From: Jim Thompson
Subject: Re: How to write the transfer function of the circuit?
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Date: Fri, 27 Sep 2002 00:28:51 GMT
NNTP-Posting-Date: Thu, 26 Sep 2002 20:28:51 EDT
Organization: Cox Communications
On Thu, 26 Sep 2002 17:20:26 -0700,
In Newsgroup: sci.electronics.design,
Entitled: "Re: How to write the transfer function of the circuit?",
Wrote the following:
|Granted there are capacitances that will make the transfer function a little
|more complicated than a simple summer. But in that transfer function a simple
|summation is highly dominant. I refer you to the schematic of a 741 at
|The error integrating capacitor C1 is pretty much fixed at three diode drops to
|V- through Q15, Q17, and Q22. The output voltage is highly dependent on the
|voltage accross C1, which is adjusted by the current mirror pair chain Q8 and Q9,
|Q10 and Q11, and Q12 and Q13.
Wrong, it's *two* diode drops: Q15 + Q17. Q22 comes into play *only*
during short-circuit current limiting and is there to protect Q15 and
Q17... the famous "output stage is protected but the driver smoked"
C1 is more correctly referred to as a pole-splitting capacitor,
compensating the two poles of the opAmp into one dominant pole and one
pole that is moved out beyond the GBW of the OpAmp.
However you are correct in that C1 does *indeed* dominate the negative
PSRR at high frequencies; but it helps positive PSRR.
| James E.Thompson, P.E. | mens |
| Analog Innovations, Inc. | et |
| Analog/Mixed-Signal ASIC's and Discrete Systems | manus |
| Phoenix, Arizona Voice:(480)460-2350 | |
| Jim-T@analog_innovations.com Fax:(480)460-2142 | Brass Rat |
| http://www.analog-innovations.com | 1962 |
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