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.
Externally biased MMICs provide the gate and drain connections separately. The user supplies a negative gate voltage (Vgs) and a positive drain voltage (Vds) independently. This allows optimization of the bias point: decreasing Ids reduces noise figure (useful for LNA mode), increasing Ids improves linearity (useful for driver mode), and adjusting Vgs over temperature compensates for gain drift.
The choice depends on the application: self-biased for general-purpose gain blocks where performance optimization is not needed, externally biased for LNAs, drivers, and any application where the operating point must be controlled or varied.
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.