Maker Pro
Maker Pro

transistors - emitters connected, caps & resistors ??



Jan 1, 1970
Hi All,

I've been investing how this remote my father made back in the late
1970's works - am trying to get a picture up of the circuit board etc.
etc, and am slowly piecing together concepts. It is a RF transmitter
operating on 27.445 Mhz. The idea was the remote could turn on two
mains lights in the house before coming inside (i.e. coming at night
time etc.). It may be a run-of-the-mill thing, I dont know - but I'm
learning about electronics now too so its good to build up basic
concepts, especially with transistors to amplify signals, crystals etc.

In the meantime though - there are four transistors. Three are C828 NPN
- from what I read lower power, general purpose transistors, and one
A564 PNP - which is connected to the inductor.

Anyway my question relates to the three C828 transistors. On the first,
a resistor is connected to the base, and one to the connector
(obviously to establish correct bias?). With the second and third, the
collector's and bases both have resistors biasing them, but also
capacitors from the collector (some small pF ones and one 150Z one,
ceramic caps). Lastly, all the emitters go into the next (i.e. copper
tracks link one to the other, to the other).

Questions - 1) Where amplification is needed such as this situation
(i.e. an RF transmitter), does the base and collector always need to be
biased with resistors?
2) What is the purpose of the capacitors - do they filter the
collector-emitter current in some way ????
3) Are the emitters connected so that essentially, the last one would
have a current equal to all the amplification (i.e. all the
amplification of the other added to the last one)?

Sorry if I sound newbie - am learning fast!


Larry Oravetz

Jan 1, 1970
You might be looking at a bandpass filter to eliminate spurs from getting
too large as to cause problems with the FCC