On 28.11.2011 17:44, Lemon Tree wrote:
How difficult are transistor with an Ft of 5GHz to use? There are
some really cheap ones around these days like the 2sc3356 for
example. I'd like to some to make a regenerative receiver but I worry
that even after careful PCB layout I would still have problems.
Anyway I am starting a new website where I intend to present many
useful circuits. It is located at http://circuitprofile.scienceontheweb.net
At the moment there is only XOR circuit, but that XOR circuit should
be helpful for anyone making a huff and puff stabilized oscillator
circuit.

In most cases high Ft transistors are ok in lower frequency
applications, high Ft means they have current gain which is flat in
frequency and is still some near 1 at Ft.
Oscillations are probably one chance if low frequency design allows
feedback or is not bandwidth limited.

One way out of these is to add extra loss at high frequencies which
lowers gain and lessen probability for oscillations.
Resistor 10..47ohm at collector (base) or tiny ferrite bead has high
impedance at high frequencies but don't affect low frequency
amplification significantly.

In most cases these values need experiments if needed at all, as
instability depends gain used and feedback available.

Gl Kari B

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..Thanks for all the feedback. I am interested in the low base
diffusion capacitance aspect as this has important effects on a
regenerative receiver.