Hi Charles, In my circuit, the VCC is 5v. I've noticed my bias and emitter resistor is something need to be changed. I will play with the resistors and see if it improves. Thanks.
2014-12-27 6:42 GMT+08:00 Charles Steinmetz <csteinm...@yandex.com>: > 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 > > > _______________________________________________ > time-nuts mailing list -- time-nuts@febo.com > To unsubscribe, go to > https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts > and follow the instructions there. > _______________________________________________ time-nuts mailing list -- time-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there.