Hi, i've seen a lot of schematics with this opmap used as audio amplifier. I have some questions about:
It was designed for low cost and low part count for very basic applications.. What else would you expect?
I remember years ago, the ECG line had a cross reference to a amp that I have no idea what the original number was, crossing to, but it only had 4 pins in a small package with staggered pins. It was basic, INPUT, OUTPUT, COMMON and Supply... with a fixed gain and only a few mw but enough to drive a small speaker in little portable devices.
I haven't seen that package style used in years.
Jamie
Two main reasons: it has enough output current source/sink capability to drive a small speaker directly; it can do 325 mW into 8 ohms. Also, the output biases up to half the supply voltage, which makes it convenient as a power amplifier that can operate from a single supply (just add a DC blocking capacitor.)
It's not actually run open loop, the LM386 contains an internal feedback network that by itself gives the chip a gain of 20. Without this internal feedback network the gain would be much higher; when you add an external capacitor what you're doing is bypassing a part of this internal feedback network to increase the gain. Because of these differences, though the chip does have + and - inputs it is _not_ an opamp, and attempting to use it as one in standard opamp circuits without consideration/modification of the circuit will cause the IC to behave weirdly best case and destroy itself worst case. I've managed to do the latter...:)
Perhaps the fact that this _amplifier_ is used as an audio amplifier can me figured out from what National Semiconductor calls it: "Low Voltage Audio Power Amplifier".
I've noticed a trend by newbies these days to call any monolithic amplifier an "opamp". Why is that? Do folks who do that not realize that "opamp" means "operational amplifier", and specifies a certain specific set of characteristics and (wide) set of applications? Do these same folks even realize that the "op" in "opamp" has a meaning? Have they forgotten that not all amplifier circuits have an opamp in them? What?
So, to clarify:
Operational Amplifier. AKA "op amp", "opamp". A 'signal' amplifier (i.e. it is not primarily indented to supply power to a load) that has tons of gain, and is intended to be used in circuits where the nominal circuit characteristics are entirely defined by the characteristics of the amplifier feedback. Makes a fine universal building block for lots and lots of things.
Power Amplifier. AKA "amplifier", "audio amplifier", "video amplifier", "driver". An amplifier intended to actually drive something on its output. May be designed to function as an operational amplifier as well (in case it will be called a "power operational amplifier"). This is what the LM386 is, for all that it's a wimpy little thing.
There is some overlap: many video amplifiers can be used as operational amplifiers, and many high-speed operational amplifiers do, of necessity, have the ability to drive significant current into a load. So one can use such amplifiers 'outside of the data sheet'.
-- Tim Wescott Wescott Design Services http://www.wescottdesign.com Do you need to implement control loops in software? "Applied Control Theory for Embedded Systems" was written for you. See details at http://www.wescottdesign.com/actfes/actfes.html
Yea, its a 'audio power amp', not an opamp. That's why its used as described in the Datasheet.
Cheers
Though the datasheet does include an example of it being used sort of like an opamp as a "power square wave oscillator." It seems to works OK like this with a single supply and a blocking cap on the output, even though the voltage at the non inverting input goes below ground.
"lionelgreenstreet"
** Funny, the graph I see is flat from DC to 300kHz at the -3dB point ** Funny how the ( non adjusted) gain is only 20 .......... Phil
Thanks for your answer... Last question:
It's correct or not? Thanks
Yabbut.
The part is specified for AC in and AC out. So that's 5Vp-p, and
25mVp-p. Where the offset is is very likely _not_ 25mV.-- Tim Wescott Wescott Design Services http://www.wescottdesign.com Do you need to implement control loops in software? "Applied Control Theory for Embedded Systems" was written for you. See details at http://www.wescottdesign.com/actfes/actfes.html
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