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Low Frequency Wien Bridge RC Sinewave Oscillators

2016-05-15 18:42  
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Low Frequency Wien Bridge RC Sinewave Oscillators
Low Frequency Wien Bridge RC Sinewave Oscillators
Three examples of Wien Bridge oscillators are shown below. The first uses three bipolar transistors. The second uses a bipolar and JFET, and the third is the more popular type using an op-amp for minimal parts. The idea is to generate a 360 degree phase shift at some particular frequency using 2 resistors and caps of equal value. One cap and resistor are in series, while another cap and resistor are in parallel. The signal loss through the network is about 66 percent so the amplifier gain needs to be around 3 for a loop gain of 1. The gain of the amplifier is critical since too much gain will produce a clipped (distorted) waveform and not enough gain will not sustain oscillation. This is almost impossible to achieve without some automatic gain control (AGC) to regulate the gain and produce stable operation. The usual AGC is accomplished with a small light bulb where resistance increases as the signal level rises and reduces the gain. The lamp used here is a 1819 (28 volt 40mA) variety found at Radio Shack, part number 272-1119. Another lamp that might be useful is the GE394, 12 volt 40mA, but a little harder to find. In the first example, the lamp is placed in series with a 1000uF cap and connected across the emitter resistor of the 2N2219A so as the signal level rises, the total resistance increases reducing the gain. The gain of the 2N2219A stage is approximately the collector resistor (100) divided by the emitter resistor (51 in parallel with the lamp 75) or maybe 100/30 = 3.3, The first stage (2N3904 on the left) provides a high impedance to the RC network so it doesn't load down the input much. The second stage (2N3904) in the middle, provides a 180 degree phase inversion and not much voltage gain. So, the overall phase shift is 360 degrees, 180 from the middle stage and another 180 from the 2N2219A stage. The overall gain can be adjusted with the 750 ohm resistor at the collector of the center stage. The example shows 2 (20K) variable resistors which are ganged together for frequency adjustment of about 10KHz to 400 Khz. Lower frequencies can be obtained using larger capacitors. The frequency of oscillation is f = 1/(2 * Pi * R * C). The circuit was built sucessfully and also simulated using LTSpice version IV. A copy of LTSpice can be downloaded from the following link.