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Op‐Amps • Active device: V0 = a(V+-V-); note that it is the difference of the input voltage! • a=open loop gain ~ 105 –106 • Most applications use negative feedback. • Comparator: no feedback • Active device requires power. No shown for simplicity. • Classics op-amps: 741, 357 ~ $0.20; one, two or four in a package.
5 - Active (Current Source) Load JFET Amplifier Figure 3.4 shows a bootstrapped drain load, which makes the JFET's drain current almost constant. As with BJTs and valves, using a current source load improves performance, usually resulting in higher gain and better linearity.
USING REF200 AND ONE EXTERNAL OP AMP: Current Source or Sink With Compliance to Power Supply Rail and Current Out >100µA You can build a programmable current source of virtually any value using two resistors, an op amp, and a 100µA current source as a reference. The current source shown in Figure 12 can be programmed
Op-Amp. The JFET constant current source A constant current source using a JFET is shown in figure 6. This circuit configuration has many useful applications ranging from charging circuits for integrators or timers to replacing the source resistor in the differential amplifier shown in figure 5. The current provided by the constant current ...
Each circuit configuration describes a two-port network having an input and an output. The transfer function of each is also determined by the input and output voltages or currents of the circuit. The most common configuration for the JFET as an amplifier is the common source circuit.
The single JFET CCR provides a suitably high dynamic load and low noise performance. The CCR is biased to the desired operating point by selecting R2 for the desired current flow through the JFET. We talk more about this in a moment as well.
An example of a current source that has an even larger current supply (100 mA or greater) is in the circuit shown in . Figure 4. This circuit uses an op amp output driver push-pull stage to supply current to the load. When a positive voltage is applied to VIN, the output voltage of the control op amp rises, which turns
