Gain Stage; Output Stage; Figure - Keithley 6512 Instruction Manual

Theory of Operation
Q308
INPUT
R334
R333
HI
A
LO
R335
-5V (BOOTSTRAPPED)

Figure 6-4

Simplified schematic of input stage

6.3.2 Gain stage

Input preamplifier gain is provided by a single IC operational
amplifier, U309, as shown in Figure 6-5. The input signal
from the input stage is applied to the inverting and non-in-
verting terminals of the op amp, while the output from the IC
is applied to the output stage. The feedback capacitor, C319,
is one of several components that provides stability by limit-
ing the bandwidth of the amplifier.
FROM
INPUT
STAGE
Figure 6-5
Gain stage

6.3.3 Output stage

The output stage takes on one of two configurations, depend-
ing on the selected function. In the volts and ohms modes,
the output is optimized for voltages as high as ±210V, while
a different configuration (one necessary for high current) is
used in amps and coulombs.
A simplified diagram of the output stage in the volts and
ohms modes is shown in Figure 6-6. Four transistors (Q301,
6-4
+0.4V (BOOTSTRAPPED)
+5V
-5V
-5V
R314
BOOTSTRAPPED
R352
O
C
O
R351
B
O
A
O
W303
R342
+5
B
BOOTSTRAPPED
BOOTSTRAPPED
GROUND
TO GAIN
STAGE
R336
C319
U309
TO OUTPUT
STAGE
Q303, Q304, and Q305) are used in this configuration. Each
transistor pair is used for one-half the output voltage swing.
Q301 and Q304 are used for the positive half, while Q303
and Q305 are used for the negative half. Because of the
±210V voltage swing requirement, ±210V supplies are used.
Each transistor pair is operated in series to provide the nec-
essary device breakdown voltage and power rating.
R302
INPUT
FROM
GAIN
GAIN
STAGE
STAGE
OUTPUT
GAIN
STAGE
COMMON
R321
Figure 6-6
Output stage configuration in volts and ohms
Circuit biasing components include R301, R302, R320, R321,
R325, CR315, and CR316. Meanwhile, CR318 and CR314
provide protection for the ±210V supplies in case a high exter-
nal voltage is applied to the PREAMP OUT terminal.
Output voltage V from the gain stage causes a current to flow
through the emitter resistor R
equal to V /R . This same current flows through the load re-
IN E
sistor, R , which produces a buffered output voltage of V
L
and is non-inverting.
Output stage configuration for the amps and coulombs func-
tions is shown in Figure 6-7. Q306 and Q307 are the active
devices for this configuration. Since the power supply volt-
ages are much lower (±24V), only a single pair of transistors
is required. As with the high-voltage configuration, signal in-
put is applied through the two diodes to the bases of the tran-
CR318
R301
+210V
Q304
Q301
R E1
R325
R309
OUTPUT
CR315
R323
R E2
CR316
R L
Q303
LO
Q305
-210V
R320
CR314
, which is approximately
E1
IN