EMRFD Message Archive 8369
Message Date From Subject 8369 2013-03-19 21:51:55 lmeeny 7 MHz RF amplifier EMFRD Chapter 8 Figure 8.22 Hello,
I'm confused on how the amplifier has any net gain.
Can I make the following assumptions?
1. The antenna source impedance is 50 Ohms
2. The input impedance seen at mixer TUF-3 RF input is 50 Ohms.
if so then ...
The grounded gate configuration exhibits no current gain, currents through the source and drain are equal for all intents and purposes so the same current from the antenna flows to the mixer. Since both in and and see impedances of 50 Ohms why isn't the gain approximately 1?
I'm missing something.
8377 2013-03-20 03:40:57 kb1ckt Re: 7 MHz RF amplifier EMFRD Chapter 8 Figure 8.22 Been a while, but iirc a mixer is not 50 ohm as much as it whatever impedance is presented at the other port. At low frequencies (where the wires inside the mixer are short relative to wavelength) the mixer is being driven to hard on pretty much all the time--for one half of the LO signal the mixer output is in phase with the input signal, and the other half of the period the output is 180 degrees phase shifted. The diodes (or FET's) are driven to be hard on, and ideally are low impedance.
A common gate amplifier can be used as an impedance transformer, or as a 1:1 transformer. In this case (don't have the book in front of me) i believe it is being as an isolation amplifier, so as to prevent the antenna from radiating any reflected energy from the mixer--there should be very little gain going the opposite way through the amplifier. Due to the switching noise of the mixer (think about the transition times of the LO signal reversing phase) lots of spurious signals can be made.
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8380 2013-03-20 09:19:48 John Marshall Re: 7 MHz RF amplifier EMFRD Chapter 8 Figure 8.22 Ed,
Your assumptions are OK but you are ignoring the impedance transformations provided by the input and output filters. I simulated them using SimSmith and came up with about 200 ohms at the FET source and 600 ohms at the drain. Since the signal current in the drain is the same as in the source, the 3:1 impedance step-up will give a voltage gain of 3 and a power gain of 9. Not exactly the 10 dB shown in the block diagram, but "in the ballpark".
BTW, I used SimSmith not because it's better than the tools provided with EMRFD but because it runs seamlessly
8381 2013-03-20 11:36:16 PY8ELO - Silva Mini Circuits XFRM TC1-1-13MG2+ Hi all,
I am looking for the Mini Circuits XFRM TC1-1-13MG2+ and I has found it
on the manufacturer web site for a cheap price but the shipment to
Brazil is very expensive.
I was thinking about purchase 10 pcs and theminum order is 20pcs.
Can somebody send me few pcs for a reasonable value please?
Thanks in advance and Best 73's,
Brazilian Ham Radio - ARRL Member
Grid Locator: GI25ar
Address: POBOX 151 CEP 68180030
ITAITUBA - PA - BRAZIL
"The quieter You become the more You are able to hear"
8383 2013-03-21 20:10:54 Todd F. Carney / ... Re: 7 MHz RF amplifier EMFRD Chapter 8 Figure 8.22 8384 2013-03-21 20:10:55 lmeeny Re: 7 MHz RF amplifier EMFRD Chapter 8 Figure 8.22 John,
Thanks for the detailed reply. I've been using LTSpice and now that I know my simulation's lied to me perhaps I'll learn something new.
Again, thank you,