Paul Burridge wrote:

Hi all,

Firstly, does anyone bother designing with Y-parameters *at all* these
days?

Then... (talking of the common-emitter configuration in this case)
The only variable according to the Ebers-Moll transistor model apart
from the device-specific "Is" which has any effect on Ic is the
potental difference applied across the B/E junction. The signal
voltage thus applied is loaded by the resistance of this diode. At
DC., the loading is at a maximum and the entire PD appears across it.
Right so far? As the applied signal voltage increases in frequency,
the feedback capacitance (B-C) and the B-E junction capacitance form
an AC bypass path across the B/E resistance above-mentioned. The two
capacitances acting in concert shunt more and more of the applied
signal voltage to ground, bypassing the emitter diode resistance,
lowering the device input impedance and resulting in less and less
applied Vbe across this diode and consequently less and less Ic output
swing?

What I'm getting at is that Ebers-Moll is still good at RF, *provided*
one allows for the bypassing of the emitter diode's resistance by the
combination of Cb and Ce. Correct?
And CJC and CJE are the relevant Spice model parameters?

Thanks,

p.
--

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

At RF, the base spreading resistance can be large when compared with
the calculated emitter resistance; this makes a serious contribution to
input noise and the NF of the stage.
So the particular version of the model one uses can be rather poor in
determining real-life NF.
BTW, noise measurements at audio frequencies using different collector
currents can be used to determine the transistor's base spreading
resistance.
Once that is known, and the collector current used in the RF amplifier
(for determining Re), one can then calculate noise (or NF) and be rather
close to measured values!

On Thu, 22 Jul 2004 10:32:55 GMT, Robert Baer
wrote:

At RF, the base spreading resistance can be large when compared with
the calculated emitter resistance; this makes a serious contribution to
input noise and the NF of the stage.

Sorry guys, I did *mean* to include BSR in series with the B/E
junction resistance, so any reference I made to this junction
resistance should be taken to mean the total of the two together.

So the particular version of the model one uses can be rather poor in
determining real-life NF.

NF isn't a consideration in this instance; please ignore it.
And I am well aware of the pi-model. I just want to know if I have it
right that Ebers-Moll will work accurately into UHF provided one
allows for the feedback capacitance and emitter junction capacitance
shunting the input signal around BSR+EBR and thereby reducing the
signal voltage developed across them. Do I have this right?

Thanks,

Paul
--

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

On Thu, 22 Jul 2004 12:24:54 +0100, Paul Burridge
wrote:

NF isn't a consideration in this instance; please ignore it.
And I am well aware of the pi-model. I just want to know if I have it
right that Ebers-Moll will work accurately into UHF provided one
allows for the feedback capacitance and emitter junction capacitance
shunting the input signal around [EBR alone] and thereby reducing the
signal voltage developed across it. Do I have this right?

Sorry! Corrected above. IOW: whilst the emitter diode resistance is
bypassed at RF by these two capacitances, the base spreading
resistance *isn't* - apart from that, the rest of the post is now
correct, yes? 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...
--

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

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.