From: Paul Burridge
Subject: Re: Transistor amplifiers v. frequency in SPICE
X-Newsreader: Forte Agent 1.8/32.548
Date: Mon, 13 Jan 2003 14:18:33 +0000
NNTP-Posting-Date: Mon, 13 Jan 2003 15:46:11 GMT
Organization: ntlworld News Service
On 13 Jan 2003 11:44:57 GMT, Rick Hellicar opined
>It's more like a good circuit gone awry:
>1. The input resistance is NOT well-defined, because beta/gm is
> comparable, or even less than, than 150k | 150k; this makes the
> input high-pass -3dB frequency sensitive to beta variations.
I assume Zin of this stage would be Beta x (re+ZE) rather than just
>2. Re is very high, making the emitter voltage very high, thus
> reducing the voltage-swing available at the collector - I'd
> personally reduce the base voltage and reduce the emitter resistor.
Yes, but the high collector voltage is not a problem for this lash-up,
since the input voltage signal was only ever intended to be (for
experimental purposes) a fixed 0.002v. Hence Vc swing isn't a
>3. The emitter sees |Xc|, which is much larger than 1/gm (re) at lowish
> frequencies, so the gain is lower than the expected value of gm*Rc.
I've been forced to pick specific values for the coupling & bypass
caps solely for the purpose of posting the circuit, but in practice I
was selecting more appropriate values to suite the specific
frequencies under test for each run.
I think some of you chaps are judging me a little bit harshly. But I
don't mind provided I get some useful and informative feedback!
"What is now proved was once only imagin'd"
- William Blake, 1793