LMV321 datasheet(20/27 Pages) NSC

20 / 27 page

Application Notes (Continued)

4.6 CURRENT SOURCE AND SINK

The LMV321/358/324 can be used in feedback loops which

regulate the current in external PNP transistors to provide

current sources or in external NPN transistors to provide

current sinks.

4.6.1 Fixed Current Source

A multiple fixed current source is show in Figure 20.A

voltage (V

REF = 2V) is established across resistor R3 by the

voltage divider (R

3 and R 4). Negative feedback is used to

cause the voltage drop across R

1 to be equal to VREF. This

controls the emitter current of transistor Q

1 and if we neglect

the base current of Q

1 and Q2, essentially this same current

is available out of the collector of Q

Large input resistors can be used to reduce current loss and

a Darlington connection can be used to reduce errors due to

the

β of Q

The resistor, R

2, can be used to scale the collector current of

2 either above or below the 1mA reference value.

4.6.2 High Compliance Current Sink

A current sink circuit is shown in Figure 21. The circuit

requires only one resistor (R

E) and supplies an output cur-

rent which is directly proportional to this resistor value.

4.7 POWER AMPLIFIER

A power amplifier is illustrated in Figure 22. This circuit can

provide a higher output current because a transistor follower

is added to the output of the op amp.

4.8 LED DRIVER

The LMV321/358/324 can be used to drive an LED as shown

in Figure 23.

10006076

FIGURE 19. Squarewave Generator

10006080

FIGURE 20. Fixed Current Source

10006082

FIGURE 21. High Compliance Current Sink

10006079

FIGURE 22. Power Amplifier

10006084

FIGURE 23. LED Driver

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