Author Topic: Basic building blocks  (Read 1346 times)

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Offline David Aurora

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Basic building blocks
« on: April 27, 2012, 02:13:10 pm »
Sorry Dave, I'm sure you've been asked this a zillion times by now, but I'd love to see some vids from you about basic circuit building blocks that people should know. In my limited experience so far the building blocks I've used the most would be RC filters, transistor amps, op amps, flip flops, but I'm sure there's plenty of others you could think of. I know a lot of this information is out there already, but a lot of it quite frankly sucks to follow for beginners. The reason I ask is that I've read explanations of the equations for decibels before and gone "Huh??", but when I saw your explanation on Youtube a while back it was so goddamn simple to follow (that's actually what brought me here to begin with). That seems to be the case with a lot of things you explain, it looks like shit when I read about something in a book, but you explain it in 2 seconds flat and make it sound easy.

I think the key is that you tend to make things practical. So many explanations I see either never actually use real values, just formulas, so it's easy to think you get the method but you can't follow along and confirm that you're doing it right. That, or the values they use are so fricken ridiculous and impractical that when you're trying to experiment you can't follow the values listed to try the circuit out for real (I constantly see this with audio circuits... oddball potentiometer sizes/tapers, capacitor values in filters that just flat out don't exist but looked good on paper, etc.). It makes it really hard when learning to be able to build up a circuit for real and match the principles up with observation.

The thing that got me wishing for this is transistor bias. I'm pretty fine with a lot of electronics stuff these days, but biasing BJT's still doesn't sit right in my head. I seem to have worked out some bizarre method of my own that works for me just fine, but it doesn't match the formulas I see elsewhere. The circuits sound good, look good on a scope and look good in simulation. But when I try to analyse my circuit using equations I've seen suggested for transistor bias, my values on paper are completely ass upwards. So I'm clearly misunderstanding something in the formulas presented, yet by my own ass backwards methods I'm making the circuits work well in practice. Very annoying. I'd bet dollars to donuts that if you did a video on the subject within about 9 seconds I'd go "Oh... of course! THAT'S where I was making a mistake in the formula".

So yeah, anyways, if you felt like doing some vids like that it would be awesome and I would definitely owe you a beer or three.

Offline vxp036000

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Re: Basic building blocks
« Reply #1 on: May 12, 2012, 11:39:44 pm »
For biasing transistors, you either follow the gm curves given on the datasheet or you need to characterize the device yourself to come up with an accurate model.  There really is no reason to try to come up with an exact model, though.  All you need to know is what parameter to tweak and in what direction to get the results you're looking for.  This is how the equations are used; they tell you what bias needs to be adjusted, in what direction, and by how much.  For a BJT in an amplifier, for example, I just use a pot to adjust the base voltage to get the amount of gain I'm looking for.  Not enough gain, decrease the base voltage.  Not enough linearity, increase the base voltage.  This can be figured out just by looking at the equations.

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