class c rf amp with bfr93a

ameters

ced kink

t two or

ley

w

s.

=A0 =A0(Richard Feynman)

Thanks for all the help guys. I was not expecting that much info , but it's great. Gives me a good starting point. I'm only using a BFR93 , because that's wot I got. Maybe I'll spend some time and look at something else. Thanks again to all. Cheers Rob

=A0 =A0(Richard Feynman)

Hi there. Where did you get the input impadance of 300+j76 ohms from. Can you get it from LTSpice , or do you have a datasheet that shows that. I can't find it on the datasheet that I have. They do have a S11 plot , but it does show anything like that. Maybe I'm just being a dumbo. A distinct possability :0) Same for the collector impedance of 700 ohms.How do you get it?

Cheers Rob

kink

or

Can't access a.b.s.e. :-(

Falk Willberg from a German NG has set up his own news server with binary access but that would require everyone who wants to look to have the access info (which Falk freely gives out to anyone who requests it).

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On Mon, 30 Aug 2010 06:29:37 -0700, neddie wrote:

From an LTSpice simulation of the circuit, using an .ac analysis with a .net directive.

The S-param plots are for the device alone. The model gets near(ish) the measured values in the datasheet (assuming they *are* measured!), but I'd say no cigar.

it?

It's not the looking-backwards collector impedance, that's more or less irrelevant, despite what some people would have you believe. Forget maximum power transfer theorem (Joerg has anecdotes about that, and his onetime Prof).

700 ohms is the load that happens to give about 50mW in this case, using a Vcc of 6V.

The usual rule of thumb is:

Rl=Vcc^2/(2*Pout), for a class C amplifier.

This can be out by quite a lot, depending on other factors, but it's a start. In fact, it gives 360 ohms, which gives about 100mW in this case, so I throttled it back.

The design procedure I use is this:

Select the collector feed inductor to resonate with the device output capacitance.

Determine what load resistance will result in the desired power.

Design a network to match that resistance to your desired load, with your desired Q, usually about 10, any more and tuning gets difficult. Much less, and harmonics become a problem.

Unfortunately, stepping up 200 ohms to 700 ohms at Q=10, using a simple RL network results in practically unrealizable capacitance values, so my circuit isn't much use outside of a theoretical simulation.

Why 200 ohms, BTW?

Here's the circuit, for what it's worth (not much).

Version 4 SHEET 1 1300 2476 WIRE 48 144 -256 144 WIRE -256 192 -256 144 WIRE 48 192 48 144 WIRE -256 304 -256 272 WIRE -224 304 -256 304 WIRE -96 304 -160 304 WIRE -48 304 -96 304 WIRE 64 304 32 304 WIRE 192 304 128 304 WIRE -256 320 -256 304 WIRE 192 320 192 304 WIRE -96 352 -96 304 WIRE -464 368 -672 368 WIRE -352 368 -400 368 WIRE -320 368 -352 368 WIRE -352 384 -352 368 WIRE -672 480 -672 448 WIRE -368 480 -672 480 WIRE -352 480 -352 464 WIRE -352 480 -368 480 WIRE -256 480 -256 416 WIRE -256 480 -352 480 WIRE -96 480 -96 432 WIRE -96 480 -256 480 WIRE 48 480 48 272 WIRE 48 480 -96 480 WIRE 192 480 192 400 WIRE 192 480 48 480 WIRE -368 496 -368 480 FLAG -368 496 0 FLAG 48 480 0 SYMBOL voltage -672 352 R0 WINDOW 3 32 84 Left 0 WINDOW 123 -97 53 Left 0 WINDOW 39 24 44 Left 0 SYMATTR Value SINE(0 3 433meg) SYMATTR Value2 AC 1 SYMATTR SpiceLine Rser=50 SYMATTR InstName V1 SYMBOL ind -272 176 R0 SYMATTR InstName L1 SYMATTR Value 190n SYMBOL voltage 48 176 R0 SYMATTR InstName V2 SYMATTR Value 6 SYMBOL npn2 -320 320 R0 SYMATTR InstName Q1 SYMATTR Value BFR93A SYMATTR Prefix X SYMBOL ind -368 368 R0 SYMATTR InstName L2 SYMATTR Value 47n SYMBOL res 176 304 R0 SYMATTR InstName R1 SYMATTR Value 200 SYMBOL ind -64 288 M90 WINDOW 0 5 56 VBottom 0 WINDOW 3 32 56 VTop 0 SYMATTR InstName L3 SYMATTR Value 597.5n SYMBOL cap 128 288 R90 WINDOW 0 0 32 VBottom 0 WINDOW 3 32 32 VTop 0 SYMATTR InstName C1 SYMATTR Value 0.18p SYMBOL ind -112 336 R0 SYMATTR InstName L4 SYMATTR Value 137.5n SYMBOL cap -224 288 M90 WINDOW 0 0 32 VBottom 0 WINDOW 3 32 32 VTop 0 SYMATTR InstName C2 SYMATTR Value .8p SYMBOL cap -400 352 R90 WINDOW 0 0 32 VBottom 0 WINDOW 3 32 32 VTop 0 SYMATTR InstName C3 SYMATTR Value 1n TEXT -552 280 Left 0 !.tran 0 60n 10n 100p TEXT -160 536 Left 0 !.lib BFR93A.sub TEXT -408 536 Left 0 !;.net I(R1) V1 TEXT -872 280 Left 0 !;ac dec 1000 350e6 550e6

--
"For a successful technology, reality must take precedence 
over public relations, for nature cannot be fooled."
                                       (Richard Feynman)

Just a word of caution here: Don't do this for a design that has to go into mass production. The capacitive device parameters are lot dependent and vary with temperature.

