Wien bridge
A Wien- Robinson bridge is named after Max Wien bridge circuit in which a bridge path through a band pass, the other is formed by a 2:1 voltage divider.
Filter
The AC voltage is always supplied to unbalanced differential voltage evaluated Out1 - OUT2 at the frequency
Shows a minimum. Requirement is that the two resistors and capacitors in the bandpass are chosen to be equal. There is also a phase shift of -90 ° takes place after 90 °. Therefore, the Wien-Robinson bridge can be used in conjunction with an operational amplifier as a blocking filter. Since no coils are required, the filter is also used for audio frequencies and below.
The photo shows two frequency responses are shown: the clear differentiation curve ( black) for ideal components, the flat curve (blue ) when R1 is increased by 5%: Even small tolerances will impair the quality factor of the bridge circuit in the vicinity of the phase jump and shift the frequency.
Oscillator
In the Wien-Robinson oscillator a Wien-Robinson bridge is used in an oscillation circuit as a frequency determining element. When the phase shift of the Wien-Robinson member at a certain frequency disappears, and the amplifier also generates 0 ° phase shift, a stability criterion is satisfied by the Barkhausen. Because the output signal is only 1/ 3 of the input voltage in each branch of the Wien- Robinson bridge, the amplifier must also have the gain of 3.
Strictly speaking however, then the bridge diagonal voltage is zero, and the operational amplifier does not supply a signal. Therefore, the bridge must be slightly out of tune.
Amplitude control
The pictured here simplified circuit has the following disadvantage:
- Is the gain of the operational amplifier is less than 3, not start oscillation.
- But the gain is larger than three, the amplitude of the generated AC voltage rises still further, to the operational amplifier is limited. Then, the output voltage is not more sinusoidal.
Therefore, it is always an amplitude control is required to reduce the gain when the amplitude exceeds a certain value. Only then can a sinusoidal signal with low harmonic distortion can be produced.
The amplitude regulation may be performed by two anti-parallel diodes D1 and D2 in the simplest case. The resistor R3 is slightly larger than R4, so that the gain is slightly greater than 3, and the circuit oscillates. When the voltage at R3 is about 0.5 V is reached, current starts to flow through the diodes; thus R3 is seemingly small and the gain reduced until the vibration is just just maintained. With the specified components, the frequency is 159 Hz.
Because the differential resistance of the diode depends on the voltage, the THD is still substantial. With better control elements such as photo resistor and junction field- effect transistor can be - with increased circuit complexity - extremely low values reach up to 0.0003 %.
For a sine wave generator having an adjustable frequency Stereopotentiometer or small capacity, a double - tuning capacitor is used for R1 and R2.
Historical
The bridge circuit was discovered in 1891 by Max Wien. The oscillator circuit is the result of the PhD thesis of William Hewlett in 1939 at Stanford University in 1939. Instead of the time not yet invented operational amplifier he used a differential amplifier with electron tubes. To gain control the PTC behavior of a tungsten filament served in a light bulb.
To market his invention, William Hewlett founded with David Packard, Hewlett -Packard, the first product of the Vienna bridge oscillator HP200A was.