Vine Bridge Generator

Wiring diagram for the generator with the Wien bridge * The Wien bridge is highlighted in green. * An operational amplifier is used as an active element . * In this scheme, R ₁ = R ₂ , C ₁ = C ₂ .

Hewlett-Packard HP200A * sine-wave voltage generator * At the center of the figure is a four-section capacitor of variable capacitance , which tunes the frequency of the quasi-resonance of the Wien bridge and, thus, the frequency of the generated voltage.

The generator with the Vina bridge is a type of electronic generators of sinusoidal oscillations .

The frequency-defining part of this generator is made on a capacitive-resistive band - pass filter , first proposed by Max Vin in 1891 to measure the impedances of electrical circuits and is now called the Vin bridge .

The generator is an electronic amplifier covered by frequency-dependent positive feedback across the Wien bridge. When changing the parameters of the Wien bridge, the generator can generate voltage in a wide tunable frequency range and generates a sinusoidal voltage with small differences from the ideal sinusoidal signal.

Content

1 History
2 Principle of work
3 Stabilization of amplitude and waveform
4 Application
5 See also
6 References

History

The circuit electronic implementation of the generator was first described in the dissertation of William Hewlett for a master's degree, which he defended in 1939 at Stanford University .

Subsequently, Hewlett, together with David Packard, founded the company Hewlett-Packard . The company's first industrial product was the HP200A Precision Sine Wave Generator with Wien Bridge. The HP200A was one of the first commercially available laboratory sinusoidal voltage generators with such low sine wave distortion.

Principle of Operation

An electrical circuit consisting of connected according to the drawing $R_{1},R_{2},C_{1},C_{2}$ ${\ displaystyle R_ {1}, R_ {2}, C_ {1}, C_ {2}}$ are usually called the Vin bridge.

If the resistance values $R_{1}$ ${\ displaystyle R_ {1}}$ and $R_{2}$ ${\ displaystyle R_ {2}}$ as well as tanks $C_{1}$ ${\ displaystyle C_ {1}}$ and $C_{2}$ ${\ displaystyle C_ {2}}$ do not differ too much, then such a circuit has a smoothed quasi-resonance, that is, the voltage transfer coefficient from the right one according to the output circuit $R_{1}$ ${\ displaystyle R_ {1}}$ (input signal) to the connection point $C_{1},C_{2},R_{2}$ ${\ displaystyle C_ {1}, C_ {2}, R_ {2}}$ (output signal) has a maximum at a certain frequency.

The simplest formula for the quasi-resonant frequency occurs with the equalities:

R_{1}=R_{2}=R

{\ displaystyle R_ {1} = R_ {2} = R}

and

C_{1}=C_{2}=C,

{\ displaystyle C_ {1} = C_ {2} = C,}

the frequency of quasi-resonance is equal to:

f={\frac {1}{2\pi RC}}.

{\ displaystyle f = {\ frac {1} {2 \ pi RC}}.}

At the quasi-resonance frequency, the phase shift of the output signal of the Wien bridge relative to the input signal is zero, and the modulus of the transmission coefficient is 1/3. If we include in the feedback loop covering the input and output of the Wien bridge an active non-inverting amplifying element, ideally without a phase shift, with a transmission coefficient of more than 3, then self-oscillations increasing to infinity in amplitude will occur in the loop, since this loop does not stability criterion for linear systems.

In practice, in real generators, the amplitude of sinusoidal oscillations does not increase to infinity, but is set at a certain level due to the nonlinear properties of the active amplifier element, for example, the natural limitation of the supply voltage supplying the amplifier. With a nonlinear limitation of the amplitude, the shape of the initially occurring sinusoidal voltage during distortion is distorted, and, in the end, becomes far from sinusoidal, for example, close to trapezoidal.

When the transmission coefficient in the feedback loop is less than 3, randomly generated oscillations decay, since in this case the system is stable.

Thus, to maintain sinusoidal oscillations with small deviations from the sinusoid in this generator, it is necessary, after establishing the oscillations with the desired amplitude, to strictly maintain the voltage transfer coefficient of the active amplifier element exactly equal to 3.

In the above diagram, an example is shown as an active amplifier element of an operational amplifier (op amp) included for a generated signal according to a non-inverting amplifier circuit . Voltage transfer coefficient $K_{U}$ ${\ displaystyle K_ {U}}$ non-inverting op amp amplifier:

K_{U}=1+R_{3}/R_{4}.

{\ displaystyle K_ {U} = 1 + R_ {3} / R_ {4}.}

Thus, stable generation of a sinusoidal signal with small distortions and without amplitude fluctuations is provided when:

R_{3}=2R_{4},

{\ displaystyle R_ {3} = 2R_ {4},}

the frequency of the generated voltage will then be equal to the quasi-resonant frequency of the Wien bridge.

The above relations are valid for ideal passive components - resistors and capacitors and ideal active amplifying elements. In practice, the main deviations from ideality are made by the amplifier, mainly due to the internal phase shift of the output signal relative to the input signal, which increases with increasing frequency. Therefore, at a certain high frequency, the “onset” of the phase shift will turn the positive feedback into negative. Therefore, the frequency range of the generated oscillations is limited from above by practically several MHz.

Amplitude and waveform stabilization

Maintaining the indicated ratio of resistors $R_{3}$ ${\ displaystyle R_ {3}}$ and $R_{4}$ ${\ displaystyle R_ {4}}$ in practical schemes of such generators, it is implemented by introducing the dependence of the resistance of these resistors on the amplitude of the voltage across them, that is, by using non-linear resistors.

As non-linear resistors, thermistors with a negative coefficient of thermal resistance (TCS) or metal thermistors with a positive TCS are used.

The essence of stabilizing the resistance ratio is to reduce the resistance $R_{3}$ ${\ displaystyle R_ {3}}$ with increasing amplitude of the generated voltage, or increasing resistance $R_{4}$ ${\ displaystyle R_ {4}}$ with increasing amplitude, or, accordingly, vice versa, with increasing amplitude.

Since the power released in the resistor is proportional to the square of the effective voltage on it, and the steady temperature of the resistor is proportional to the power, to stabilize the amplitude $R_{3}$ ${\ displaystyle R_ {3}}$ with negative TCS, - semiconductor thermistors , or $R_{4}$ ${\ displaystyle R_ {4}}$ with positive TCS - for example, incandescent lamps with a tungsten radiation body.

For the nonlinear properties of thermally dependent resistors to manifest in order to stabilize the amplitude and shape of the generated voltage, it is important that the steady-state temperature in them, caused by the heating of the current flowing through them, significantly exceeds the ambient temperature. It is also important, to ensure small distortions, that the intrinsic thermal time constant of the applied thermally dependent resistances is many times greater than the period of the generated oscillation. An additional requirement is the operation of the active amplifying element within the linearity of its transfer characteristic.

In addition to the described popular thermoresistive nonlinear negative feedbacks, such generators often use parametric negative feedbacks through two-terminal devices with a non - linear current-voltage characteristic , for example, zener diodes or servo systems for amplitude self-regulation, where field-effect transistors are used as voltage-controlled resistances in the feedback loop. and optocoupler photoresistors .

Application

The traditional use of such generators is as standard measuring signal generators. Also in various electronic devices where high frequency stability is not required with small distortion of the sinusoidal signal.

Vine Bridge Generator

Content

History

Principle of Operation

Amplitude and waveform stabilization

Application

See also

Links

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