What is this electronic schematic symbol?

[slackwarelinux]

High school is out for the summer, so I am trying to entertain myself. Yesterday I made a simple audio amplifier the LM386, and now I am trying to make the same circuit, but without the LM386. I am using the connection diagram, which is on the page I linked to, to try to make the replacement for the LM386 using transistors, resistors, and diodes. However, there is one part in the diagram I can't identify. I uploaded a picture of it onto imageshack, if you want to see it in context, here it is, in the upper right.

I tried searching google, I tried asking an EE, but I got no leads. So, does anyone have any idea what part the symbol I linked to on imageshack symbolizes?

 

[PottedMeat]

Thats a current source/sink.

I don't think you'll be able to make a discrete replacement for the opamp using that schematic without a lot of work ( not enough info ). I think it's just for the user to know basically how the pins are connected and how to interface with the IC.

 

[f95toli]

I agree with PottedMeat. Schematics like that does NOT tell you the thing actually works, in some cases you can use them to create SPICE models of a component but that is all.

 

[slackwarelinux]

Originally posted by: PottedMeat

Thats a current source/sink.

I don't think you'll be able to make a discrete replacement for the opamp using that schematic without a lot of work ( not enough info ). I think it's just for the user to know basically how the pins are connected and how to interface with the IC.

So, doing this likely won't work? I suppose I should look for a plan for another audio amplifier then.

How would I build a current source/sink?

Edit: Never mind, I did some more research. Thanks for the quick responces!

 

[Loki726]

Try to find a schematic that gives precise part numbers for the transistors/diodes/etc or at least gives gross device parameters like current gain for transistors and junction voltage for diodes so you can find parts with the same characteristics.

Barring that, you could just go with what they give you in that schematic and refine it until you get something that works. Start with a SPICE simulation with ideal transistors and diodes and start swapping them out for real components until you get something that works.

 

[blahblah99]

That there is a current source... you can make a cheap and easy version by replacing that with a resistor, or do an active one with a transistor that's biased with resistors.

You cannot replicate a working circuit based on just that schematic alone.. they purposely left out the details so you cannot copy it 1:1.

A quick look shows that the input stage is a differential stage, then a voltage amplification stage, and finally class-ab output stage, with the current source being used to bias the output stage to prevent crossover distortion.

If you do some analysis, you can find the overall voltage gain with the feedback in the loop with the values they have given you.

But just to repeat - you're not going to get a working circuit based on that schematic alone. There are a lot of components left out.

 

[slackwarelinux]

Try to find a schematic that gives precise part numbers for the transistors/diodes/etc or at least gives gross device parameters like current gain for transistors and junction voltage for diodes so you can find parts with the same characteristics.

But just to repeat - you're not going to get a working circuit based on that schematic alone. There are a lot of components left out.

Thanks, I ended up just making an audio amplifier based off of a schematic on <a target=_blank class=ftalternatingbarlinklarge href="http://

Try to find a schematic that gives precise part numbers for the transistors/diodes/etc or at least gives gross device parameters like current gain for transistors and junction voltage for diodes so you can find parts with the same characteristics.

">this site.</a>

 

[davesaudio]

low powered amplifiers can be much simpler than that.
lookup ZEN - or diyaudio dot com will deluge you with schematics of alternatives

 

[krotchy]

Building an op-amp from individual components is impossible by any practical standard. Mainly because many of the transistors need to be matched with another one in the circuit at the fabrication level, and you dont know which matched pairs there are inside the op-amp anyways. Plus two discrete BJT chips might be the same type/make/model/year and still have different on resistance characteristics and whatnot.

 

[bobsmith1492]

This is a good website for discrete audio amplifiers:
http://www.sound.westhost.com/projects.htm#index

I built the ~100W amplifier, and it works great (never got that much power, though, due to the lack of an adequate power supply).

I also built a "switching" DC-DC supply from the site, which works though I haven't tested it under load yet.