It is shown here how to build practical Wien-Bridge Oscillator with automatic amplitude control using diodes only, using JFET and diodes and finally using limiting or clamping diodes. In the previous tutorial How Wien-Bridge Oscillator Works we showed how basic Wien-Bridge oscillator works. The basic Wien-Bridge oscillator suffers from amplitude stabilization due to component value drift in actual real life circuit which unbalances the bridge circuit. The amplitude drift is due to the changes in the gain resistors values. In this tutorial we show how non-linear elements like diodes and transistors can be used to build a practical Op-Amp based Wein-Bridge oscillator with automatic amplitude stabilization.
Automatic Amplitude Control Wien-Bridge Oscillator with Diodes
The following shows circuit diagram of Op-Amp based Wien bridge oscillator with diodes for automatic amplitude control.
In the above Wien-Bridge oscillator circuit diagram, a simple diode-resistor network is added to control the effective value of the feedback resistor RF. When the signal level is low the diodes are turned off and so the resistor R4=100KOhm has to effect on the value of RF. In this case the condition for sustained oscillation is met because the ratio of RF/R3>2 and so oscillation will start. When the signal level is high enough and at its extreme level, the diode are forward biased(>0.65V), the effective value of the feedback resistance is RF||R4 = 22KOhm||100KOhm = 18KOhm so that the Transfer Function magnitude is T(jw)<0.9 which is below the value for sustained oscillation. However, before reaching this value the signal amplitude is automatically stabilized.
The Wien-Bridge oscillator frequency formula is as follow.
\(F_{r}= \frac{1}{2\pi RC}\) ------------->(1)
where, R=R1=R2=1.6KOhm and C=C1=C2=100nF=0.1uF in the above wien bridge oscillator schematic diagram.
With the value of R and C in the above RC Wien-Bridge oscillator, the frequency of sine wave output around 1KHz frequency. This can also be calculated easily using the
Automatic Amplitude Control Wien-Bridge Oscillator with JFET & Diodes
The following is Wien-Bridge Oscillator circuit diagram implemented with JFET transistor and Diode for automatic amplitude control.
In the above automatic amplitude control Wien-Bridge oscillator circuit diagram, an 2N7000 N-channel JFET and 1N5817 switching diode along with capacitor(C3) and resistors(R4 and R5) are used to control the stabilization of the amplitude. When power is applied to the oscillator, initially, the capacitor is still uncharged and therefore the JFET gate voltage is near 0V(see how JFET transistor works)When the gate voltage is low there is little resistance by the JFET transistor and so it effectively shorts the resistor R4=100KOhm to the ground. In this moment, the effective feedback resistance is RF/R3=RF/(R3||R4)=20KOhm/(10KOhm||100KOhm)>2 and oscillation will start. The diode 1N5817 D1 and the capacitor C3 forms a negative peak detector. As the oscillation grows the voltage across the diode and capacitor becomes gradually more negative and therefore the JFET conductivity decreases as oscillation grows. At the extreme of JFET cutoff we will have RF/R3=20KOhm/10KOhm=2 or less than 2. But for sustained oscillation we should have RF/R3>2 but at the intermediate signal level the JFET is brought to conduction and automatic amplitude stabilization happens.
Wien-Bridge Oscillator Automatic Amplitude Control with Clamping Diodes
Below is another Wien-Bridge Oscillator automatic amplitude control circuit diagram implemented with clamping diodes.
In this automatic amplitude control using limiting diodes, the diodes are cutoff when the signal level is low(around >0.65V for silicon diodes). In this situation the ratio RF/R3=20KOhm/10KOhm is greater than 2 and oscillation starts. The oscillation grows and the two diodes become alternatively conductive. Due to the symmetrical placement of the biasing resistors(R4=R5, R6=R7) the output signal amplitude is limited in amplitude symmetrically.
Practical Wien-Bridge oscillator
The above illustrated Wien-Bridge oscillators are practical Wien-Bridge oscillator. A real life Op-Amp Oscillator circuit have component drift due to temperature variation, due to environmental condition, component manufacturing defects or symmetry. Hence real life practical Wien Bridge incorporates automatic amplitude variation compensation which is achieved using diodes and transistors.
