From: John Woodgate
Subject: Re: Transistor amplifiers v. frequency in SPICE
Date: Mon, 13 Jan 2003 17:19:47 +0000
Organization: JMWA Electronics Consultancy
Reply-To: John Woodgate
NNTP-Posting-Date: Mon, 13 Jan 2003 18:47:27 +0000 (UTC)
X-Newsreader: Turnpike (32) Version 4.01 <5Z8C9wtxbnpWyFnyfFzqmVF739>
I read in sci.electronics.design that Paul Burridge
wrote (in <4kh52vk90vskk0ef9ck9bsltk9knnue
email@example.com>) about 'Transistor amplifiers v. frequency in SPICE', on
Mon, 13 Jan 2003:
>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!
No, Win is, in fact, letting you down very lightly, and it's not a
matter of harsh judgement but of trying to steer you to success.
You have 2 mV from a high impedance source at 40 MHz (however surprising
that may be). You don't say how much gain you want, but you design a
stage that has 500 times at low frequencies but use 1 nF capacitors,
more suitable for 40 MHz. That high gain means a lot of noise, even if
you following circuits reject it. You run with a very low Vce because
you don't need much output voltage swing, but you haven't noticed that
you have only a sniff of collector current. You have a 150 kohm
collector load, which the tiniest bit of capacitance shunts into
oblivion at 40 MHz. Your base bias circuit resistors give 75 kohms input
resistance, but the base looks like maybe 16 kohms, beta-dependent. The
device you have chosen, BC 337, is a low-frequency device, with
capacitances to suit, not the best choice for a 40 MHz amplifier.
So, with the best will in the world, you are starting from a LONG way
back, and need a lot of guidance.
Regards, John Woodgate, OOO - Own Opinions Only. http://www.jmwa.demon.co.uk
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