I just reread the webpage notes, he say T2 collector should be between 6 and 7 volts, and the adjustment is the 1.2K resistor.
Yes, I see a voltage gain of 20 or 10 if terminated with 50 ohms. But why did the designer say gain of 17? Is there some alteration caused by the emitter feedback? I think that would increase the gain. (slightly) Mikek
Ahh, ya!
Ya, start see a drop at a little over 7 Mhz, still not 3db at 10 Mhz, although I didn't measure it.
Yeah, I was wondering about some of the things in that circuit.
In particular, there are several places (including the emitter of the fast transistor) in which the circuit places two caps in parallel... a large value (e.g. 10 uF electrolytic) and a small value (something in the nF range). Although this is a not-uncommon practice ("use a big cap to handle the low frequencies, and bypass around it for the high frequencies") I've read several people say that it can actually be a somewhat dangerous practice. The big ('lytic) cap has a good deal of parasitic inductance in its foil and leads, and (reportedly) this can form a resonant tank circuit with the small-value capacitor.
Putting a parallel-resonant tank in between a fast transistor's base and emitter seems like a certain recipe for instability.
Since even the author of the circuit warned that the fast transistor might oscillate, and that a slower one could/should be substituted if that were to occur, this would seem to be the right thing to do at this point. Cleaning up the parasitic tank circuit (maybe replacing the two existing caps with a single part) might also help.
--
Dave Platt AE6EO
Friends of Jade Warrior home page: http://www.radagast.org/jade-warrior
I do _not_ wish to receive unsolicited commercial email, and I will
boycott any company which has the gall to send me such ads!
Yes, but leave a little headroom for Vce as well. A transistor pulled too close to saturation will become sluggish during that phase.
Seems like the web site is Dutch. That's a high tax country, maybe they taxed away some of the gain ;-)
Not really, the emitter mostly follows the base unless it pegs somewhere.
-- Regards, Joerg http://www.analogconsultants.com/ "gmail" domain blocked because of excessive spam. Use another domain or send PM.
The right 47ohms is in the DC path and that (plus the input divider) sets the DC levels. The left 47ohms is in parallel only for AC so the two together and the 470ohms determine AC gain.
I wish people would use designators in such schematics.
Ah, loading!
I must have not had my morning coffee yet. You are right, there's the bias resistors of the following stage and both sides go in parallel, RF-wise. That in parallel again to 470ohms.
-- Regards, Joerg http://www.analogconsultants.com/ "gmail" domain blocked because of excessive spam. Use another domain or send PM.
Ok, guys, I tried soldering a piece of sheet brass across the open area on my board. It seemed to work, then I thought I'd lift a resistor and cut a new brass to cover the area completely. Then it oscillated! I took the brass sheet off. I removed the 5Ghz transistor installed a 2n3904 and no oscillation. The voltage gain was 13, I want 17, I changed the collector resistor from 470 ohms to 620 ohms and the 1.2k on the base to 1k. Now the voltage gain is 17. The voltage on the collector is 6.6 volts.I get about 7vpp output before distortion, and the 3db point is about 8 Mhz, This is fine, I'm only interested in the AMBCB. Now, on to the FET preamp. Thanks, Mikek
ElectronDepot website is not affiliated with any of the manufacturers or service providers discussed here. All logos and trade names are the property of their respective owners.