Isolation - Stanford Research Systems SIM954 Operation And Service Manual

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2.5 Isolation

Because each SIM954 channel is an inverting current feedback am-
plifier, the input node is connected via an e ective
the virtual ground node of the amplifier, which itself is connected to
the output via an e ective
Because the amplifier's transimpedance gain is finite, the isolation
between the output and the input port is also finite. As the loop gain
diminishes at higher frequencies, the output to input isolation will
decrease, and a larger fraction of the RF energy at the output will
appear at the input of the amplifier.
While this is generally of limited concern, it can become a problem
if this RF energy can leak into high gain or high Q (quality factor)
circuits connected to the amplifier input.
High impedance, high Q resonant circuits (e.g. tanks, open transmis-
sion lines, crystals etc.) can be excited, and oscillation of the ampli-
fier and the frequency selective element can occur. Limited isolation
properties are more likely to become a problem if the output is in-
correctly terminated as well, where the load reflects RF energy back
into the amplifier. Since the phaseshift between input and output
changes at higher frequency, making the feedback more "positive,"
parasitic oscillations due to limited isolation are most likely to occur
near the amplifier's bandwidth limit.
When multiple amplifiers are connected in series to increase the
gain, or used in parallel to increase output current or voltage in
a bridge circuit, the finite isolation can destabilize the amplifiers
even in wideband, low Q circuits. Again these oscillations are most
likely going to occur at frequenices close to the amplifier's bandwith
limit(i.e., in the 100 MHz to 300 MHz range).
If oscillations (or an increase in noise gain) are observed, isolation
between the amplifier and the driving ot terminating circuits has to
be increased. This can be accomplished with attenuators (to reduce
overall gain), isolating power splitters (to isolate multiple inputs)
or by using frequency selective circuits like lowpass and bandpass
filters (to reduce gain at the highest frequencies at which isolation is
worst).
The following diagram shows the measured isolation between a
SIM954 output and its input. The measurement was made with a
network analyzer by connecting the source to the amplifier's output
and the network analyzer input to the amplifier's input.
Both the isolation in amplifier 'on' and amplifier 'o ' configuration
are shown. With the amplifier powered on, the isolation gets increas-
SIM954 300 MHz Dual Inverting Driver Amplifier
General properties
220 feedback resistor.
50 resistor to