CA3140AS datasheet(15/19 Pages) INTERSIL

15 / 19 page

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 R1 = R2 = R

and C1 = C2 = C, the frequency equation reduces to the

familiar f = 1/(2

πRC) and the gain required for oscillation,

AOSC is equal to 3. Note that if C2 is increased by a factor of

four and R2 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, RS, is commonly replaced with some variable

resistance element. Thus, through some control means, the

value of RS 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 (RF 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 1

µF 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 16VP-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 2k

Ω and 30pF is shown in the schematic.

In this circuit, the storage compensation capacitance (C1)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:

π R

1C1R2C2

-------------------------------------------

OSC

-------

--------

OUTPUT

FIGURE 21. BASIC WIEN BRIDGE OSCILLATOR CIRCUIT

USING AN OPERATIONAL AMPLIFIER

CA3109

DIODE

ARRAY

+15V

0.1

µF

0.1

µF

-15V

CA3140

SUBSTRATE

OF CA3019

0.1

µF

7.5k

Ω

3.6k

Ω

500

Ω

OUTPUT

19VP-P TO 22VP-P

THD <0.3%

1000pF

1000

R1 = R2 = R

50Hz, R = 3.3M

Ω

100Hz, R = 1.6M

Ω

1kHz, R = 160M

Ω

10kHz, R = 16M

Ω

30kHz, R = 5.1M

Ω

FIGURE 22. WIEN BRIDGE OSCILLATOR CIRCUIT USING

CA3140

+15V

3.5k

Ω

30pF

CA3140

SIMULATED LOAD

NOT REQUIRED

100k

Ω

INPUT

0.1

µF

0.1

µF

-15V

Ω

400

Ω

200pF

CA3080A

0.1

µF

+15V

-15V

200pF

Ω

STROBE

SAMPLE

HOLD

-15

30k

Ω

1N914

Ω

FIGURE 23. SAMPLE AND HOLD CIRCUIT

------

----

0.5mA 200pF

⁄

2.5V

µs

⁄

CA3140, CA3140A

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