Biasing MMIC amplifiers (GALI-5)

An electronic current source dissipates heat just like a resistor - the advantage is the current level is more stable with varying supply voltage than a simple resistor and the effective impedance much higher.

Your best bet IS to use the recommended 40 ohm resistor with a 7 volt regulated supply & use a switching regulator to drop the voltage from your main supply.

......... Phil

The bias inductor is often unable to provide a very high impedance, so that the bias network shunts RF power (gain loss) and degrades output VSWR. In those cases, the resistor also serves to raise the RF load impedance of the bias network. Any current source you design will need to present a high impedance beyond the frequency band of interest, unless your series inductor can do the entire job below its self resonant point.

There are constant current designs using certain adjustable linear regulators. The sense/set resistor is the "upper resistor" and the "lower resistor" is your load. You'd need a regulator that has the fixed voltage drop across the upper resistor, not the lower one. Assuming the typical

Good luck Steve

I think I'll try a current mirror with either PNP bipolar transistors or P-channel FETs. I've done this before when I wasn't as concerned about excess power disspipation, and I could use a fairly high bias voltage. The problem in the current case is that excess bias voltage leads to a proportional amount of excess power consumption that I want to minimize. So I will have to determine how low I can go with the supply voltage while maintaining a reasonably constant current.

Check the LM317 datasheet. Such a regulator and one lone resistor can make a nice current source. Personally I'd stay away from LDOs because they are expensive and most have documented as well as undocumented pathologies. Some of them of the kablouie category, pieces flying around and stuff.

As Steve wrote you need to verify what load resistor the MMIC wants for RF purposes. You can use an inductor plus load resistor but watch out for resonances. Provide good bypassing at the other end of that.

The ultimate Uncle Scrooge method in power economy would be a switcher that gets the current as feedback instead of a voltage. Then you could keep the collector voltage at the far end of the inductor precisely where it needs to be for your 60mA.

The extreme power-saving case would be to pull up the output through an inductor, from a stiff supply, and sense/servo the operating current by applying a bit of DC to the input. An opamp and a few passives can do that.

Otherwise, any classic current source will work.

I think Zetex and maybe TI make integrated current mirrors that might do this with less parts. Just be careful they don't oscillate, given the inductor load.

Incidentally, most of these parts have a below-50 ohm input impedance at recommended operating current, and go up at lower currents.

John

A fet would probably have more output capacitance than a bipolar. One good combination is a small PNP, a ferrite bead, then an inductor. The whole mess will be a fairly good broadband high Z.

Be careful with Minicircuits. They outsource these parts and occasionally change vendors, and the new parts only vaguely behave like the old ones. And they tend to tell us to drop dead when we have problems. I'd go with W-J or Sirenza or somebody dependable.

John