can't find info about buffers - help!

[mg.audio]

I can't seem to find a good source of information that can tell me exactly what a buffer is (and when/why they are needed). Can anyone help me with this?

[velbright]

A buffer increases the impedance... I think... and you use it to get a signal from a circuit without influencing the original circuit... or at least that's my understanding...

For example if you wanted the sawtooth output from a 555, you can't just put a wire there, because it will affect the cycle of the 555, so you have to buffer it in order for the 555 to function properly, so it's like separating the circuits, so that one can operate without damaging the function of the other.

A buffer can be an op-amp, or it can be simpler, with a transistor.

I only know the very basics, but I'm sure colin or ve3wwg will have a more comprehensive answer.

Hope I've helped
Velbright

[mg.audio]

Good stuff, thanks. That makes perfect sense to me now. So, in the case of building effects pedals and rack units, I want to keep my circuits simple. I'm wondering if effects with only an output buffer would be stable in combination with most (preferably all) other guitar/audio equipment.

[velbright]

Not really sure there. Sorry

[mg.audio]

No problem. I'm sure this is all going to boil down to experimentation in the end. 

[ve3wwg]

A buffer stage, whether a single transistor or an opamp stage provides some isolation between input and output. Frequently there is power amplification involved as well.

Using the earlier 555 example, you don't really want to attach any external circuit to the timing capacitor. To do so would alter it's timing/frequency, because it would discharge or charge the capacitor differently.  A simple way to visualize the harm is to imagine placing a 1000 ohm resistor between the timing cap's top end and ground. Depending on other components, the cap might not ever charge high enough (any more) to trigger the next cycle- and then it stops oscillating.

When you attach a circuit, you are effectively putting resistances and capacitances in parallel with that point. Thus you're changing the source circuit, unintentionally. Thus it needs a buffer stage.

A buffer stage uses a high input impedance. This way when you attach it to a component like the 555's timing cap, it is like attaching a 1 Megohm resistor in parallel with it (instead of 1000 ohms).  The resistance of 1 Meg is so high, that it will only alter the original charging circuit a teensy weensy bit.

If the buffer is an opamp, the output impedance will be low (< 50 ohms I think). This makes it good for driving your next stage (or pedal), since the amplified signal has more current behind it.  The voltage might not be higher, but the current has been amplified. Amplify either voltage, current or both, and you amplify the signal's power.

So buffers avoid "loading" the input side, and have more power to drive the output side. Buffers often don't include voltage gain but usually offer some current gain (overall, power gain is increased).

A 1 transistor buffer is actually easier to build than using an opamp, by the way. Direct couple the input to the base (no capacitor).  The voltage appearing at the transistor's emitter lead will be about base voltage minus 0.6 volts. This tells you the voltage across R1. Say it is 6 volts- compute R1 to conduct between 1 and 10 ma. So R1 = 6 volts / 0.001 (1 ma) => 6 kohms approx (600 ohms for 10 ma).

Then your buffered output signal is available at the emitter. Use a coupling capacitor between there and the next stage (usually). This transistor circuit is called an emitter follower. It too has low output impedance of near 50 ohms or less.

[mg.audio]

So, let's see if I'm understanding this correctly...

Say I'm building an effect with two different parts to it, for example, a distortion circuit followed by a resonant lowpass filter. Would signal flow look like this...?

input > buffer > distortion stage > buffer > filter > buffer > output


This seems like too many buffers to me, but of course I'm learning here.

[ve3wwg]

The answer is ït "depends".

It depends on the stage prior to the next one. If you know the output impedance of the last stage is low, then it can probably handle it fine. Separate effect and pedals are designed to be chained together without buffers. The buffers if necessary, are "built in" to each already.

What you're describing may or may not need one. You'll have to analyze the distortion stage (1 transistor, or perhaps opamp). In either case, I suspect you won't need one. Unless you use a very unusual circuit for that, I can't imagine you needing one.

But sometimes you're taking a signal from an oscillator say. The osc has to maintain a delicate balance, so taking a signal from it changes it. Then you may need a buffer.

[mg.audio]

I guess my next question would be: How do I measure/identify impedance at the beginnings and ends of circuit stages? (also what are the ranges separating high/low impedance)

Also also, when looking at schematics, I see lots of circuits with buffers only at the output, and only a few that either have buffers at both ends or no buffers at all (maybe I'm confusing an output stage with buffers). Is this yet another example of "it depends"?

[ve3wwg]

Input and output impedances at each stage seems like rocket science at first. But it is within grasp of anyone who has an understanding of ohms law, and a little about how the different stage configurations work.

The first thing you need to do is to learn about "Thevenin's Theorem". In a nutshell, you look at all voltage sources and short them out. If you see any "current sources" (they are rarer and harder to recognize), you treat them like an open circuit.  After you do all that, you compute your resistors in parallel etc., to simplify it to a single impedance (resistance without caps and inductors). The following is just one resource on the subject:

http://hyperphysics.phy-astr.gsu.edu/hbase/electric/thev2l.html

I wish I had understood this theorem decades ago.

The only other short note I can add is that the input/output impedance varies by transistor stage configuration:

• common emitter
• common collector (emitter follower)
• common base

Output Impedance:

For the emitter follower, you can probably just assume about 50 ohms for rough calculations. For the common emitter stages, it is usually just assumed to be about the amount of the collector's resistor.

Input impedance is more complicated. Generally speaking, emitter followers have reasonably high input impedance when directly coupled, while the common emitter stages may often be just the bias resistors in parallel (apply Thevinin's theorem). But these are imprecise answers to your question. If you bias a stage with a resistor from the collector, you have the added complication of feedback calculations.

All of this is fun science to learn about!

[mg.audio]

ve3wwg, thanks so much for the tips. This is about 70% gibberish to me right now, but you're right, it's a lot of science that I will be having a lot of fun learning about here. Time to go geek out on this stuff for a while.


Cheers.
_matt

[ve3wwg]

If you only get two books on analog electronics, I highly recommend these:

• The Art of Electronics - Paul Horowitz and Winfield Hill
• Microelectronic Circuits - Adel S. Sedra and Kenneth C. Smith

The first is hard to obtain at a decent price. But if you go to some place like abebooks.com and search for them used, sometimes the international editions can be purchased for a fraction of the usual price. The paper quality isn't as good but for hobby purposes I can justify those prices.  The 2nd book is very good also and presents a lot of derivations but otherwise covers the same material in a different way. I found that multiple approaches was helpful.

Don't worry too much about the age/edition of any book 1980+. For discrete electronics, it hasn't changed too much. The same is less true of linear and opamp ICs - go for more recent texts if you're looking at those (but money can be saved on a preceding edition).

I'm always looking for another old electronics textbook to read. I love books. It's a minor addiction.

Warren