28-06-2017, 05:43 PM
(28-06-2017, 04:36 PM)Nowhere-Man Wrote: This is changing the base bias of a GE transistor and fitting a Silicon. This would make life easier for me as I got kind of tired chasing around after GE transistors and being directed towards E-bay. Let's take the example I posted earlier in the thread of a 3 volt supply through a voltage divider. Let's assume we have a PNP GE transistor that needs 0.3 volt to "kick in". We located a suitable silicon equivalent but that requires 0.6 volt. Thus, by my reckoning, substitution without thought would mean the base of a silicon transistor staying idle because only 0.3 volt is reaching the base. What about altering the voltage divider? Or is there some known loophole to enable the base bias to be adjusted?
A lot of the time, there's nothing to change. Many circuits designed for germanium work just fine with silicon replacements.
If we'd stuck with the tutorial a couple of months back, we would have got to that. In short, the circuit with 4 resistors is known as "fully stabilised bias", meaning that the DC conditions don't change radically. To prove it, just build it. Or, re-run the exercise, but substitute 0.3 for 0.6V.
Just like designing for a particular value of current gain (Beta, or Hfe) is bad practice, it's also bad practice to require a particular value of VBE. Some circuits do - and might require matched or selected transistors - but you're not likely to encounter those in a transistor radio.
Remember; 0.3 and 0.6 are just simplified assumptions - in reality VBE depends on base current. Just like the voltage dropped across a diode depends on current...
There are places where things are more critical, but in a typical transistor radio, we're only talking about the output stage where you might expect to re-bias.
