Op Amp - What is the purpose of the 1 ohm resistor and cap to ground

ian field

Joined Oct 27, 2012
6,539
In this sample non-inverting op amp circuit from Texas Intruments from the LM675 datasheet..
http://www.ti.com/lit/ds/symlink/lm675.pdf

What is the purpose of having the 1 ohm resistor and 0.22uF capacitor in parallel to the load?


View attachment 105626
That's very similar to an op-amp, but its actually an AF power amp.

The CR network is often called a Zoebell network - its sometimes called by another name that I can't remember right now.

AFAICR: it damps spurious oscillation on the output stage.
 

crutschow

Joined Mar 14, 2008
23,802
It is being used as a audio amp to drive a speaker, and the resistor and capacitor roll off the high frequency response above the audio range to stabilize the amp, avoiding oscillations due to the reactive speaker load.
 

AnalogKid

Joined Aug 1, 2013
8,229
The schematic symbol for many audio speaker amplifiers look like normal opamps. They are not. Audio speaker amplifiers are weird, squirrely, and tempermental. They require careful decoupling plus things like the zobell network to prevent breaking into oscillation. Just a warning...

ak
 

crutschow

Joined Mar 14, 2008
23,802
The schematic symbol for many audio speaker amplifiers look like normal opamps. They are not. Audio speaker amplifiers are weird, squirrely, and tempermental. They require careful decoupling plus things like the zobell network to prevent breaking into oscillation. Just a warning...
The spec sheet lists it as a power op amp, so I think it's a more general purpose amp then those labeled as audio power amps.
 

ian field

Joined Oct 27, 2012
6,539
The spec sheet lists it as a power op amp, so I think it's a more general purpose amp then those labeled as audio power amps.
There's a sort of grey area between power op-amps and audio power amplifiers, some are equally at home in either application - some are a little more fussy.

Sometimes ultra fast buffers look the same as a schematic symbol, but they're not the best choice for audio amplifiers.

Made for the job audio amplifiers usually have a bootstrap pin so a capacitor can couple the output back to the base bias chain. This increases the gain and linearity of the emitter follower pair.

These days most people go for at least a BTL amplifier, unless they need a common terminal for a headphone jack.

Class-D is rapidly gaining ground - you can get a few watts from a DIL or SOIC package with no heatsink whatsoever.
 
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