I need a good text on low noise amplifier design ASAP

I'm struggling with an amplifier design for a radio front end.

Ham 600 meter band (475kHz)

The antenna is a ferrite loop (can't change that) with a resonating capacitor on the primary, and a few turns on the secondary, so I have a pretty low input impedance.

I have several designs in spice, with common emitter and common base configurations, but I am struggling to get the noise below 300nV/Hz^1/2. I need to get the noise as low as practical, as my input signal is somewhere less than 300nV. Basically, how low can I really go in a practical sense? I need to stay with transistors that are currently available, vintage unobtanium won't work for me.

This is followed by a mixer and crystal filter with 200hZ bandwidth that is performing well, but the front end is just not hacking it. I keep orbiting the drain with gain and noise.

The amp has to run from 2.8V.

In the end, I will need three preamps for three antennas, and the ability to switch inputs quickly without hammering the IF, but that's a secondary problem.

So, can someone point me to a text, preferably available online (Kindle ok) so I can get through it this weekend?

Reply to
kc6ete
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A BF862 jfet has noise around 1 nv/rthz. A decent bipolar should be similar.

Too bad the antenna sec turns are so few.

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John Larkin         Highland Technology, Inc 
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John Larkin

You get low voltage noise with lots of junction area, to reduce the noise voltage. The cool vintage unobtanium solution is to use an LM194. I don't know if there's a current part available, but the LM194 is, more or less, a bazzilion low-noise transistors all on a die in parallel with emitter loading resistors.

What about stepping the voltage up with a transformer so that your antenna impedance is closer to the ideal noise match for a transistor that you do have?

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Tim Wescott 
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Reply to
Tim Wescott

I wasn't paying attention to his actual figures. Dammit.

Yes, that noise figure seems high -- maybe just use a nice low-noise op- amp with a GBW product of 10MHz or better? I've used 'em for IF amplifiers at 455kHz.

I don't know how the situation is now, but in the past I've never gotten the impression that SPICE is really good at predicting noise performance. I could be wrong, but I think this is one area where you're better off scrutinizing data sheets and doing some pencil-and-paper work.

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Tim Wescott 
Wescott Design Services 
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Reply to
Tim Wescott

I've done noise analysis in LT Spice and the numbers looked reasonable and the hardware worked about right. Phil has noted that Spice might not always be trusted for noise.

He'll probably need a tuned LC drain load on a jfet, to run at that low a supply voltage. Or maybe just a biggish inductor.

A manual analysis should be pretty simple here.

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John Larkin         Highland Technology, Inc 
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John Larkin

My favorite...

...Jim Thompson

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| James E.Thompson                                 |    mens     | 
| Analog Innovations                               |     et      | 
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Jim Thompson
[snip]

Spice is as good at predicting noise only as accurately as the MODEL portrays it.

See "InputReferredNoise.zip" on the Device Models & Subcircuits page of my website for elements that can be added to just about any active device model so that the model portrays noise properly. ...Jim Thompson

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| James E.Thompson                                 |    mens     | 
| Analog Innovations                               |     et      | 
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Jim Thompson

Hi, Dave. Second the BF862- that's really what it's designed for. You should be able to get it to operate from 2.8V since typically cutoff is 800mV (1.2V max).

--sp

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Spehro Pefhany 
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Reply to
Spehro Pefhany

How low? What value capacitor? You want to match impedance, after all, or the best low-noise design won't give you low signal-to-noise.

Maybe I'm being naiive, but the low frequency should mean you might be able to neglect self-resonance of the ferrite...

Reply to
whit3rd

** So you have not built anything yet ?

I suspect you are making a bad error somewhere, as common audio pre-amp circuits have noise figures under 10nVrtHz.

.... Phil

Reply to
Phil Allison

Another LC, after the antenna, can do an impedance match. I'm guessing that an impedance/voltage step-up might be appropriate into a jfet.

--
John Larkin         Highland Technology, Inc 

lunatic fringe electronics
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Reply to
John Larkin

And the best amplifier in the 120-page low-noise chapter in The Art of Electronics, 3rd edition, is 0.06nV/rt-Hz.

--
 Thanks, 
    - Win
Reply to
Winfield Hill

Have you any idea what the actual gain (directivity x efficiency) is for that ferrite antenna ? For a typical ferrite antenna at those frequencies, the gain is often worse than -60 dBi, Thus, the signal is attenuated by 60 dB, but also the band noise is attenuated by 60 dB. The band noise is huge at those frequencies. You really should measure the band noise(density) through your antenna. The amplifier noise floor needs to be only a few dB below the band noise and design the amplifier accordingly.

With very low sorcery impedances, common base stages are sometimes used or even multiple common base stages in parallel. Some have experimented with a big transistor (TO-220) common base stage.

external step up transformer ? This will allow more amplifier topologies.

However, since you are using such low 2.8 V power supply, the amplifier output can handle less than 1 Vrms. If the amplifier gain is

20 dB, any input signal (wanted or unwanted) must be below 100 mVrms.

Inserting an external step-up transformer ahead of the amplifier will increase loading on the antenna LC circuit, reducing the Q. This may or may not be a problem. A larger antenna bandwidth reduces the need to tune the LC over the band, but on the other hand, will allow strong out of band signals through.

Reply to
upsidedown

It's easy to spoil a low-noise amplifier by doing something wrong. Show us your circuit!

300nV/rtHz? That almost *has* to be the noise level at the amplifier output, no? It's customary to state noise levels referred to the amplifier input.

Spice can plot the contributions of individual circuit elements to the overall output noise. There's a good chance that there's one single component that's the source of almost all your noise and I'd guess that it's not the transistor.

Jeroen Belleman

Reply to
Jeroen Belleman

Motchenbacher and Fitchen is a fine text.

LM194 and LM394 can be sometimes found on ebay.

That Corporation may have some low noise equivalents to the LM1/394 parts (300 series).

TomC Detroit, Mich,USA

Reply to
chrpkwcz

** Yeah - the OP is an obvious, software massaging half wit.

The only purpose of this NG seems to be to foster and placate such fools.

Puke ....

..... Phil

Reply to
Phil Allison

If you want to cover the whole 472..479 kHz band without retuning the antenna capacitance, you need to keep the _loaded_ Q below 50, thus the whole band is within 3 dB attenuation.

However, watch out for any strong local AM broadcast signals just above 520 kHz, which might saturate the amplifier due to your low 2.8 V supply voltage.

Reply to
upsidedown

I used the Motchenbacher & Fitchen text when I started in low noise design, but another that I highly recommend is Amplifying Devices and Low-pass Amplifier Design by Cherry and Hooper. The former is way overdue a reprint.

Reply to
JM

Cherry & Hooper can be downloaded (free) from...

...Jim Thompson

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| James E.Thompson                                 |    mens     | 
| Analog Innovations                               |     et      | 
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Jim Thompson

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