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LM386 output RC purpose

The datasheet for an LM386 amplifier shows a resistor and capacitor in series at the output:

250 uF LM386 +|| output --------|---||--- (pin 5) | || | .|. | 10 | | | __ /| ohm | | -| | | '|' -|__| | | \\| | | | | .05 --- | uf --- | | | === === GND GND

(created by AACircuit v1.28.6 beta 04/19/05

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I've been trying to ascertain exactly what the purpose of this RC combination is. My first guess was attenuation of higher frequencies (decreasing AC resistance due to decreasing capacitive reactance with increasing frequency). Next, a search lead to a site which indicated it's a roll-off network with no future explanation. Additional searching revealed it might be a impedance compensation network (Zobel). However, my calculations indicate that for a Zobel the capacitor value implies a speaker inductance of ~5 uH whereas I think speaker inductances are closer to 1 mH (and therefore the capacitor would be in the 10-20 uF range).

So can anybody tell me the actual purpose?

TIA.

Reply to
Michael
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It's a method of making the highly inductive loudspeaker load 'look' more resistive to the amplifier.

Graham

Reply to
Eeyore

It is part of the feedback stability requirement. The amplifier has internal output voltage feedback, to set the gain. But, as frequency rises, the phase shift through the amplifier increases, and before the open loop gain falls below 1, the phase shift gets large enough to turn negative feedback into positive feedback. That takes only a half cycle of phase shift.

The output RC network (called a Zobel network, if you want to Google for more), rolls off the open loop gain (as frequency rises) with the capacitor load, but terminates this roll off with the series resistor, just as the amplifier is really cranking up the phase shirt. This gets the loop gain below 1 before the phase shift inverts the sense of the feedback, stabilizing the amplifier, especially when it is loaded with a speaker that has an inductive component (less load at higher frequencies, that causes the open loop gain to rise.)

I hope that gives you a little feel for its purpose.

--
Regards,

John Popelish
Reply to
John Popelish

"Michael"

** Yep - 50nF and 10 ohms.

** It is so the amplifier sees a defined load ( ie 10 ohms) at frequencies above 300kHz.

Otherwise, it may become unstable at such frequencies and impose spurious oscillations on the output signal.

....... Phil

Reply to
Phil Allison

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