Datasheet

MAT03
–7–
REV. C
to bias each side of the differential pair. The 5 kΩ collector
resistors noise contribution is insignificant compared to the volt-
age noise of the MAT03. Since noise in the signal path is referred
back to the input, this voltage noise is attenuated by the gain of
the circuit. Consequently, the noise contribution of the col-
lector load resistors is only 0.048 nV/√Hz. This is considerably
less than the typical 0.8 nV/√Hz input noise voltage of the
MAT03 transistor.
The noise contribution of the OP27 gain stages is also negli-
gible due to the gain in the signal path. The op amp stages
amplify the input referred noise of the transistors to increase the
signal strength to allow the noise spectral density (e
in
× 10000)
to be measured with a spectrum analyzer. Since we assume
equal noise contributions from each transistor in the MAT03,
the output is divided by √2 to determine a single transistor’s
input noise.
Air currents cause small temperature changes that can appear
as
low frequency noise. To eliminate this noise source, the mea-
surement circuit must be thermally isolated. Effects of extraneous
noise sources must also be eliminated by totally shielding the circuit.
SUPER LOW NOISE AMPLIFIER
The circuit in Figure 3a is a super low noise amplifier with
equivalent input voltage noise of 0.32 nV/√Hz. By paralleling
three MAT03 matched pairs, a further reduction of amplifier
noise is attained by a reduction of the base spreading resistance
by a factor of 3, and consequently the noise by √3. Additionally,
the shot noise contribution is reduced by maintaining a high
collector current (2 mA/device) which reduces the dynamic
emitter resistance and decreases voltage noise. The voltage noise
is inversely proportional to the square root of the stage current,
and current noise increases proportionally to the square root of
the stage current. Accordingly, this amplifier capitalizes on
voltage noise reduction techniques at the expense of increasing
the current noise. However, high current noise is not usually
important when dealing with low impedance sources.
Figure 3a. Super Low Noise Amplifier