[...]

--
Regards, Joerg

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a

RL

=A0 =A0(Richard Feynman)

The only reason I've gor a load of 200 ohms is... The anternna is going to be a piece of track on the pcb. Analysing it in Sonnet shows it has an impedance of about 200 ohms at

433Mhz I can change the track dimensions to change the impedance to something else. I actually only need 10mw radiated ( to keep within legal limits) but pcb antennas are not very efficient. That is why I'm aiming for higher powers. I also want to have some filtering to kill 868Mhz and above harmonics. Cheers Rob

Are you sure that it is 200 ohms _resistive_ at that frequency, i.e the reactance tuned out ?

With traditional antennas, simple monopoles are about 35-50 ohms, dipoles about 60-75 ohms and folded dipoles 240-300 ohms.

Since the amplifier is unbalanced and the antenna most likely balanced, it would make sense to install a balun between the transmitter and antenna for proper antenna performance and predictable radiation pattern.

With a 4:1 balun and 200 ohm antenna, the transmitter would be loaded by 50 ohms.

Fortunately. there's always a bit of wiggle room, plus, my designs are never more than a few hundred off.

I leave mass produced stuff to those with the relevant mindset (ITCFWI);-)

--
"For a successful technology, reality must take precedence 
over public relations, for nature cannot be fooled."
                                       (Richard Feynman)

s/RL/LC

-- "For a successful technology, reality must take precedence over public relations, for nature cannot be fooled." (Richard Feynman)

S-parameters

kink

or

Cool. Thanks Falk.

^^^^^^ Not found. Please persplain.

a

RL

=A0 =A0(Richard Feynman)

Hi there. With regards to the ltspice cct. What does the .net directive give you. It just produces a netlist , or am I missing something. I've managed to get an impedance plot using the ac analysis and plotting V/I. That gives me the impedance in polar form , which is great. I sometimes get a odd number like when I convert a number like

200 < -170 ( < , I can't draw the angel sign) , gives me -196 -j34.7 Somewhere in the back of my mind , from years ago , I remember subtracting the angle (-170 in this cage) from 180degrees , if it is bigger than 90, or something like that. That gives me 196 - j34. So I assume I'm on the right track. I've done some searching on polar to rectangular conversion , but I can't seem to find anything about this>? I must be missing something again :0(

I did a new track layout and remoddeled it in Sonnet. It gives me an impedance of 5.5 +j28 ohms for the impedance at 433Mhz. Much easier to match. Q is only about 4 , but it's a start. This rf stuff is tough , especially if it is not something you do often :0(

S-parameters

kink

two or

If you want access you can request it from Falk at:

nntp at consult42 dot com

--
Regards, Joerg

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Use another domain or send PM.

If The Cap Fits, Wear It

;-)

--
"For a successful technology, reality must take precedence 
over public relations, for nature cannot be fooled."
                                       (Richard Feynman)

First, you need to uncomment the .net card, and uncomment the .ac card. You'll get a barf about two analyses at once, so pick the .ac one. Run the analysis, then from the plot screen, take a look at "Visible Traces". You'll see that there are a few more than usual for an .ac analysis, nice things like S-params, Y-params, and Z-params. You plot these as you would anything else, using whatever format suits best - Bode, Nyquist, or Cartesian.

Now, go RTFM to see how it all works, then load up /examples/Educational/S-param.asc to get a feel for it.

How many angles can dance on the head of a pin ;-)

I find it difficult to believe that someone who has Sonnet, doesn't have a calculator that does polar/rectangular conversions :-)

Much harder to match. Notwitstanding that losses in inductors and tracks are starting to look significant, stepping up 5 ohms to the value you'll need for 10mW at a reasonable Q is probably not feasible. I'd go with Joerg's idea, and forget class C, use straight class A (who cares about efficiency, it's only 10mW?). That also gives you the option of setting output power by adjusting drive with a couple of resistors.

Not high enuf. Harmonics will not be nice. You'll need filtering, I was getting -20dB on second harmonic with Q~~10. I don't know the spec. but at

20dB down on 10mW (100uW), you might get away without additional filtering.

--
"For a successful technology, reality must take precedence 
over public relations, for nature cannot be fooled."
                                       (Richard Feynman)

That would be for the engineering side of business. I once heard a similar word of wisdom for the business side of business, and almost spewed my coffee all over the place: Find a parade and get in front of it :-)

--
Regards, Joerg

http://www.analogconsultants.com/

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S-parameters

kink

two or

I get a.b.s.e natively on my Astraweb account. But i gotta respect Falk for providing the service.

Thranx