"Jim Thompson" wrote in
message ...
Is S.E.D actually sci.electronics.DUMMIES ??

When I saw this original post....

From: "powerampfreak"
Newsgroups: sci.electronics.design
Subject: Somebody explaining this design?
Date: 9 Mar 2007 12:08:35 -0800
Organization: http://groups.google.com
Lines: 11
Message-ID: .com

I commented, "Looks like crap to me ;-)"

The response from the hot air crowd, you know, the ones posing as
guru's, was an implication that I was incorrect.

So I posed a simple question... do a hand analysis of the gain of the
circuit.

It's a simple analysis (if you're not a faker :-)

It's been about 40 hours since I posted that request/taunt.

Nary a peep.

So I think this is TRULY...

sci.electronics.DUMMIES ;-)

...Jim Thompson
--

Best, at the moment that I can guess.....

Short circuit the gain setting pot/capacitor and throw away the NPN
transistors. It's a differential pair. The gain is gmRC. gm is set by the
'tail' resistor which is R2||R6.

Put the NPNs back in again. It's still a differential amplifier but the NPNs
'pin' the PNP currents at Vbe/Rbe which makes gm some different value......,
gm' less than before.

However the NPNs 'boost' that value by approximately their Beta....Bnpn [1]
So

Av = BnpnRCgm' = BnpnRCIcpnp/25E-3 = BnpnRCVbe/25E-3Rbe

That might be a factor of two out because it's a diferential amplifier.

Open circuit the gain setting network and it's just a pair of 'thingy'
amplifiers with

Av = RC/RE

In between........ I couldn't be bothered.


[1] Wildish guess that 'feels' right because there is some feedyback thing
going on and I think gm is a measure of re, or something.

I won't push my luck further on 'second' order effects because I probably
haven't got the first order ones right.

On the basis of the above analysis..... the gain is going to vary all over
the shop as Bnpn (and other stuff) varies. So yes it is crap.

DNA