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#10
July 22nd 04, 11:33 AM
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Rick Karlquist N6RK wrote:
You didn't even mention base spreading resistance, a major figure of merit for RF transistors. Newer designers have considerably lower r-sub-bb-prime than legacy transistors. Look up the hybrid-pi transistor model. SiGe technology's claim to fame is low base spreading resistance. Rick N6RK "Paul Burridge" wrote in message ... 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. Beat me to it; see my comment. |
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