4
dc1416fa
DEMO MANUAL DC1416
DC1416 F02
–
+
LT1222
2V
+12V
–12V
R3
2.49k
D
S
G
V
OUT
NXP
BF862
R1
1MΩ
–12V
–
+
LT1222
+12V
–12V
R7
3.01k
D
S
G
V
OUT
NXP
BF862
R1
1MΩ
SOURCE FOLLOWER
JFET IN GAIN
–
+
DC1416 F03
–
+
LT1222
2V
+12V
–12V
R3
2.49k
R6
221Ω
R11
10MΩ
R11
10MΩ
D
S
G
V
OUT
NXP
BF862
R1
1MΩ
R10
1k
C9
10nF
C9
10nF
–12V
–
+
LT1222
–
+
LT1793
LT1793
+12V
–12V
R7
3.01k
D
S
G
V
OUT
NXP
BF862
R1
1MΩ
SOURCE FOLLOWER WITH INTEGRATOR
JFET IN GAIN WITH INTEGRATOR
Figure 2. The Two Basic Types of JFET Configuration. The Left Shows the JFET as a Source
Follower, Simply Buffering the Feedback Resistor to the Op Amp’s Inverting Input. The Right
Shows the JFET In Gain, with Source Grounded. Because the JFET Inverts, the Feedback Is Now
Applied to the Op Amp’s Non-Inverting Input. In Both Cases, the Effective Input Offset Voltage
Is One JFET V
GS
(About –400mV). The Source Follower Configuration Is the Simplest and Most
Versatile, but the JFET In Gain Configuration Offers the Highest Achievable Gain-Bandwidth
Product and the Lowest Voltage Noise. Output Noise at Low and Medium Frequencies (10kHz to
100kHz) Is 130nV/
√Hz
, Dominated Entirely by the Feedback Resistor
Figure 3. The Two Basic Types of JFET Configuration Again, but Shown with LT1793 Integrators
which Zero Out the Overall Input Offset Voltage. On the Left, the JFET V
GS
Is Forced to the
LT1793 Non-Inverting Input. On the Right, the Integrator Puts JFET V
GS
at the Source Directly.
In both Cases, the 10M Sensing Resistor R11 Injects 40fA/
√Hz
of Current Noise, which Is
Discernible but Relatively Small Compared to the 130fA/
√Hz
of the 1M Feedback Resistor. The
Output Noise at Low to Medium Frequencies Is about 136nV/
√Hz
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