Datasheet CA3140, CA3140A (Intersil) - 16

CA3140, CA3140A Wien Bridge Oscillator OUTPUT 19V
Another application of the CA3140 that makes excellent use
P-P TO 22VP-P +15V THD <0.3% R
of its high input impedance, high slew rate, and high voltage
2 C2 1000pF 7
qualities is the Wien Bridge sine wave oscillator. A basic Wien
0.1
µ
F CA3109 3 + 8 9
Bridge oscillator is shown in Figure 21. When R1 = R2 = R
DIODE CA3140 6 ARRAY
and C1 = C2 = C, the frequency equation reduces to the
R C 1 1 2 - SUBSTRATE
familiar f = 1/(2πRC) and the gain required for oscillation,
OF CA3019 1 1000 4 6 pF 2 0.1
µ
F
AOSC is equal to 3. Note that if C2 is increased by a factor of
3 7
four and R2 is reduced by a factor of four, the gain required
0.1
µ
F -15V
for oscillation becomes 1.5, thus permitting a potentially
7.5k
Ω
5 4
higher operating frequency closer to the gain bandwidth
R1 = R2 = R
product of the CA3140. 50Hz, R = 3.3MΩ
C R 2 2 3.6k
Ω NOTES: 100Hz, R = 1.6MΩ 1 f = --------------------- 1kHz, R = 160MΩ 2π R C R C
500
Ω 1 1 2 2 10kHz, R = 16MΩ
+
30kHz, R = 5.1MΩ
OUTPUT
C R
-
A = 1 1 + ---- 2 + ---- OSC
FIGURE 22. WIEN BRIDGE OSCILLATOR CIRCUIT USING
C R 2 1
RF CA3140 Simple Sample-and-Hold System C1 R
R
1 R
A = 1 F + ----
S
CL RS 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
FIGURE 21. BASIC WIEN BRIDGE OSCILLATOR CIRCUIT
and low feed-through transmission switch (see Note 13).
USING AN OPERATIONAL AMPLIFIER
System offset nulling is accomplished with the CA3140 via its offset nulling terminals. A typical simulated load of 2kΩ Oscillator stabilization takes on many forms. It must be and 30pF is shown in the schematic. precisely set, otherwise the amplitude will either diminish or reach some form of limiting with high levels of distortion. The
0 SAMPLE
element, R
30k
Ω S, is commonly replaced with some variable
STROBE
resistance element. Thus, through some control means, the
-15 HOLD 1N914
value of RS is adjusted to maintain constant oscillator output. A FET channel resistance, a thermistor, a lamp bulb, or other
+15V
device whose resistance increases as the output amplitude
1N914 +15V 5
is increased are a few of the elements often utilized.
0.1
µ
F 3.5k
Ω
2k
Ω
7 INPUT 3 + 7
Figure 22 shows another means of stabilizing the oscillator
CA3080A 6 + 3 CA3140 6 2
with a zener diode shunting the feedback resistor (R
-
F of
4 2 - 0.1 4
Figure 21). As the output signal amplitude increases, the
1
µ
F 0.1
µ
F 5
zener diode impedance decreases resulting in more
2k
Ω
100k
Ω
-15V
feedback with consequent reduction in gain; thus stabilizing
2k
Ω
-15V
the amplitude of the output signal. Furthermore, this
200pF C1
combination of a monolithic zener diode and bridge rectifier
200pF 400
Ω
2k
Ω circuit tends to provide a zero temperature coefficient for this
0.1
µ
F
regulating system. Because this bridge rectifier system has
30pF SIMULATED LOAD
no time constant, i.e., thermal time constant for the lamp
NOT REQUIRED
bulb, and RC time constant for filters often used in detector networks, there is no lower frequency limit. For example, with
FIGURE 23. SAMPLE AND HOLD CIRCUIT
1µF polycarbonate capacitors and 22MΩ for the frequency determining network, the operating frequency is 0.007Hz. In this circuit, the storage compensation capacitance (C1) is only 200pF. Larger value capacitors provide longer “hold” As the frequency is increased, the output amplitude must be periods but with slower slew rates. The slew rate is: reduced to prevent the output signal from becoming slew- dv rate limited. An output frequency of 180kHz will reach a slew --- I = -- = 0.5mA ⁄ 200pF = 2.5V ⁄ µs dt C rate of approximately 9V/µs when its amplitude is 16VP-P. NOTE: 13. AN6668 “Applications of the CA3080 and CA 3080A High Performance Operational Transconductance Amplifiers”. 16 FN957.10 July 11, 2005