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The bi-polar transistor at RF

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July 22nd 04, 04:18 PM

Paul Keinanen

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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.

For VHF applications, a high current SiGe device operating at
impedance levels well below 50 ohms would give good IP3 figures, but
apparently the noise figure increases quite rapidly with high
collector currents.

Paul OH3LWR

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