Pont de Wien

The bridge of Wien is an electrical circuit made up of two impedances Z1 and Z2 in series. Z1 consists of a R1 resistance and a C1 condenser in series, Z2 of a R2 resistance and a C2 condenser in parallel.

The bridge of Wien can be used as filter.

Oscillator with bridge of Wien

It can also be used to produce a producing oscillator of the sinusoidal signals with weak a Distorsion.

Let us recall that an oscillator is composed of two parts:

an amplifier: this one has, according to the times, summer carried out with a vacuum tube, one or more bipolar transistors or with field effect; those can be integrated on a chip;
a circuit of reaction, placed between the exit of the amplifier and its entry; this circuit implements various impedances: resistance S, condensing S, winds S, quartz.

It is the circuit of reaction which determines the frequency of oscillation. Indeed, this one occurs at a frequency where the condition of oscillation n.Go = 1 is satisfied. N and Go, both complexes, represent the “profit” of the circuit of reaction and the profit of the amplifier.

At the frequency $f= \ frac {1} {2 \ pi \ sqrt {R_ {1} R_ {2} C_ {1} C_ {2}}}$ is $f= \ frac {1} {2 \ pi {RC}}$ , the “profit” of the filter of Wien is worth 1/3 and the output signal east in phase with the entry signal. By connecting the filter of Wien between the exit and the entry of an amplifier of profit 3 (an operational Amplifier in the figure), one obtains an oscillator which produces a sinusoid at the frequency indicated.

In general, one takes $R_ {1} = R_ {2}$ and $C_ {1} = C_ {2}$ .

Stabilization of the amplitude of the oscillations

The profit of the AOP depends on resistances R3 and R4; to have a profit of 3, one will take R3 = 2 R 4.

But the inaccuracies of the values of R3 and R4 make that this condition never is completely met. That it occurs then:

if R3 < 2 R 4, the oscillator does not oscillate;
if R3 > 2 R 4, the oscillation starts well, the amplitude grows up to the limiting value, determined by the supply voltage of the AOP; the problem, it is that under this condition the form of wave is distorted, the tops are flattened.

To cure this problem, one replaces R3 or R4 by a CTP or a CTN (resistances whose value grows or decrease with the temperature). The amplitude is stabilized with a value such as R3 is equal to 2 R 4.

That functions in the following way: let us suppose that R4 is a CTP. If, for an unspecified reason, the amplitude grows slightly, the power dissipated in R4 increases, which makes thus grow its value and reduced the profit of the AOP, which brings back the amplitude to its correct level.

Short history

The bridge of Wien was developed at the origin by max Wien in 1891. At that time, Wien did not have the means of carrying out an amplifier circuit and thus could not build an oscillator. The modern circuit is derived from the thesis of mastère of William Hewlett in 1939. Hewlett, with David Packard, Co-founded Hewlett-Packard. Their first product was the HP 200A, an oscillator based on the bridge of Wien. The 200A is a known traditional instrument for the low distortion of the output signal.

The CTP used was simply a filament of incandescent lamp. The modern oscillators with bridge of Wien use, the place of a filament of bulb, field-effect transistors or electric eyes. Rates of distortion about some parts per million can be obtained by slightly improving the original circuit of W. Hewlett.

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