Stanford Research Systems BJT SIM911, Operation And Service Manual

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4 – 2 Circuitry
4.1 Circuit Discussion
4.1.1 Input amplifier and protection circuitry
The front-end amplifier Q102 is a matched transistor pair biased
with 3 mA per side. Closed-loop feedback is provided by U105
for an overall first-stage gain of 10.
The MAT02 is run at this
collector current to optimize input voltage noise. The transistors
require
∼ 4
µ
A of base current for this operating point. When the
amplifier is operated with AC coupling, the user is isolated from this
bias current.
The transistor inputs are protected by TVS101. A pair of boot-
strapped diodes, D101 and D102, isolate the amplifier from this
device. Series input resistors R103 and R104 (10
Ω
each) provide
some passive input protection as well, and limit the input current
when TVS101 turns on. For excessive input overvoltages, one or
both of these resistors may be damaged or destroyed.
4.1.2 Programmable gain stages
To preserve pulse shapes while changing amplifier gain, a “fixed-
gains with attenuators” topology was chosen for the SIM911. Low
impedance precision divider ladders (R201
− R204, and R207 − R211)
provide programmable gain steps without introducing excessive
noise at lower gains. Gain allocations are noted on the schematic,
indicating which switches within U201 and U203 are closed based
on gain.
The overall gain of the SIM911 is trimmed at U204, the gain of which
is adjustable by
± 10 %. The final output stage rolls the gain o
ff
slowly
above 1 MHz, and includes a high-current output bu
ff
er (U207) ca-
pable of driving long coaxial cables and other reactive loads. Note
that the 50
Ω
output resistor R225 is common to both the front- and
rear-panel output connectors. Because of this, at most one of the
outputs may be terminated with an external 50
Ω
load.
4.1.3 Digital control
The SIM911 is controlled by microcontroller U405. Amplifier config-
uration is set by shift registers U301 and U302, while the front panel
LED indicators are driven directly by processor port pins. U406 is a
serial EEPROM providing non-volatile memory for amplifier config-
urations.
A critical aspect of the design is the clock-stop circuitry implemented
by U403 and U404. A simple RC-oscillator is enabled or disabled at
pin 1 of U403, which is driven by synchronizing flip-flop U403B to
SIM911 BJT Preamp