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.
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
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.
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 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.
> 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.
N7ZWY / WDX3HLB
Senior Member IEEE