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:
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.
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
