This has been the subject of extreme hair pulling for me for a while now and I've decided to give in and ask someone else about it.
An amplifier I've put together has the most interesting characteristic of oscillation that I've ever seen.
To start, it has a darlington output stage and operates in class B.
First off, with a turnoff resistor from base to base of the last main output transistors, the amplifier oscillates with a clean sine wave of about 200mV at 7MHz.
Second, with this resistor removed from the circuit, there is no oscillation during idle; it is when peaks of audio appear that the oscillation shows up. It appears on only the positive swings, and is proportional to power output. (This only occurs when loaded with a reactive load; if loaded resistively, the amplifier does not have this unusual 'peak' oscillation.)
It is more interesting because with the turnoff resistor in place, the 'peak' oscillation does not manifest itself, nor does the signal look different on the scope when the load is installed. It simply remains a good audio signal with 7MHz on it and doesn't change no matter what load I put on.
Does anyone possibly have any ideas at all???
Thanks
An amplifier I've put together has the most interesting characteristic of oscillation that I've ever seen.
To start, it has a darlington output stage and operates in class B.
First off, with a turnoff resistor from base to base of the last main output transistors, the amplifier oscillates with a clean sine wave of about 200mV at 7MHz.
Second, with this resistor removed from the circuit, there is no oscillation during idle; it is when peaks of audio appear that the oscillation shows up. It appears on only the positive swings, and is proportional to power output. (This only occurs when loaded with a reactive load; if loaded resistively, the amplifier does not have this unusual 'peak' oscillation.)
It is more interesting because with the turnoff resistor in place, the 'peak' oscillation does not manifest itself, nor does the signal look different on the scope when the load is installed. It simply remains a good audio signal with 7MHz on it and doesn't change no matter what load I put on.
Does anyone possibly have any ideas at all???
Thanks