When the output voltage VO is first at its high, VOH, the ca-
pacitor C is charged toward VOH through R2. The voltage
across C rises exponentially with a time constant τ = R2C, and
this voltage is applied to the inverting input of the op amp.
Meanwhile, the voltage at the non-inverting input is set at the
positive threshold voltage (VTH+) of the generator. The ca-
pacitor voltage continually increases until it reaches VTH+, at
which point the output of the generator will switch to its low,
VOL which 0V is in this case. The voltage at the non-inverting
input is switched to the negative threshold voltage (VTH−) of
the generator. The capacitor then starts to discharge toward
VOL exponentially through R1, with a time constant τ = R1C.
When the capacitor voltage reaches VTH−, the output of the
pulse generator switches to VOH. The capacitor starts to
charge, and the cycle repeats itself.
10006077
FIGURE 18. Pulse Generator
Figure 19 is a squarewave generator with the same path for
charging and discharging the capacitor.
10006076
FIGURE 19. Squarewave Generator
CURRENT SOURCE AND SINK
10006086
The LMV321/LMV358/LMV324 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.
FIGURE 17. Waveforms of the Circuit in Figure 16
As shown in the waveforms in Figure 17, the pulse width
(T1) is set by R2, C and VOH, and the time between pulses
(T2) is set by R1, C and VOL. This pulse generator can be made
to have different frequencies and pulse width by selecting dif-
ferent capacitor value and resistor values.
Fixed Current Source
A multiple fixed current source is shown in Figure 20. A volt-
age (VREF = 2V) is established across resistor R3 by the
voltage divider (R3 and R4). Negative feedback is used to
cause the voltage drop across R1 to be equal to VREF. This
controls the emitter current of transistor Q1 and if we neglect
the base current of Q1 and Q2, essentially this same current
is available out of the collector of Q1.
Figure 18 shows another pulse generator, with separate
charge and discharge paths. The capacitor is charged
through R1 and is discharged through R2.
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 Q1.
The resistor, R2, can be used to scale the collector current of
Q2 either above or below the 1 mA reference value.
19
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