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a
b
I z
I z
y 2
z
V 0
y 2
z
V 0
DVCC
DVCC
y 1
y 1
x
x
I x
I x
D 2
D 1
R 1
R 2
R 2
k R p
+
v c
Rp
C
+
v c
C
c
V 0 (t)
V +
V TH
V c (t)
V TL
V
T on
T off
(off duty cycle period)
(on duty cycle period)
T 1
T 2
Fig. 15.12 The relaxation oscillators using a single DVCC and their relevant waveforms [ 15 ](a)
circuit with 50 % duty cycle (b) circuit with variable duty cycle (c) corresponding waveforms
2 R 2
R 1
T 2 ¼
R 1 C ln
1
ð
15
:
52
Þ
Thus, the frequency of the output square wave is found to be:
1
T 1 þ
1
f 0 ¼
T 2 ¼
ð
15
:
53
Þ
2 R 2
2 R 1 C ln
R 1
1
A slightly modified form of this circuit to facilitate non-50 % duty cycle
(Fig. 15.12b ) obtained by splitting the resistance R 1 in to two parts through a
potentiometer (kR P ) and ensuring that during the charging and discharging of the
capacitor C only one of these two parts is coming into picture. From the diagram, it
can be seen that the diode D 1 is ON and diode D 2 is OFF during the charging of the
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