### Author Topic: Basic building blocks  (Read 1346 times)

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#### David Aurora

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##### Basic building blocks
« on: April 27, 2012, 02:13:10 pm »

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.

#### vxp036000

• Regular Contributor
• Posts: 167
##### 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.

Smf