Li Ang wrote:
RF pnp transistor is harder to get. I would like the front end works
at 300MHz.
My questions:
1) why the difference of DC bias of the 2 NPN matters? I thought only the
frequency part is useful to a counter, amplitude information is useless
right?
You want the circuit to switch near the mid-point of the input sine
wave, and at exactly the same place every time. How you bias the
transistors determines how well this is accomplished.
You also want the output to switch fast and cleanly between a low
voltage very near 0v ("ground") to a high voltage very near 3v (Vcc,
logic high). An NPN cannot do that, biased the way that you have
them connected (the emitter of the output transistor Q301 can only
pull the output down to a little less than 1v due to R315, which may
sort of work but is not a proper way to run 3v logic). This
operation also saturates Q301, which is bad for performance. See
simulated results below.
In order for an NPN to provide a useful output for 3v logic, (i) its
emitter must be grounded, and (ii) it must either be run into
saturation or use a Baker clamp. Running the transistor into
saturation must be avoided, particularly if you want to reach 300MHz,
and a Baker clamp raises the "logic low" output voltage to >0.5v (not
a good thing with 3v logic). So, it is very much better to use a PNP
differential pair. For a 300MHz circuit, I would use BFT93 (and even
that barely gets you to 300MHz).
2) what's is the C4 in your circuit for?
C4 makes Q1 and Q2 a differential (emitter-coupled) pair at RF
frequencies, but not at DC. So, the circuit has no gain at DC and
therefore the DC errors between Q1 and Q2 cause much less output
error than they would if the emitters were connected directly together.
3) If the noise is more important than the gain, what kind of transistor
should I choose? The Ft near 300MHz ones(BFS17, 2SC9018) or Ft far beyond
300MHz ones(BFP420, BFP183,BFR93) ?
Far beyond. The Ft is the frequency where a transistor completely
runs out of gain. You want to operate at a much lower frequency
where the transistor still has substantial gain, particularly with
fast RF transistors, which generally have much lower DC hfe than
general-purpose transistors like 3904 and 3906. Note that the
simulation of the circuit you published (simulated results below)
barely works at even 20MHz. As I noted above, even the BFT93 barely
gets you to 300MHz with a 1Vrms input.
Best regards,
Charles
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