Wien bridge

Wien Bridge with asymmetrical power supply

Normally, a Wien bridge oscillator contains two identical capacitors and two identical (variable) resistors. That being the case, the transfer factor of the bridge in Fig. 1 is 1:3. For example, when a potential of 1 V is applied to the non-inverting input of the opamp, the output voltage of the amplifier will be 3 V. In many cases, a smaller transfer factor is required. With reference to Fig. 2.

Up/Uo = 1(1 +R1/R2+C2/C1)

from which it follows that the factor becomes smaller if the value of R1 or C2 is increased. The frequency is altered when the value of both capacitors or of both resistors is changed. That makes it possible to vary the frequency by using a dual-gang potentiometer in place of the two resistors. Since the two resistances are then always identical, the ratio Up/Uo will be 1:12 when C2=10C1. To ensure sufficient positive feedback for the oscillator to start the amplification of the opamp must be >12. With values as shown in Fig. 1, the amplification is

a= 1+ (R5+P1)/ R3 = 13.8

Wien Bridge with asymmetrical power supply Circuit diagram:

Wien bridge

Stability of the output voltage is ensured in the traditional manner by two anti-parallel connected diodes in the feedback loop. Preset P1 is adjusted so that the sinusoidal output voltage is just not clipped by the supply voltage.

The frequency of the output signal may be set between 150 Hz and 1500 Hz with P2, higher frequencies may be obtained by altering the values of C1 and C2.

The supply voltage, which must be regulated, may lie between 9V and 12V. When the oscillator is not loaded , it draws a current of about 6 mA.

Wien bridge

Parts List:

Resistors:

R1, R2 =1kΩ

R3=4.7 kΩ

R4= 10Ω

R5=10kΩ

R6=47Ω

P1=47kΩ preset

P2= 10kΩ dual-gang linear preset

Capacitors:

C1=33nF

C2=330nF

C3, C5=47µF, 16 V, radial

C4=10 µF,16V, radial

C6=47nF

C7=100µF ,16 V, radial

Semiconductors:

D1,D2= 1N4148

Integrated Circuit:

IC1= LM386N-4

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