MAT03

–7–

REV. C

to bias each side of the differential pair. The 5 k

Ω 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

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