Hi Guys,
Bypass capacitors have always been somewhat of a mystery to me so I thought I'd do a little research. All I know is I often have trouble if I don't add them. Here are a few bits of info I found that I thought were interesting. Hopefully I'm not hijacking your thread too much but I think it might be related.
I found a few references on the inductance of a loop of wire such as:
Estimating wire & loop inductance: Rule of Thumb #15 | EDN
The rule-of-thumb seems to be around 1uH per meter of wire.
So for example if your wiring from your supply to your clamp circuit is 20inches (each way) then the inductance in the wiring would be about 1uH.
The formula of impedance, X, of an inductor at a given frequency, F, is:
X = 2 Pi F L
For sake of discussion an oscillation at 1MHz would see ~6 ohms
A current of +/-1 amp at 1 MHz would cause the supply voltage to change +/- 6V !!
It only takes the Supply Voltage to drop by ~0.7V for the circuit to turn on the clamping transistor.
I also found this article on Ceramic vs Electrolytic Caps. They don't seem to be as much different as I expected. From this eetimes article
Power Tip 51: Be aware of capacitor parasitics | EE Times
There is this chart:
At 1MHz the ceramic capacitor has an effective resistance of ~0.008 ohms
At 1MHz the Electrolytic has an effective resistance of ~0.03 ohms
Either one would be much better than the 6 ohms without any capacitor.
Actually because of the bigger value the Electrolytic is more effective than the Ceramic until 300KHz
And that chart is for a 22uF Ceramic. The 0.1uF I recommended still acts properly as a capacitor beyond 1MHz but its low value makes its impedance ~1 ohm at 1MHz. So a big Electrolytic would be more effective. The 0.1uF Ceramic is more appropriate for killing 10MHz oscillations. Here is a plot from an AVX 0.1uF Ceramic Capacitor:
Its hard to say if the circuit is breaking into oscillation what the frequency would be without scoping it. I believe the standard approach to cover the entire frequency band is to use a big Electrolytic + 10uF Tantalum + 0.1uF Ceramic.
I'm not sure I'm being of much help
Regards