Noise - Stanford Research Systems SIM954 Operation And Service Manual

2 – 8

2.3 Noise

The SIM954 amplifier stages are compound amplifiers. The RF power
amplifier contains four THS9031 current-feedback operational ampli-
fiers per channel. These amplifiers have 2 nV
input voltage and 14 pA
above 100 kHz) each. The parallel operation e ectively halves the
input voltage noise and doubles the current noise. Amplifier noise
accounts for 1 nV Hz and 28 pA
noise matching resistance of 1 nV
close to the source resistance, and the amplifier noise contribution is
low.
Including the feedback resistors, this compound amplifier can be
calculated to have a theoretical Johnson noise floor of 1.95 nV
when driven with a 50
Noise measurements on SIM954 stages have yielded Johnson noise
data between 1.85 nV
obtained at 100 MHz, while the larger number coincides with slightly
higher noise at 160 MHz. The increase in noise (gain) at higher fre-
quencies can be attributed to the increasing (capacitive) mismatch of
the SIM954 input to the driving impedance and parasitic impedances
in the amplifier's feedback. The chosen compensation optimizes a
combination of gain flatness, bandwidth and step response and sac-
rifices noise performance close to the bandwith limit.
The SIM954's Johnson noise is better than 3 nV
driven by 50 sources.
The resulting noise figure for the ideal amplifier is about 8 dB, while
the guaranteed noise figure does not exceed 11 dB. Actual production
models will be somewhere in-between.
Because of its relatively low gain and medium noise figure, the
SIM954 does not qualify as a low noise amplifier, but it will still
yield reasonable noise performance in applications which can toler-
ate its modest 11 dB noise figure while requiring only small gains at
large amplitudes, a domain which is usually poorly covered by other
amplifiers.
SIM954 300 MHz Dual Inverting Driver Amplifier
Hz typical current noise (Johnson noise
Hz input noise. The resulting
Hz 28 pA
source.
Hz and 2.45 nV Hz. The lower figure was
General properties
Hz typical equivalent
Hz 36 is very
Hz
Hz for amplifiers