At 9/10/2004 05:30 AM, you wrote:
>Steaven Rogers, W4YI wrote:
>>Kevin,
>>
>> This is perfect. I have only one more question. What is the
>> component labeled 386. I would expect it to be an op-amp or something
>> along that line. What is the actual part number that I would need to look
>> for.
>
>Hi Steaven,
>The part is a LM-386, available at Radio Shack.
>
>While this part resembles an op-amp, it's actually a power amplifier
>intended to run loads down to 16 ohms.
>There are two things I have found that are a must when using this
>amplifier IC. You must use a 100 uF or larger filter capacitor on the
>power supply rails placed near the IC, and you must use the 10 ohm - .05
>uF network. These components suppress parasitic oscillations, and keep
>the amplifier stable.
I just designed & built & little 4 ohm speaker amp using the LM386. It
works fine provided you use the 10 ohm & .05 µF cap on the output as Kevin
notes (I didn't have the latter in my parts bins, so I used a 0.1 µF). The
filter cap is also required, though I found 10 µF to be more than
sufficient in my case. I found 0.1 µF was enough to squelch the
oscillations, so I figured a factor of 100 more would be good enough.
>Many common op-amps cannot source a load below 2K, the LM-741 and common
>TLO series are an example. These op-amps become unstable when loaded
>below 2K, and proper design requires a build-out resistor to ensure the
>op-amp's stability.
Looking at the datasheet for the TL084, I can't find anything that suggests
that the op amp's minimum output load resistance is 2K. I've used them
successfully in many 600 ohm applications. You do lose a significant
amount of output voltage swing at 600 ohms (only +/- 10 volts for a +/- 15
volt supply, one reason why I prefer the TLV2374).
The "build-out" resistor prevents capacitive loads from causing internal
high-frequency oscillation due to reduction of phase margin; even a long
shielded audio cable directly connected to the output of an op amp can
cause it to break into oscillation due to the cable capacitance. 220 ohms
should be sufficient, as a 220 ohm resistive load to ground would still
allow for almost half the output voltage swing as compared to having no
load (at least for the TL084, the TL064 appears to be much worse in this
regard). The datasheet for the TLV2374 recommends an external resistor of
only 20 ohms, so maybe less than 220 ohms would work. Unfortunately, the
datasheets I have for the TL084 & TL064 do not discuss this issue.
> The build-out resistor is lossy when driving low impedance loads and
> the use of this chip eliminates the need for a build-out resistor, and is
> stable down to 16 ohms.
Yes, they are nice amps. But they have limited gain since they aren't op
amps & AFAIK won't handle DC tasks such as a comparitor.
Bob NO6B
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