I was reading on Capacitive Coupling a bit and become confused when reading the Parasistic section, specifically regarding two claims.
If a high-gain amplifier's output capacitively couples to its input it often becomes an electronic oscillator.
I imagine this to be similar to the situation when you have input from a microphone and output through an amplifier. The output is then coupled back into the microphone until the output of the amplifer rails, clips, or whatever happens that may cause the loud ringing (what is happening exactly?).
My only problem with the electrical example is regarding how the system may couple its output back into its input. Is this a simple case of simple feedback? How does simple feedback relate to capacitive coupling?
It seems that the output would need some ripple voltage to begin with that would be amplified. Without a clear example, I imagine an unstable amplifier with a small margin of ringing to begin with, but through feedback eventually oscillates much more significantly. Is this correct? As in:
One rule of thumb says that drivers should be able to drive 25 pF of capacitance which allows for PCB traces up to 0.30 meters.
What does this driving requirement have at all to do with capacitive coupling? To give some context, the article was talking about the parasitic coupling that exists between PCB traces, breadboard metal strips, and PCB traces to ground. I imagine that type of coupling to exist like this between traces:
And this between a trace and ground:
What exactly would "..drivers should be able to drive 25 pF of capacitance.." be defined as in terms of specifications for a driver, and how can one verify compliance?
Best Answer
An 1 Mohm 0805 resistor will have self capacitance of about 0.2pF and this, when applied in the feedback loop of an op-amp will reduce the gain from the 3dB point of 795 kHz. This means that the transimpedance amplifier you are designing will not work well at 10 MHz because there is too much capacitance - and that's just from a single surface mount resistor - imagine what capacitance there is between tracks and if this is not properly catered for by good PCB design, you'll be lucky to get 100 kHz flat operation from the above circuit.
The above is the case of negative feedback causing a signal to become smaller on the output due to parasitic capacitance.
The minutest noise will be enough to trigger oscillation if the feedback is positive. Noise is present in all components at temperatures above absolute zero.
Regards driving tracks having capacitance this is not directly related to the possible cause of oscillations and additionally the rule of thumb in your question doesn't take into account many, many factors such as the power capabilities of the driver, and the track dimensions and whether the track is terminated. At low frequencies there is no real rule at all.