Current Sources and Voltage References

Modified current sources and example applications

Using BJTs to Create Current Sources

107

Once the voltage across the capacitor has reached the comparator’s upper trip-point

(established by the voltage between R

2

and R

3

), the flip-flop changes state and

switches on Q

5

. The I

REF

current via Q

1

and Q

2

establishes the current into the current

mirror Q

3

and Q

4

. With D

2

now effectively reverse-biased, the capacitor discharges at

a constant rate via Q

4

and Q

5

. Once the lower threshold is reached (established by

the voltage between R

3

and R

4

), the flip-flop changes state again, and switches Q

5

off. The oscillator’s duty cycle is fixed because the charge and discharge ...