The Wien Bridge Oscillator

The Merits of Wien Bridge Oscillator types

The Wien Bridge oscillator is an oscillator type used in many circuits which rely on a sine wave to drive or act in part on a circuit designed for another purpose.

The Wien Bridge oscillator is possibly one of the more versatile and easier to build circuits, used often to generate a 20Hz to 20kHz sine wave for testing audio equipment.

The Wheatstone Bridge

But first of all, let’s look at the Wheatstone Bridge, another very important circuit and used to accurately measure very low (or high) resistances.  Most Ohm Meters or resistance checkers are not accurate when going down to below a few Ohms and the Wheatstone Bridge is the solution.

Wheatstone Bridge
Wheatstone Bridge

The circuit above works on the theory that if the potential difference across a Volt meter (galvonometer, a device which measures electric current), there would be no deflection of the needle as no current flows through the meter.

Now if there are two known resistors in circuit plus a carefully calibrated potentiometer, VR1 we can use the above circuit to calculate an unknown resistance.  If this was a device used to measure very low resistances, in the order of 0.1 Ohms we would have a current limiting resistor in the circuit, possibly in the (+) line of the PSU.

E.g R1 = 4.7 Ohms, R2 = 2.2 Ohms and the meter shows no deflection when VR1 is set to 4 Ohms the formula above would give an output, read off the potentiometer indicator as 1,87 Ohms.   Read up on Kirchoff’s Law.

This circuit is used for impedance measurement as well. Below is another variation, the Wien Bridge:

Wien Bridge (for capacitor check)
Wien Bridge (for capacitor check).

The Wien Bridge, known after Max Wien is similar to the Wheatstone Bridge where the potential divider networks need to have an equal ratio to null the network. This is often used to determine, capacitive, inductive (Maxwell-Wien), frequency and resistive values. In an AC circuit things become more complicated and a damned sight more interesting than DC. Both capacitive (Farad) and inductive (Henry) values need a known frequency to determine the capacitive or inductive reactance, resp. Xc or Xl. Xc = 1/(2πfC) and Xl = 2πfL.

In the circuit above there are two variable components, the capacitor C1 and VR1 in parallel. The component being checked is a capacitor which also has a high internal resistance.  By adjusting VR1 and C1 a null will be reached where the values of the two variable controls are read off and the parallel impedance calculated, and likewise the values of unknown components calculated through the null value balanced equation Z1/Z2 = Z3/Z4.

The Wien Bridge Oscillator

Basic Wien Bridge Oscillator
Basic Wien Bridge Oscillator – note the series R1C1 and parallel RC networks R2C2

The Wien Bridge oscillator is based on the bridge circuit.  The circuit above is the classic student’s build using a small incandescent bulb in the circuit to control the amplitude. (AGC).

The capacitors and resistors are of the same value. The result of this is C1 = C2 = circuit C and R1 = R2 = circuit R.  Frequency of oscillation is calculated as being 1/(2 * π * R * C).  Rf  is the positive feedback resistor.

A stable oscillation output is determined when the bulb filament resistance is equal to the feedback resistance / 2 or Br = Rf/2

Sine wave output is generally of good quality, undistorted and even amplitude. Limitation is generally only the lower frequency range, making it good for testing audio equipment.

Next:  Total Harmonic Distortion meter for audio equipment





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