute noise that is not fully suppressed by the gain of preceding transistors. This is the fundamental tradeoff: distributed amplifiers sacrifice noise figure for bandwidth. They are most useful in applications where bandwidth matters more than sensitivity.
Frequently Asked Questions
When should I use a distributed amplifier?
When you need gain from DC (or below 100 MHz) to above 20 GHz with a single component. Common applications: wideband test equipment, fiber-optic transimpedance amplifiers, EW receivers, ultra-wideband radar, and high-speed digital circuit drivers.
How many transistors are optimal?
4-8 transistors is typical. More transistors provide more gain but increase the input line loss (which degrades noise figure and high-frequency gain). The optimum number depends on the gain-per-transistor and the line loss at the highest frequency.
Can I cascade distributed amplifiers?
Yes. Two cascaded 10 dB distributed amplifiers provide 20 dB gain with the same bandwidth. The second stage degrades the noise figure by only its contribution divided by the first stage gain. Cascaded distributed amplifiers are used in 40+ Gbps fiber-optic receiver pre-amplifiers.