Stanford Research Systems SIM954 Operation And Service Manual page 50

5 – 2
5.1 Circuit Description
The SIM954 contains two independent amplifiers and a power con-
ditioning circuit.
Each amplifier is primarily made out of four 250mA line driver op-
amps (U101-U104 and U105-U109). The THS3091 95 family of line
drivers are made by Texas Instruments using a robust 36 V RF bipolar
process. Inside the small outline packages the dies are soldered to a
metal pad which is exposed on the bottom side of packages. These
cooling pads are directly soldered to the printed circuit board, giving
the part unusual thermal load handling capability.
The SIM954 exploits this unusually powerful part by paralleling four
of them with 13 2 output resistors (R1x6a-d).These isolation resis-
tors give this amplifier excellent stability by adding a positive re-
sistive component to all external passive loads. Even dead shorts
and perfectly lossless capacitive and inductive loads are seen by the
actual amplifier as a dissipative load that lie well within it's stability
limits.
Because of the load sharing each operational amplifier sees a worst
case load of 13 2
appears as 213.2 to each individual op-amp, which is a very benign
load condition.
In addition, the resistors reduce the power dissipation of the op-amps
in case of very low impedance loads (like shorts and DC currents
into coils transformers) driven with large currents. At 1 A output the
output resistors will absorb a voltage drop of 3.2 V or approximately
20% of the total thermal load.
Because of their 3.3 output impedance, SIM954 amplifier channels
can also be ganged in parallel if they are being driven by the same
signal. See section 2.1.2 for further discussion.
The THS3091 95 are current-feedback op-amps and the ideal gain and
feedback configuration at which these amplifiers have their largest
useful gain-bandwidth are as inverting amplifier with a gain between
4 and 5.
The DC gain of the amplifier is therefore chosen such that the 3.3
output impedance together with the 50
ical RF system form a divider which reduces the e ective gain to 4
or 12 dB. This means that the module will have a gain of 4 266 into
a high impedance load. This is equivalent to a 12.6 dB unterminated
signal gain.
Depending on the variations in wafer lots each amplifier has a feed-
back of 953–1100 (R1x4) which makes the frequency response close
SIM954 300 MHz Dual Inverting Driver Amplifier
Z 4. Specifically a 50
Load
input impedance of a typ-
Circuitry
load therefore