I need to mute (soft mute, no clicks and pops) an audio signal while routing it using a mechanical relay. I use a 2N5089 high gain NPN with a grounded emitter, collector to the signal output and the base is hooked up through a 4k resistor to a micro-controller pin so when fed 5V the current to the base is 1.25mA which is just above the saturation current. To make the transient long I added a 33uF cap to ground from the transistor's base. I hooked this up to a scope while altering the voltage at the pin that goes to the base. For the purpose of clearly seeing the rising and falling edge I connected a DC 1V (through a resistor) to the collector. The fall time of that voltage is 1.6 - 1.8msec and the raise time is about 11ms. I must admit I don't fully understand what goes on here. How come the fall time is so short (this is when the transistor switches ON) even after adding a 33uF cap from base to ground? Tau is 0.132 sec !!!
Any help much appreciated!