Okay the BF998 looks like a possibility. I'm not really interested in using one of the gates for AGC, so should I just use both gates in parallel or tie one to a fixed bias voltage?Perhaps there are plenty of BJT's with NF below at 1dB at 15MHz, but you can try using the dual-gate MOSFET BF998, which from my knowledge has about 0.7dB NF at HF frequencies (up to 30MHz).
**broken link removed**
Huh, never thought of SiGe. The only thing I can see available on digikey is the BFP 640, is that a good candidate?If you can find SiGe, use it no doubt... ( Infineon..)
Otherwise use BJT but never use GaAs or compound semicon. discretes..They are very noisy below 100-150MHz
Huh, never thought of SiGe. The only thing I can see available on digikey is the BFP 640, is that a good candidate?
Wow, that would be great. What kind of noise impedances should I expect to need for SiGe? I'm having trouble finding relevant literature on the topic (for low frequency anyways).Yes, it may be used..You'll obtain probably 0.15dB NF or less at those frquencies.
It's good enough..
My application isn't communications; but rather NMR coil reception. So throwing the whole thing in a shield is no problem for me, fortunately.Not sure what your application is but if for communication applications at thoughs frequencies the background noise will swamp out a 1 db noise figure receiver so the noise figure of the receiver does not need to be that low.
1 db NF is noise temp of 78 degs K, typical atmospherics and interference in the 7-15 MHz range is 500-1500 degs K.
Intermod performance and dynamic range are more important then NF in this RF comm range.
Only high frequency transistors seem to be specified for low noise. I've looked for lower frequency parts, but they don't tend to specify any noise characteristics.Hi,
What surprise me is that you are using high frequency transistors, for a low frequency application, i.e. for about 20MHz. Maybe you can try using just low noise transistors.
15dB is adequate, though 20dB would be nice. As for putting an attenuator on the output, that certainly wouldn't help. I'm concerned about my SNR, not the actual noise on the output.The other thing is how much amplification do you really need, i.e. if you have a 15 to 17 dB amplification, why don't you try to put an attenuator on the output, this will lower the signal, but will also lower the noise.
Yeah, I found this back when I was designing the thing and it was somewhat helpful for simulating noise figures, but ultimately I couldn't replicate its results. It always gave absurdly optimistic NF (like 0.3dB). But it was useful for biasing and stability analysis.I have also a small comment about the simulation software. I think you can look at this ( LTspice is currently free ) :
I tried a couple bipolar transistors, but they always had stability issues, even when I tried cascoding. The BF998 on the other hand never had any issues and gave acceptable results.I would either use a silicon bipolar, or use an instrumentation op amp with a low input noise voltage.
It's meant for use in small NMR receivers.May I just ask, what is the current application of this LNA ? Radio astronomy ?
Sure, that bypassing scheme would probably work too, though you'd still need some impedance on the drain.Why not using a different schematic for the drain :
+12V -> R1 -> 2.2uF -> L3 -> 5nF -> Q1 drain
[/quote]It certainly will start to increase at some point. In general I've seen that most transistors have a bathtub shaped curve of NF vs frequency. But that 2.8dB number at 45MHz seems way too high... I know that when I was first playing with it, I had it tuned to 20MHz at one point and its NF was around 1.5dB.Edited:
I just did a quick check with datasheet of BF998, it is given that NF at 45 MHz is 2.8 dB, how is it possible that you measure 0.9 dB at 10 MHz? I would expect that with lowering the frequency the NF goes high.
Datasheet from Infineon : **broken link removed**
This is meant for small scale, low field NMR (not imaging), using permanent magnets, so no cryogens available. And even in clinical superconducting magnet scanners, none of the electronics (except for those monitoring the cryogens) are supercooled (otherwise we'd never be able to tinker with them without quenching the magnet).NMR receiver, you mean for MRI application ?
Why not to cool down the transistor? Maybe you can try with cooling spray, just cool down as in the circuit, let's say to -40 degC and measure NF again. As far as I know the nuclear magnetic resonance, cooling is widely used in those machines.
The other possible option is vacuum tube, maybe there is a ready application for LNA with valve for NMR ?
Yeah, I've seen this done and might try it myself if I run into a wall and still need lower NF.Might be worth trying soldering another BF998 in parallel to the one in your circuit. At VHF the NF is further reduced using parallel transistors. Just another idea to try lowering the NF...
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