15
Wien Bridge Oscillator
Another application of the CA3140 that makes excellent use
of its high input impedance, high slew rate, and high voltage
qualities is the Wien Bridge sine wave oscillator. A basic Wien
Bridge oscillator is shown in Figure 21. When R
1
= R
2
= R
and C
1
= C
2
= C, the frequency equation reduces to the
familiar f = 1/(2πRC) and the gain required for oscillation,
A
OSC
is equal to 3. Note that if C
2
is increased by a factor of
four and R
2
is reduced by a factor of four, the gain required
for oscillation becomes 1.5, thus permitting a potentially
higher operating frequency closer to the gain bandwidth
product of the CA3140.
Oscillator stabilization takes on many forms. It must be
precisely set, otherwise the amplitude will either diminish or
reach some form of limiting with high levels of distortion. The
element, R
S
, is commonly replaced with some variable
resistance element. Thus, through some control means, the
value of R
S
is adjusted to maintain constant oscillator
output. A FET channel resistance, a thermistor, a lamp bulb,
or other device whose resistance increases as the output
amplitude is increased are a few of the elements often
utilized.
Figure 22 shows another means of stabilizing the oscillator
with a zener diode shunting the feedback resistor (R
F
of
Figure 21). As the output signal amplitude increases, the
zener diode impedance decreases resulting in more
feedback with consequent reduction in gain; thus stabilizing
the amplitude of the output signal. Furthermore, this
combination of a monolithic zener diode and bridge rectifier
circuit tends to provide a zero temperature coefficient for this
regulating system. Because this bridge rectifier system has
no time constant, i.e., thermal time constant for the lamp
bulb, and RC time constant for filters often used in detector
networks, there is no lower frequency limit. For example,
with 1µF polycarbonate capacitors and 22MΩ for the
frequency determining network, the operating frequency is
0.007Hz.
As the frequency is increased, the output amplitude must be
reduced to prevent the output signal from becoming slew-
rate limited. An output frequency of 180kHz will reach a slew
rate of approximately 9V/µs when its amplitude is 16V
P-P
.
Simple Sample-and-Hold System
Figure 23 shows a very simple sample-and-hold system
using the CA3140 as the readout amplifier for the storage
capacitor. The CA3080A serves as both input buffer
amplifier and low feed-through transmission switch (see
Note 13). System offset nulling is accomplished with the
CA3140 via its offset nulling terminals. A typical simulated
load of 2kΩ and 30pF is shown in the schematic.
In this circuit, the storage compensation capacitance (C
1
)is
only 200pF. Larger value capacitors provide longer “hold”
periods but with slower slew rates. The slew rate is:
NOTE:
13. AN6668 “Applications of the CA3080 and CA 3080A High Per-
formance Operational Transconductance Amplifiers”.
NOTES:
f
1
2π R
1
C
1
R
2
C
2
-------------------------------------------=
A
OSC
1
C
1
C
2
-------
R
2
R
1
-------++=
A
CL
1
R
F
R
S
--------+=
C
1
R
2
R
1
C
2
OUTPUT
R
F
R
S
+
-
FIGURE 21. BASIC WIEN BRIDGE OSCILLATOR CIRCUIT
USING AN OPERATIONAL AMPLIFIER
8
5 4
3
1
9
6
CA3109
DIODE
ARRAY
+15V
0.1µF
0.1µF
-15V
2
6
7
4
+
CA3140
-
SUBSTRATE
OF CA3019
0.1µF
7
7.5kΩ
3.6kΩ
500Ω
OUTPUT
19V
P-P
TO 22V
P-P
THD <0.3%
3
R
2
C
2
1000pF
1000
pF
C
1
R
1
R
1
= R
2
= R
50Hz, R = 3.3MΩ
100Hz, R = 1.6MΩ
1kHz, R = 160MΩ
10kHz, R = 16MΩ
30kHz, R = 5.1MΩ
2
FIGURE 22. WIEN BRIDGE OSCILLATOR CIRCUIT USING
CA3140
+15V
3.5kΩ
30pF
2
6
1
+
CA3140
-
SIMULATED LOAD
NOT REQUIRED
100kΩ
INPUT
0.1
0.1µF
µF
7
0.1µF
-15V
2kΩ
3
400Ω
200pF
6
4
5
7
4
+
CA3080A
-
0.1µF
+15V
-15V
200pF
2kΩ
2
3
5
2kΩ
STROBE
SAMPLE
HOLD-15
0
30kΩ
1N914
1N914
2kΩ
C
1
FIGURE 23. SAMPLE AND HOLD CIRCUIT
dv
dt
------
I
C
---- 0.5mA 200pF⁄ 2.5V µs⁄== =
CA3140, CA3140A
