Can I use the typical characteristic curves on a datasheet for my biasing analysis?

[skatefast08]

When I designed my first microwave pre-amplifier, I noticed that my bias point was totally off. I used the typical characteristic curves shown on the datasheet (for an ATF-55143 pHEMT) and based those curves for my design, which I believe was not the best way to calculate my bias network resistors. If I was to design another microwave pre-amplifier, would you think I should test the characteristic curve for my transistor before calculating resistor values?

[d123]

Hi,

Theoretically, depending on precision required, the datasheet should suffice for lowest and highest points to find the best fit - accepting a certain % tolerance/variable will be there in reality beyond calculations. I think some circuits do need the device to be tested first otherwise the results are way off. Different but similar, NFETs I have have a wide ranging VGSoff, designing a reasonably precise circuit is not possible with even a variable VGSoff in the same pack of NFETs that so far have ranged from -1.8V to -2.8VGSoff (grrrr).

[FvM]

If you look at the Vgs min/max range in datasheet, it's quite obvious that passive biasing can't achieve a predefined Id with reasonable tolerance (e.g. +/- 10 or 20 %) without adjusting the voltage divider. Active biasing however can.

No use of recording a curve, just Vgs for the intended Id.

[BigBoss]

Typical Biasing values ( Vgs and Vds ) are generally given for dedicated purpose. I mean they are for either max. Power or max. Efficiency or max. Gain or Max. Linearity
Which one you are interested in will be your biasing point.But you should check these values by IV curves.Then decide your way of designing.

[ktr]

Active Biasing is the way to go for eliminating those tolerance issues. It seems a bit contrary at first sight, adding more variables to solve variables; however, using another transistor as a switch really solves the biasing problem.

Two points to keep in mind.

- Putting a biasTee to isolate active bias from circuit

- using radial stubs and TLIN's to short out harmonics