Added Series "Dropping" Resistor on Drain Provides Temp. Compensation?

so ya i have no idea what your talking about but i had to post one of these comments for my class so dont pay any mind to it ok well bye

thats cool man nice but hey heres tyhe thing i am workin on my pc and tryin to fig out if i can go from a 754 socket motherboard to a 949 one you know if i can?

I don't see the value in that reasoning, because gm = Ic/VT, where VT = kT/q, for a negative transconductance tempco with constant current. [To eliminate gm tempco you need to use a PTAT (proportional to absolute temperature) current source. To get a PTAT CS, use a BJT with Vb = Vg0 = 1.23V (e.g. an LM385-1.2 or LM4041 reference) and an emitter resistor. Vg0 is the bandgap of silicon extrapolated to 0K, where 0K means absolute zero... alphabet soup.]

You may find saving a bunch of regulators appealing for a personal one-off, but I wouldn't do it for any serious use or for production. A low-current part would experience more voltage than intended, perhaps even damaging it, and a high-current part would have inadequate voltage, and be unable to reach the specified output power. Even if you trim the resistors for your one-off, if you ever needed to change a component, you'd be back to square one. Not good.

Check the app notes, many MMIC RF amps REQUIRE a sereis resistor to stabilize the current vs temperature...

what you reaaly want is to construct the V/I load line of the amp at various temperatures and then see what the voltage and current will be for various supply and resistor values.

Mark

I suppose my question is really this:

Will the ouput power of a MMIC vary less over temp with a series dropping resistor (more like a current source) than with a straight voltage source.

The question is complicated by the fact that the MMIC will have built in RF chokes on the die, so we are not actually tied to the drain directly.

That's not really a concern because even if the device draws zero current, the voltage from the regulator will still be less than the absolute maximum drain voltage rating on the data sheets.

And as the other poster mentions, the MMIC supplier recommends a dropping resistor in the data sheet for proper operation. It would have to be extreme lot to lot and wafer variations for us to need to adjust these during production (once we dial them in during prototyping).

Thank you for the input, Mr. Hill.

Dr. Slick

That's the reason we are using dropping resistors in the first place. It seems you agree with me then that one of the reasons for using the dropping resistor is for temp comp.

Well, if you look at an AC load line (assume linear class A for simplicity) over your DC I/V curves, you can see how lowering your drain voltage will begin to compress your amplifier sooner, at a lower output power. So if we lower the transconductance at high temp, the entire AC load line is shifted down, and the average drain current drops. But if we increase the drain voltage, then we shift the load line to the right, and can get the voltage swing back to the room temp values.

A bit simplistic of a model, i'll agree, but seems to make sense.

Slick

I suppose my question is really this:

Will the ouput power of a MMIC vary less over temp with a series dropping resistor (more like a current source) than with a straight voltage source.

The question is complicated by the fact that the MMIC will have built in RF chokes on the die, so we are not actually tied to the drain directly.

That's not really a concern because even if the device draws zero current, the voltage from the regulator will still be less than the absolute maximum drain voltage rating on the data sheets.

And as the other poster mentions, the MMIC supplier recommends a dropping resistor in the data sheet for proper operation. It would have to be extreme lot to lot and wafer variations

for us to need to adjust these during production (once we dial them in during prototyping).

That's the reason we are using dropping resistors in the first place. It seems you agree with me then that one of the reasons for using the dropping resistor is for temp comp.

Well, if you look at an AC load line (assume linear class A for simplicity) over your DC I/V curves, you can see how lowering your drain voltage will begin to compress your amplifier sooner, at a lower output power. So if we lower the transconductance at high temp, the entire AC load line is shifted down, and the average drain current drops. But if we increase the drain voltage, then we shift the load line to the right, and can get the voltage swing back to the room temp values.

A bit simplistic of a model, i admit, but seems to make sense.

Slick