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Design >> RF Design >> ft measurement discrepency
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Message started by boong on Apr 12^th, 2006, 6:36pm

Title: ft measurement discrepency
Post by boong on Apr 12^th, 2006, 6:36pm

Hello,

I evaluated ft two ways; 1) doing DC analysis and printing DC operating points for the transistor, 2) i did ac analysis for short (output) circuit current gain and evualated the frequency at which it crossed 0 dB. However, I get two very different values. ft doing the ac analysis is about 35GHz higher than from the first method. Does anyone have encountered this problem before? Which one is more accurate? Thanks

boon

Title: Re: ft measurement discrepency
Post by sheldon on Apr 22^nd, 2006, 8:12pm

Boon,

Can you provide more information, for example, the bipolar model
that you are using: Gummel-Poon, HiCUM, ... and the test circuit?

The following comments are for the gp bjt.

1) The dc operating point ft is a rough approximation, gm/2*pi*f*Cbe
where Cbe is Cpi + Cbe,junction

2) Measurements include the full device model and other factors,
Rb, Ccb, Re can effect the simulation result.

3) When you measure ft are you measuring at the frequency cross-over
or extrapolating from the 3dB frequency using 20dB/dec. When
measuring, the modeler usually extrapolates to avoid undesired
high frequency effects, perfect de-embedding is hard to achieve.
Are the characteristics of the Ic/Ib curve "well-behaved" at the
cross-over frequency?

a) In general, the simulation result should be more correct, if your
testbench looks like a real circuit.

b) You may want to build a more "realistic" testbench and extract the
ft from the s-parameters.

c) You may want to verify that the bias conditions in the your test
circuit match the conditions in your testbench.

d) You may want to review the circuit performance, for example, is
the gain lower in the testbench? For example, could Re be acting
like an emitter degeneration resistor, lowering gain, increasing
bandwidth?

Finally, remember that even if the ft result is accurate, it may not
reflect you circuits performance. Depending on what you are designing,
fmax may be a better figure of merit.

Best Regards,

Sheldon