e997019.pdf

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variable oscillator

019

C1

R1

10k

T. Giesberts

Although the oscillator in the diagram at first sight resembles a

standard Wien bridge type, it is in fact a variant of it since it is

tuned by varying only one component. This has the advantage of

not needing a carefully selected stereo potentiometer; instead, a

single potentiometer may be used. In the diagram, this is P 1 , and

with values as specified, the output frequency of the oscillator may

be varied between 340 Hz and 3.4 kHz.

The basic Wien bridge consists of R 1 -C 1 and (R 2 +P 1 )-C 2 .

Since in this variant the well-known ×3 attenuation does not occur,

the conditions for stable oscillation are met by including the current

through R 2 +P 1 in the positive feedback loop. This means that the

circuit cannot be based on a single operational amplifier, which is

the reason that IC 1b has been added to it. Diodes D 1 and D 2

ensure reasonable stability of the signal.

The design requires that the resistance of R 4 is more or less

equal to the impedance of network R 5 -R 6 -R 7 P 2 -D 1 -D 2 . Poten-

tiometer P 2 is set to a position which ensures that the level of the

output signal is just below that of the supply voltage: the distortion

is then a minimum. For best results, it may be worthwhile to exper-

iment with the values of R 5 , R 6 , and P 2 .

Frequency control P 1 may have a linear or logarithmic charac-

teristic, although the latter will normally give more linear control.

The frequency, f , is, at least theoretically, determined by

15n

R7

220k

D1

2x

1N4148

R3

10k

R6

150k

D2

R5

10k

P2

P1

R2

1k

2

500

R4

10k

1

6

100k

IC1a

R8

100

3

7

IC1b

5

C2

15n

15V

C5

C3

8

100n

µ

63V

IC1

4

C6

C4

IC1 = TL072

100n

10 µ

63V

15V

994040 - 11

52

Elektor Electronics

7-8/99

10

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f =1/2π

R 1 C 1 √ α,

parallel network D 1 -D 2 by a proper stabilization circuit.

The circuit draws a current of about 4 mA without load. When

a supply voltage of ±15 V is used, the peak output signal is 9.4 V

r.m.s. With the use of a Type TL072 as specified, the circuit can

work from supply voltages as low as ±5 V.

where α=(R 2 +P 1 )/R 1 .

Note the conditions that R 1 =R 3 and C 1 =C 2 .

The circuit has a drawback in that the frequency is dependent

to some degree on the peak value of the signal, which cannot be

nullified with the present design. It can, however, by replacing the

[994040]

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