Vienna Robinson Bridge
A Vienna-Robinson bridge is a bridge circuit named after Max Wien , in which one branch of the bridge is formed by a bandpass and the other by a 2: 1 voltage divider .
filter
The alternating voltage is always supplied asymmetrically, the differential voltage Out1 - Out2 is evaluated, which corresponds to the frequency
shows a minimum. The prerequisite is that the two resistors and capacitors in the band pass are chosen to be the same. There is also a phase jump from −90 ° to + 90 °. The Wien-Robinson Bridge can therefore be used as a blocking filter together with an operational amplifier . Since no coils are required, the filter can also be used for audio frequencies and below.
Two frequency responses are shown in the picture: the sharp curve (black) for ideal components, the flat curve (blue) when R1 is increased by 5%: Even small tolerances worsen the quality factor of the bridge circuit in the vicinity of the phase jump and shift the frequency.
oscillator
In the Wien-Robinson oscillator , also known as an RC generator and laboratory device (tone generator), a Wien-Robinson bridge is used in an oscillator circuit as a frequency-determining element. If the phase shift of the Wien-Robinson term disappears at a certain frequency and the amplifier also generates 0 ° phase shift, a stability criterion by Barkhausen is fulfilled. Because the output signal in each branch of the Wien-Robinson Bridge is only 1/3 of the input voltage, the amplifier must also have a gain factor of 3.
If you look closely, the bridge diagonal voltage is then zero and the operational amplifier does not deliver a signal. Therefore the bridge has to be detuned slightly.
Amplitude control
The simplified circuit shown here has the following disadvantage:
- If the gain of the operational amplifier is less than 3, no oscillations begin.
- However, if the gain is greater than three, the amplitude of the alternating voltage generated continues to rise until the operational amplifier limits. Then the output voltage is no longer sinusoidal.
Therefore, an amplitude control is always required to reduce the gain as soon as the amplitude exceeds a certain value. Only then can an approximately sinusoidal signal with low distortion factor be generated.
In the simplest case, the amplitude control can take place by means of two anti-parallel connected diodes D1 and D2. The resistor R3 is slightly larger than R4, so that the gain is slightly larger than 3 and the circuit starts to oscillate. When the voltage on R3 reaches about 0.5V, current begins to flow through the diodes; This means that R3 is apparently smaller or the gain is reduced until the oscillation is just maintained. With the specified components, the frequency is 159 Hertz.
Because the differential resistance of the diodes depends on the voltage, the distortion factor is still considerable. Better actuators such as photoresistor and junction field effect transistor (each controlled with the output voltage) can - with increased circuit complexity - achieve extremely low values of up to 0.0003%. A clever method is the use of the PTC thermistor behavior of the tungsten filament of an incandescent lamp, which increases the negative feedback of the amplifier when heated (cause: increasing amplitude values).
For a sine wave generator with adjustable frequency, a stereo potentiometer is used for R1 and R2 or a double variable capacitor for small capacities .
Historical
The bridge circuit was invented by Max Wien in 1891 . The oscillator circuit is the result of William Hewlett's doctoral thesis at Stanford University in 1939. He used two electron tubes as an amplifier . The PTC thermistor behavior of an incandescent lamp, which served as a cathode resistor for one of the two triodes, was already used for gain control in the patent from July 11, 1939. Such PTC thermistors were also used later in many laboratory tone generators for amplitude control until the 1980s.
In order to market his invention, William Hewlett founded the company Hewlett-Packard together with David Packard , the first product of which was the Vienna bridge oscillator HP200A . His teacher Frederick Terman later said that this oscillator was the foundation for the Hewlett-Packard company.
literature
- Ulrich Tietze, Christoph Schenk, Eberhard Gamm: Semiconductor circuit technology . 10th edition. Springer-Verlag, Berlin 1993, ISBN 3-540-42849-6 . (in the 13th edition only the filter function is included)
- W. Benz, P. Heinks, L. Starke: Book of tables electronics communications engineering . 2nd Edition. Frankfurter Fachverlag, Frankfurt 1980, ISBN 3-87234-065-4 .
Individual evidence
- ^ Max Wien: Measurement of the induction constants with the "optical telephone" . In: Annals of Physics . tape 280 , no. 12 , 1891, p. 689-712 , doi : 10.1002 / andp.18912801208 .
- ↑ Patent US2268872 : Variable Frequency Oscillation Generator. Applied July 11, 1939 , published January 6, 1942 , inventor: William R. Hewlett.
- ↑ James E. Brittain: Electrical Engineering Hall of Fame: William R. Helwett in Proceedings of the IEEE Vol. 99, No. 1, January 2011, page 234