LM386 output RC purpose

M

Michael

Guest
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 www.tech-chat.de)

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.
 
Michael wrote:

I've been trying to ascertain exactly what the purpose of this RC
combination is.
Click to expand...
It's a method of making the highly inductive loudspeaker load 'look' more
resistive to the amplifier.

Graham
 
Michael wrote:
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 www.tech-chat.de)

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?
Click to expand...
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
 
On 3ÔÂ18ČŐ, ĎÂÎç10Ęą14ˇÖ, Michael <n...@bigtoes.com> wrote:
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 04/19/05www.tech-chat.de)

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.
Click to expand...
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On 3ÔÂ18ČŐ, ĎÂÎç10Ęą14ˇÖ, Michael <n...@bigtoes.com> wrote:
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 04/19/05www.tech-chat.de)

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.
Click to expand...


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"Michael"
The datasheet for an LM386 amplifier shows a resistor and capacitor in
series at the output:
Click to expand...

** Yep - 50nF and 10 ohms.


So can anybody tell me the actual purpose?
Click to expand...

** 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
 
On 3ÔÂ18ČŐ, ĎÂÎç10Ęą14ˇÖ, Michael <n...@bigtoes.com> wrote:
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 04/19/05www.tech-chat.de)

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.
Click to expand...


Do you want access to China's massive pool of electronic
manufacturers... but lack the time to contact suppliers, negotiate
contracts, arrange shipping or monitor product quality? Don't worry -
Let seriouswholesale deal with all that for you.

*Check out the huge range of Gadgets, MP3 / MP4 Players, Car DVD /
Audio, and Computer Accessories now by visiting the online wholesale
catalog at seriouswholesale. com You'll have peace of mind thanks to
the seriouswholesale Quality Control, 12-month Warranty on all
products, and easy secure payment by credit card through Paypal.

Selling on eBay or your own online store? Send products direct from
our warehouse to your customers using our unique drop-shipping
service. You can profit by selling hundreds of different products,
without holding any of your own inventory! Any questions you have will
be answered by the seriouswholesale English-speaking customer support
team... Their aim is to make your China electronics importing business
easier to run than ever before.

Welcome to http://www.seriouswholesale.com.

seriouswholesale - Buy from the source, profit without the hassle.

- 12 Months Warranty - No minimum order restrictions - Drop-shipping
with no additional fee - Pay by safely by PayPal seriouswholesale
Wholesale Co., Ltd.: Chinas original and best online electronics
wholesaler & drop-shipper: seriouswholesale. com
 
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