Re: gEDA-user: BC547 vs. 2N3904

[Wojciech Kazubski <wk@xxxxxxxxxxxxx>]

> Do you like more BC547C for higher frequency applications or 2N3904?
> I once did a simulation for some Ronja differential limiting amplifier
> which showed that BC547C is a bit better than 2N3904 because, despite
> it being lazy, it has higher amplification.

Yo can also use BF240 (SMD variant BF840) if you want to amplify up to some 
50MHz or even BFR91 (SMD=> BFR93) if faster transistors are needed. Those 
transistors cannot be used for saturated switching. For such case try old 
2N2369 or its derivatives (plastic or SMD). But faster the transistor, more 
likely it oscillates.
>
> Is it possible to get rid of oscillating transistors by replacing the
> hand-soldered airwire with a SMD on a 2-sided PCB with a ground-plane?

Very recommended.  Use shortest possible connections to ground and try not to 
cut the ground plane with traces, especially long.
>
> What is the major cause of oscillations in high-frequency amplifiers?
> Inductance of wires?

If your circuit oscillates well above the operating frequency it is mainly due 
to inductance of wires plus parasitic capacitances inside the transistor. The 
input-output capacitance is major cause of instability of resonant apmlifiers 
(RF or IF) In this case oscillations are at or near operating frequency. 
Sometimes amplifiers oscillate on LF due to bad supply decoupling.
>
Wojciech Kazubski


_______________________________________________
geda-user mailing list
geda-user@xxxxxxxxxxxxxx
http://www.seul.org/cgi-bin/mailman/listinfo/geda-user