J
Joerg
Guest
Joerg wrote:
Note to readers of this thread: The designators refer to the posted
LTSpice simulator file, not the displayed schematic.
http://cjh.polyplex.org/electronics/RFCascodeProbe/RFProbeCascodeAmp.asc
To make it run you also have to load the BF862 and BFR93A models into
Notepad each and then store in the same directory where the simulator
file is as BF862.mod and BFR93A.sub.
--
Regards, Joerg
http://www.analogconsultants.com/
Clifford Heath wrote:
On 22/11/14 03:04, Joerg wrote:
rickman wrote:
On 11/21/2014 6:21 AM, Clifford Heath wrote:
On 21/11/14 19:15, rickman wrote:
On 11/21/2014 4:10 AM, josephkk wrote:
On Fri, 21 Nov 2014 10:51:57 +1100, Clifford Heath
no.spam@please.net
wrote:
On 19/11/14 09:02, rickman wrote:
Thanks for helping me understand this.
Now I need to figure out why the Cascode design doesn't deliver a
higher
gain. It should, right? Ah, I just found it. He left the
output on
the drain to emitter leg and the output is now on the collector!
That's
much better giving some 75 dB at 60 kHz now.
Thanks, I think I learned a few things. Now if I can bias the
circuit
properly.
I've previously mentioned my 350MHz RF probe/amplifier, which uses a
cascode. If you want to explore it in LTSpice, the full design files
and
performance measurements are here:
http://cjh.polyplex.org/electronics/RFCascodeProbe/
The bandwidth is limited by the Ciss of the FET and the probe lead
inductance.
Something I don't understand, which someone more experienced
might be
able to explain, is why I had to add such a large source
resistance to
the input to kill the resonance between the input inductance and the
Ciss. Input inductance is approximately right for a pair of 4mm
probes,
and they won't have appreciable resistance. What kills this
resonance in
real life?
Clifford Heath.
I am not sure what difference it will make but i would try capacitors
from
the drain to ground to remove any AC gain to the drain which will
have
some interesting side effects.
Yes, that sounds right. If you are using a source follower why have a
resistor in the drain leg? The only time I have seen balanced
resistors
in the source and drain legs without bypass caps is when generating
differential signals. I would say in this case the drain resistor is
simply inappropriate and is setting up the input for the Miller
effect.
Either bypass or remove R11.
R11 was only there in the schematic file - you'll see it's a piece of
wire in the photos. It's not in the LTSpice file. So the drain is
directly bypassed to ground.
10pF Ciss and 20nH (for 20mm total) is about right for the measured
350MHz. That much is explained. But if you look at the LTSpice and
remove the extra capacitance C6, you get a massive resonant peak which
isn't there in the physical measurements. I played with source
resistance as a mechanism, but I still can't get the Spice to act like
the measured curves.
I don't know that much about the models or the actual circuits really.
Maybe Joerg could give an opinion or some of the others here who know
about such things?
Source or emitter followers can be like a sports cars on a sheet of ice,
even oscillate. Hard to see the layout but a few hints. Please don't
take that as an attempt to pick on the design, just suggestions:
1. R10 sould be via'd straight to GND at its pad and not looped all the
way to the input GND terminal.
2. R11: Just jumpering it does not really take it out in the RF world.
The drain would have to be via'd straight to 9V nd that 9V should be a
plane.
3. C7 does a "loop-de-loop" and that adds unwanted inductance.
4. Parts on RF probes should be 0402 or 0603 at the most. Otherwise
there is too much inductance. For bypass caps consider side-contact ones.
5. Grund plane: It is almost broken in the neck area. Needs to be
flooded.
6. Use a 4-layer board. 2-layer is very tough because you can't have a
supply plane.
Sincere thanks for your gems, Joerg. This is my first design above HF,
which you can probably see!
Don't sell yourself short, you got it to work and do what you wanted it
to do. That's what matters.
We all start into new turf at some point, usually all the time. When I
applied for my first job (medical ultrasound) I wondered what they do
with depth sounders on humans. Luckily they didn't ask me anything
medical, it would have been a major embarrassment.
I'm not unhappy with the measured performance (it's quite well-behaved
up to and around 350MHz) even though it could easily be improved as you
point out. The behaviour that still puzzles me is what LTSpice predicts
however.
Why does Spice predict such a hump (without C6), and what needs to be
added to the simulation to make it match reality? Maybe JimT has some
thoughts, or one of the other LTSpice gurus?
It is caused by resonance (L1 with the input capacitance) and you can
already see the hump at the output of the first stage J1. When you
remove that inductor the hump is gone. Same when you increase C8.
Note to readers of this thread: The designators refer to the posted
LTSpice simulator file, not the displayed schematic.
http://cjh.polyplex.org/electronics/RFCascodeProbe/RFProbeCascodeAmp.asc
To make it run you also have to load the BF862 and BFR93A models into
Notepad each and then store in the same directory where the simulator
file is as BF862.mod and BFR93A.sub.
--
Regards, Joerg
http://www.analogconsultants.com/