Audio Function Generator
A sine wave signal is generally used in testing linear circuits such as amplifier and filters: on the other hand, a square wave input is often essential for testing digital circuit such as flip-flop, counter, and registers. In this section an audio function generator using a special designed integrated circuit, ICL8038, will be-presented. This IC produced not only sine wave and square wave but also triangle wave.
Circuit Description and working Audio function generator
Lets us see how the 8083 can be used as an audio function generator. Figure 1-1 shows a function generator that produces the sine wave, square wave, and triangle wave and has a frequency range less than 20 Hz to above 100 kHz. However, around 100 kHz the quality of output waveform starts deteriorating in that the sine and triangle waves distort and the square wave become a pulse waveform with <50% duty cycle.
The function generator of figure 1-1 is of high laboratory standard and may used for calibration purpose or in critical design applications, it is useful for beginning or in critical design applications, it is useful for beginning designers and hobbyists. It is also simple and low in cost.
In the circuit of figure 1-1, switch SW2 helps to select the desired output waveform. For the output waveform chosen, the frequency select switch SW1 is then used to select an appropriate frequency range; a special value of frequency is then obtained by adjusting potentiometer R4. R4 allows the potential at pin 8 to be varied from +VCC to +VCC/3 with respect to ground, which in turn varies the frequency of the output waveform.
Potentiometer R3 is used to vary the duty cycle of the square wave output and hence may be used to adjust the duty cycle of the square wave to 50%, especially at higher frequencies. Also, potentiometer R7 and R9, whose wipers are connected to pin 1 and 12 as shown in circuit respectively, are adjusted to minimize sine wave distortion.
The square wave output is uncommitted and is available a pin 9. Therefore, to obtained a TTL-compatible square wave output, pin 9 may be connected through a pull-up resistor to a +5 V supply if desired. However, in circuit pin 9 is connected through resistor R10 and potentiometer R11 to +12 V. The output amplitude of the square wave can be adjusted to a desired value by varying R11. Resistor R10 is used in series with potentiometer R11 so that the +VCC of 12 V is directly applied to the (+) input of the op-amp.
The triangle wave output is available at pin3, the amplitude of which is (0.33)( ± 12 V) = ±3.96 V. However, potentiometer R12 is used to vary the amplitude of the triangle wave. Finally, the sine wave output is available at pin 2. Although the amplitude of the sine wave is (0.22)( ±12 v) = ±2.64 V, it can be adjusted to a desired value with the help of potentiometer R13.
Switch SW2 helps to select one of the three output waveforms, which is then applied to the output stage composed of the op-amp and a power booster. The output stage serves two functions: (1) it provides the low output impedance and (2) it increases the output power drive capability of the 8038. Recall that without the output stage the output resistance of the triangular and suare wave is 200Ω, while that of the sine wave is 1kΩ. Because the output stage is configured as a noninverting amplifier, the output resistance is reduces to a negligible small value. In addition, the use of the power booster MC1438R (or BB3553) inside the op-amp’s feedback loop improves the output drive capability of the 8083 without degrading op-amp characteristics. The op-amp used here is a LF351 with a slew rate of 13V/µS and a unity gain bandwidth of 4 MHz. It is used as a noninverting amplifier with a gain of 11.
For smaller-amplitude output ac signals, it may be necessary to reduce the output offset voltage to zero innitially. This is accomplished by adjusting the offset null circuitry, that is, potentiometer R14 of the LF351 op-amp.
Resistors (all ¼-watt, ± 5% Carbon)
R1, R2 = 3.9 kΩ
R3 = 1kΩ Potentiometer
R4, R11, R14 = 10 kΩ Potentiometer
R5 = 20 kΩ
R6, R8, R10, R15 = 10 kΩ
R7, R9 = 100 kΩ Potentiometer
R12, R13 = 20 kΩ Potentiometer
R16, R17 = 1kΩ
C1 = 10µF
C2, C6, C7 = 1µF
C3 = 0.1µF
C4 = 0.01µF
C5 = 0.001µF
C8 = 470 pF
IC1 = ICL8038 Function generator
IC2 = LF351 op-amp
IC3 = MC1438R power booster
SW1 = Five-position switch
SW2 = Three-position switch
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