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Here is the op-amp circuit, which is found in this datasheet on page 5: enter image description here

I have boxed, in red, the part of the circuit that I am confused about. What is the point of having this? Thanks in advance.

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What Jippies says. The Boucherot cell (seen here) is a spacial case of Zoebel networks which are a matching network aimed at transforming the impedance of a frequency variable "target" to make the overall result more like a pure resistance with a wider flatter frequency response. For too much detail see Wikipedia - Zoebel network –  Russell McMahon Apr 26 at 6:54

2 Answers 2

up vote 10 down vote accepted

It is a Zobel network also known as Boucherot cell, meant to stablize the amplifier when no load (speaker) is attached. It compensates the increasing speaker impedance at higher frequencies and prevents high frequency oscillation.

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Great, thank you! –  Daniel Apr 26 at 6:54

(1) What Jippies says.

(2) The Boucherot cell (seen here) is a special case of a Zoebel network.
Zoebel networks are a matching network aimed at transforming the impedance of a frequency variable "target" to make the overall result more like a pure resistance with a wider flatter frequency response. For too much detail see Wikipedia - Zoebel network.

For speaker application see section "Zobel networks and loudspeaker drivers".
In the speaker application the component between the centre of the RC and the speakers notional LR is missing (as the speaker's distributed LR has no physical tap - and you can swap R & C so it is no longer a Zobel bridge but still works.

Interest:

Original 1923 paper is : Zobel, O. J., Theory and Design of Uniform and Composite Electric Wave Filters, Bell Systems Technical Journal, Vol. 2 (1923), pp. 1–46.

Image of full 1923 article here:
http://alcatel-lucent.com/bstj/vol02-1923/articles/bstj2-1-1.pdf

Munted text version here: http://archive.org/stream/bellsystemtechni02amerrich/bellsystemtechni02amerrich_djvu.txt

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Interesting background information! –  jippie Apr 26 at 8:06

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