Inputandgain Stages; Input Stage And Gain Stage - Keithley 595 Instruction Manual

THEORY OF OPERATION
7.5.2 input and Gain Stages
tion of jumper W301 and potentiometer R3l5, the voltage
at the gate of 4302 will be from -2Onii to +2OmV, directly
The preamplifier input stage and gain stage (U305) is shown
offsetting the voltage error which would otherwise appear
in figure
7-11.
The tiput~stape consis~ of a pmpriatary
JFET at the input-side gate of 4302.
amplifier circuit, offset voltage circuit and input bias cur-
rent compensation. The gain stage simply consists of a
single IC operational amplifier (U305).
Input bias compenation is provided to optimize the input
bias current of the itwinmwnt. The compofients involved
are RW RT301, R322, R'I302, R321 and R3l6. The wiper of
The JFET amplifier circuit is comprised of Q302, R323, R324,
lW7 is adjustable to k5V. This voltage is divided with Kl3Ul
' RX& R3l3 and R314. This biasing configuration results in
andR322by20andagainby50withRT302andR321.KI301
lower input biis current (gate current).
and RI302 are thermistors whose combined temperature
dependence tracks the JFEX leakage. The voltage applied
to R316 is then nominally *5mV, but changin with
lb compensate for the offset voltage of this stage, the non-
temperature. Since IW.6 is 25OGQ and it is connecte 1 to the
inverting gate of 4302 is connected to the divider formed
virhml ground input of the amplifier at its other terminal,
by R320,
R319, FGl5, I?318 and W3Ul. Depending on the posi-
i2OfA compensation at 23T is possible.
c, I
INPUT
HI
R323
Figure 7-11. Input Stage and Gain Stage
7-10