In article ,
Paul Keinanen writes:
On Thu, 22 Jul 2004 03:09:10 +0000 (UTC), (John S.
Dyson) wrote:
One big disadvantage of the typical SiGe transistors is
that their breakdown voltage is low.
Unless the transistor is too fast for a given layout, SiGe
can be used at low frequencies (e.g. VHF) while still avoiding
the low frequency noise problems that are common from GaAs FETS
and even other fast BJTs.
While the IP3 and compression figures may good to comparable devices
working in the GHz bands, the huge gain with the low Vce (typically
less than 2.3 V) will damage the input IP3 values quite quickly at
VHF. This can be a problem in the VHF and lower UHF bands, in which
signal levels can be quite high and multiple strong signals may pass
the front end selectivity.
"Noiseless feedback' is very helpful to mitigate the excess amounts
of gain, while pushing the return loss match (the impedance match)
closer to the noise match (the input impedance where the noise is lowest.)
A simple emitter (source) inductor and a little bit of parallel, noisy
feedback can be used to tame some of the interesting UHF+ components.
(The emitter (source) inductor is a case where a small amount of
inductance is much better than too much, because instability can ensue
with too much series feedback (too large an emitter (source) inductor).)
I do agree with your implication that a high current device can be helpful
at low frequencies, but some SiGe components do seem to maintain
reasonable noise performance at high currents.
In any case, the SiGe components do give the GaAs type components a
run for their money. In some cases, the SiGe components are actually
better. The PHEMTs (HP 54143) are also an interesting variation on
the 'fet' theme, which helps to mitigate some of the problems WRT
GaAs. For example, a PHEMT can provide a good noise match at 50ohms,
a transconductance of almost 1MHO at 60ma, and reasonable IP3 (38dBm isn't
impossible.) Feedback can be especially helpful with the high
transconductance FETs.
John