impedance for idiots?

[CynicalMan]

Yes, that was very comprehensible. Thank you.   It is interesting when you actually do the math at each end of the spectrum.

Now, on the input impedance, if we change your circuit to use a .01 uf instead of a .1 uf, we have a much more dramatic swing of capacitive reactance ranging from 795K at 20 Hz vs. 795 ohms at 20 kHz.   In fact, I have noticed doing the math that a very small value capacitor will result in a HUGE swing in the capacitive reactance ranging from 20 Hz to 20 kHz, but the larger the cap will result in  much less of a differential.  For example, if we now use a 1000 pf capacitor the reactance of the cap ranges from  7.9M at 2 Hz down to a measly 7.95 ohms at 20 kHz.  

Granted, in this particular circuit the swing large or small of the cap might not make much of a difference, but in other circuits it might.

Well the reactance is proportional to the reciprocal of the frequency. In other words, if you multiply the frequency by 1000, the reactance is divided by 1000. This is true for any capacitance. So the reactance does vary more with lower value capacitors, but it always varies by the same factor. This is another electronics way of thought: it's often more useful to think about the ratio between numbers, not their difference.

You're right, it might matter. So these are the things you have to look at: the output impedance from the previous stage, the frequency range that you want to pass, and the input impedance of your circuit. With that information, you should be able to choose an input capacitor. Once you get going in design, you'll start just using rules of thumb. For example, there are very few effects where you can't just use a 0.1u or 1u input capacitor without problems. Coupling capacitors between stages sometimes need more thought, but you just use the same design method.

1000pf is 10nF so it's NOT 7.95R but 7.95k .

1000pF is 1nF, but you're right about the reactance.

> The output impedance of the TL071 is around 160 IIRC

The butt-naked output of TL072 is hundreds of ohms, yes. But in a NFB amplifier connection this is reduced by the excess gain lost to NFB. At low guitar frequency the TL072's gain is 100,000 roughly. Your circuit is unity-gain, so the output is say 160/100,000 or 0.002 ohms. At high guitar harmonic the TL072's gain has fallen to roughly 1,000. 160/1,000= 0.2 ohms. The small-signal output impedance of all the opamps under NFB at most reasonable gains over the audio band may be penciled as "well under 10 ohms". Since all useful loads are much-much greater than 10 ohms, this may be ignored for nearly all practical purposes.

Aaah.. that makes sense. Never thought of it in that way. Thanks for the correction.

[ntblade]

Remember that the input capacitor shown in the buffer circuit is really just there as a AC coupling (DC blocking) capacitor.  It couples or passes the frequencies of interest to us and blocks an DC that might be present on the input signal as well as preventing the 1/2 supply voltage at the voltage divider "going back out" to the previous stage which, in our case may be a guitar pickup or effect.

The circuit shown is a unity gain first order high pass filter whose rolloff frequency is given by:

1/(2*PI*R*C) where R is the parallel combination of of the voltage divider resistors and is

1/ (2*3.14*500E3*0.1E-6)
=3.18Hz.

Now this is plenty low for guitar and bass and with the two 1M resistors still give a decent high input impedance.  In this case, the input impedance of the op=amp is so high compared to the CR circuit that we can ignore it.

It's often easier to look at circuit examples where the power is supplied from a split supply as there's no 1/2 power reference to confuse things!

Hope I've not confused thigs!