[time-nuts] Ultra low noise amplifiers.
Dr Bruce Griffiths
bruce.griffiths at xtra.co.nz
Fri Mar 30 19:49:37 EDT 2007
The following papers may be of some interest to anyone contemplating
building a low noise low frequency (dc -100kHz) amplifier.
Such an amplifier would be useful, for example, after a double balanced
mixer in a conventional phase noise measurement system.
http://arxiv.org/PS_cache/physics/pdf/0503/0503012.pdf
http://arxiv.org/PS_cache/physics/pdf/0512/0512082.pdf
In a correlation measurement system the white noise floor can be lowered
by averaging, whilst the flicker noise cannot. Thus it is important to
select amplifiers with low flicker noise for such systems.
Although some of the aspects of the amplifier design are questionable
(e.g. lower noise current sources are possible), the very low frequency
noise measurements illustrate the importance of thermal lagging and
using a light tight enclosure. The seals around the kovar leads in the
metal dual transistor packages transmit light into the package. All
transistors are photosensitive unless the junctions are shielded. The
LED in the current source is also photosensitive, however the resultant
modulation of the current source output is a common mode signal. Since
the LED emits light it can also generate photocurrents in all of the
metal can packaged transistors. It would be interesting to look at the
effect of placing a light tight shield over the LED. Another approach
would be to use equivalent dual transistors (SSM2220) in a light tight
package.
Other useful information is the noise data for various opamps in the
flicker noise region. This shows that the intrinsic low frequency noise
of devices can be significantly lower than that specified in their
datasheets. The datasheet noise specs are usually based on measurements
by the manufacturer using a setup which isn't optimised to minimise
thermal fluctuations or air currents.
Since the close in phase noise of a bipolar RF amplifier is primarily
due to modulation of collector base (and other) capacitances by bias
voltage fluctuations, and the collector base voltages of the various
transistors is temperature dependent, it is important to shield the
amplifier from air currents and thermally lag the amplifier by enclosing
it in a massive metal enclosure, if one wishes to achieve low close in
phase noise.
Bruce
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