EMRFD Message Archive 6898

Message	Date	From	Subject
6898	2011-11-26 14:10:03	Ray	Feedback amplifiers in series
EMRFD Fig 2.57 shows a feedback amplifier with 20.5 dB gain. The text says it is a good match for 50 ohms input and output. If I wanted to put two of them in series for 40 dB gain, should I put a 50 ohm resistor to ground between the output of amp #1 and amp #2? I would terminate amp #2 in 50 ohms. According to the associated text on feedback amplifiers this should make the input of amp #2 a good match for 50 ohms. But I'm not sure whether the input is looking for a 50 ohm source to be a good match of if the impedance as seen at the input is 50 ohms. Ray AB7HE
6899	2011-11-26 15:33:58	Shawn Upton	Re: Feedback amplifiers in series
If amp #2 is terminated into 50ohms, then the input of amp #2 will look like 50ohms.  And amp #1 will "see" 50ohm loading.  And since it sees 50ohms as a load, then the input of amp #1 will look like 50ohm also.  No need for a terminated resistor between the two stages. Shawn Upton, KB1CKT NAQCC 4723 ________________________________
6900	2011-11-26 16:29:58	Ray	Re: Feedback amplifiers in series
Shawn, Thanks for clarifying that. I thought that was the case but I wanted to be sure. Ray AB7HE
6901	2011-11-26 16:34:59	Thomas S. Knutsen	Re: Feedback amplifiers in series
What frequency are you operating on? I would have choosen to include an circulator or perhaps an attenuator between the amplifiers. Since circulators get large
6905	2011-11-26 18:28:12	Ray	Re: Feedback amplifiers in series
Thomas, I am operating at 10 MHz. I actually have 3 of the amplifiers and was going to try to series them to get up to 60 dB gain. If I have oscillati
6906	2011-11-26 18:42:24	Thomas S. Knutsen	Re: Feedback amplifiers in series
I would say to build them in separate screened boxes, 60dB is a lot of gain! I try to limit the amount of avaible gain in one box to 20-30dB
6919	2011-11-27 09:14:21	Chris Trask	Re: Feedback amplifiers in series
> > I am operating at 10 MHz. I actually have 3 of the > amplifiers and was going to try to series them to get > up to 60 dB gain. If I have oscillation problems I will > try adding the 3 dB pads between amplifiers. > I was just about to jump in and mention that. I have a 30dB LNA that I use for NF measurements, and it is made with a cascade of five 6dB Norton feedback amplifiers. Each of them has a resistor going from the transistor collector to the supply voltage. Can't remember the value at the moment. Chris Trask N7ZWY / WDX3HLB Senior Member IEEE http://www.home.earthlink.net/~christrask/