Tektronix 3A9 Instruction Manual page 23

Error Error Amplifier
R2
Modifier ———>- RI
L
q
Fig. 3-5. Basic differential feedback amplifier,
®
leakage current associated with the gates of the input
FET's and the overload protection diodes increase rapidly
To compensate this increase, a temperature-sensitive input
current balancing network is included, using thermistors
As the voltage across R113 and R213 increases due to
increase in temperature of the active devices, an equal
voltage change is produced in the thermistor compensating
circuit, maintaining the FET gate level at zero volts.
The gate current compensation becomes inoperative if
the straps are removed for high input impedance operation.
Preamp
The feedback amplifiers in the + and — inputs are
identical except for circuit numbers. Except where needed
for clarification, only the amplifier in the -++input will be
described in detail.
Fig. 3-8 is a partial diagram of the input amplifier illustrat
ing the current paths.
The feedback amplifier in the +-input consists of Q133A,
Q144A and @Q254. The feedback path is from the source
of Q254, through the divider (modifier) consisting of R253,
R251. R251 is the gain setting resistor and since approxi-
mately the same signal current flows through both R25]
and R257, the gain may be caluculated by the ratio of
3-5