I read in sci.electronics.design that Paul Burridge
wrote (in 8afvf05guvnvmjgqtafgau2d2li3ckn
) about 'The bi-polar transistor at RF', on Thu, 22 Jul
2004:
IOW, as the signal frequency increases, the BSR becomes the dominant
component of the device's input impedance... Phew! Unless of course,
someone knows otherwise...
Emitter lead inductance?
--
Regards, John Woodgate, OOO - Own Opinions Only.
The good news is that nothing is compulsory.
The bad news is that everything is prohibited.
http://www.jmwa.demon.co.uk Also see http://www.isce.org.uk

On Thu, 22 Jul 2004 14:41:45 +0100, John Woodgate
wrote:

I read in sci.electronics.design that Paul Burridge
wrote (in 8afvf05guvnvmjgqtafgau2d2li3ckn
) about 'The bi-polar transistor at RF', on Thu, 22 Jul
2004:
IOW, as the signal frequency increases, the BSR becomes the dominant
component of the device's input impedance... Phew! Unless of course,
someone knows otherwise...
Emitter lead inductance?

Er, yes, but I'm only interested in the *internal* characteristics of
the device here, so even the bonding wires' inductance isn't an issue.
Thanks for giving me the chance to clarify, though.
--

"What is now proved was once only imagin'd." - William Blake, 1793.

I read in sci.electronics.design that Paul Burridge
wrote (in 61kvf05660gk62pnm7o3bthepkujnep
) about 'The bi-polar transistor at RF', on Thu, 22 Jul
2004:
Er, yes, but I'm only interested in the *internal* characteristics of
the device here, so even the bonding wires' inductance isn't an issue.
Thanks for giving me the chance to clarify, though.

A bit of the emitter lead is inside the encapsulation, and a bit is on
the die.
--
Regards, John Woodgate, OOO - Own Opinions Only.
The good news is that nothing is compulsory.
The bad news is that everything is prohibited.
http://www.jmwa.demon.co.uk Also see http://www.isce.org.uk

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

On Thu, 22 Jul 2004 15:46:25 +0100, John Woodgate
wrote:

I read in sci.electronics.design that Paul Burridge
wrote (in 61kvf05660gk62pnm7o3bthepkujnep
) about 'The bi-polar transistor at RF', on Thu, 22 Jul
2004:
Er, yes, but I'm only interested in the *internal* characteristics of
the device here, so even the bonding wires' inductance isn't an issue.
Thanks for giving me the chance to clarify, though.

A bit of the emitter lead is inside the encapsulation, and a bit is on
the die.

Thank you, John, that gives me another chance to re-state the question
more succinctly. I'm not concerned with inductances here at all,
though.
Does the Ebers-Moll equation hold good at UHF+, provided the value one
inserts for Vbe is adjusted to account for the loss of signal voltage
the B/E junction will suffer as much of it (the applied signal
voltage) is shunted around it via the device's internal capacitances?

There! I think I've nailed it this time!

--

"What is now proved was once only imagin'd." - William Blake, 1793.

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

On Thu, 22 Jul 2004 16:01:02 +0000 (UTC), (John S.
Dyson) wrote:

"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).)

Has anybody actually used noiseless feedback with these devices at
VHF? After all, the fT of these SiGe transistors are in the 30 GHz+
range, so I would guess that the parasitics would mess the situation
quite badly, especially when using feedback components.

How critical is the layout compared to for instance MAR-x series MMICs
(that are essentially darlingtons) ? Can these SiGe transistors used
with dual sided boards and microstrips or do they require multilayer
boards and full striplines in order to use feedback ?

Paul OH3LWR