What is the difference between a self-biased and an externally biased MMIC amplifier?
MMIC Bias Options
Self-biased MMICs (like the Mini-Circuits ERA and Analog Devices HMC xxx series gain blocks) include an internal FET with a source resistor that provides automatic current stabilization. The device needs only a positive supply connected through a current-limiting resistor: Rbias = (Vcc - Vdevice)/Idevice. No negative gate supply is needed because the source resistor provides the gate-source bias. These devices are designed for maximum simplicity in integration.
| 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
Can I adjust the performance of a self-biased MMIC?
Very limited. The internal bias network fixes the operating point. Changing the supply voltage slightly adjusts the current (within the supply tolerance), but this is not intended as a performance adjustment. Use an externally biased MMIC if you need to optimize NF, gain, or linearity.
What about power consumption?
Self-biased MMICs typically draw a fixed current (5-100 mA) that cannot be reduced without removing power entirely. Externally biased MMICs can be set to a lower current for power saving (at the cost of reduced performance). This is useful for battery-powered or duty-cycled systems.
Do self-biased MMICs need sequencing?
No. The internal bias network handles the power-up sequence safely. This is a major advantage for simple system integration. Externally biased depletion-mode MMICs always require gate-first sequencing.