Oscillators are devices capable of repeating two opposing actions in a period regular. Example: movement a pendulum.
An example of the oscillator in the area of \u200b\u200belectronics, is the change in voltage or current at a specific point.
An LC circuit (inductor-capacitor) is capable of producing this oscillation at its natural frequency of resonance.
oscillators Applications:
- digital circuits (clock)
- Transmission and reception of radio
One type of oscillator called oscillator feedback and swing it should be in the positive feedback circuit.
The characteristics of the oscillators with feedback
Amplification 1 .- 2 .-
positive feedback loop
3 .- Circuit for controlling the frequency
A feedback oscillator is a circuit that uses an amplifier to supply the necessary energy to the oscillator and a feedback circuit to maintain the oscillation. It is in this feedback loop where you lose the power it has to supply the amplifier for the continued operation of the oscillator.
As the swing begins?
The starting voltage is generated by the same components of the oscillator. The resistors generate noise voltage is sinusoidal frequencies higher than 10,000,000,000,000 hertz. When the circuit starts the frequencies generated are amplified and appear at the output excites the resonant circuit that responds only one, which is fed to the input of the circuit with the proper phase to start the operation.
types of oscillators:
- phase shift oscillator
- Oscillator Armstrong (not widely used because of its instability)
- Hartley Oscillator
- Oscillator Colpits
Positive feedback
- Vi = input voltage
- Vo = Output voltage
- B = gain feedback circuit
- Ao = gain open loop amplifier with Ao = Vo / Vi (not taking into account the feedback). See graph
- Vf = voltage feedback
- Ac = closed loop gain
- Bao = The product (B x Ao) is called loop gain
For positive feedback, the closed loop gain is:
Ac = Ao / [1-bao]
If the product B x Ao is close to "1", the denominator of the above formula tends to "0" and consequently gain Ac closed loop tends to infinity. These gains as high oscillations.
phase shift oscillators by
The phase shift oscillators generally used in the feedback network (B) consisting of passive components (resistance and capacitors). See graph.
The amplifier stage (A), is an inverting amplifier with operational amplifier A2, so that your input signal is shifted 180 degrees.
Entondes can use a network (B) three-stage RC (R1C1, R2C2, R3C3. Each RC network moves 60 degrees) for the remaining 180 º and 360 º and add the necessary.
Figure A1 is an amplifier that is used to prevent phase shift network load input inverting amplifier A2. This is so because the amplifier A1 has a high input impedance. The output of A1 is the same phase as its input (it lags).
The oscillation frequency is given by the following formula:
The amplifier A2 provides the gain needed to maintain the oscillation and can be calculated with the formula : Gain = - R5 / R4, where the minus sign means phase reversal. With R2 = R1 = 36K and 1K, the gain is 36.
If the attenuation caused by the RC network, is less than expected, the loop gain is greater than 1 (the open loop gain desabled is 1). The output signal then grows until the amplifier amplified distortion.
As the noninverting input of amplifier A2 is grounded and the inverting input of the amplifier is a virtual ground, the inverting input is kept close to 0 volts.
To prevent the gain is greater than 1, there are two diodes (D1 and D2) that lead when the output sine A2 in the positive direction is greater than 0.7V, and lower negative a - 0.7V.
When the output of A2 is approximately 0.7 V, D1 leads putting resistors R5 and R6 in parallel, the same happens when the signal is approximately -0.7, D2 leads paralleling the same strength. Entoces the gain of A2 is = (R5 / / R6) / R4 = (36k / / 8.2K) / 1K = 6.5. Gain is less than 36 earlier. Thus the output voltage is approximately 1.4V peak to peak.
Note:
- / / means parallel
- A1, A2: general-purpose operational amplifiers IT LM348N (4 operational)
Wein Bridge Oscillator: Gain, feedback network
gain, feedback
Oscillator Wien bridge oscillator is used to generate sine waves ranging from 5 Hz to 5 MHz.
Unlike the phase shift oscillator, has fewer components and adjusting the oscillation frequency is easier, why which is most used.
The basic circuit consists of an amplifier and a network of Adelaide / being late, composed of two RC networks, each series parallel. The two values \u200b\u200bof resistors and capacitors are equal.
Gain
The amplifier gain is given by the resistors R1 and R2.
The gain that must have this amplifier must compensate for the attenuation caused by the RC network (positive feedback network connected to pin noninverting operational amplifier). This gain must be above 1 to ensure oscillation.
gain obtained with the first formula. As the gain must be greater than 1, the equation is simplified and we obtain the second formula: 
see that for this to give the ratio of R2 and R1 should be equal to or greater than 2.
Red feedback and lag
The output of the feedback network behaves as follows way:
- For frequencies below the oscillation frequency is large attenuation and phase by 90 °
- A resonant frequency voltage gain is 1 / 3 (maximum) and no phase shift.
- Holm For frequencies of oscillation frequency, the attenuation is large and the phase is delayed 90 °.
Wein Bridge Oscillator
oscillation frequency
To achieve oscillation, it is necessary that the time lag or phase shift is 360 degrees or whatever it is, that the gap is 0 °.
To derive the oscillation formula, follow the steps shown in Fig.
The first equation is that for it to be equal to 0, the contents of paréntesisi must be equal to 0.
The equality of the second equation allows w clear after frequency f. At the end of the simplification is that the frequency depends on the values \u200b\u200bof capacitor C and resistance R. Remember that w = 2Pif
A real Wien bridge oscillator
The values \u200b\u200bof resistors and capacitors of the RC networks, R3 = R4 = 16.2K and C3 = C4 = 0.01uF.
also see the inverting amplifier with resistors R1 and R2 set the gain of the amplifier. R1 = 10K and R2 is made a resistor in series with a potentiometer R2 = R + P. The resistance R = 18K potentiometer P = 5K.
The potentiometer is connected as a variable resistor and when it has its minimum value, (0 ohms), the value of R2 = 18K. When the pot has its maximum value (5K), R2 = 23K.
With these data the amplifier gain varies from 1.8 to 2.3 (greater than 1)
The box formed by the diode bridge and diode aims at limiting zener operational amplifier output to a maximum positive and negative 7 volts to -7 volts.
The diode bridge provides a voltage of 5.6 volts for both the negative cycle and to the positive.
This tension coupled to two voltage drop of two diodes (0.7 +0.7 = 1.4), add the above 7 volts.
Hartley oscillator
The Hartley oscillator is an oscillator type widely used in transistor radios easily adapted to a wide range of frequencies. For his performance this circuit uses a center tapped coil.
Analyzing the diagram, we see that the branch point D of the coil L1, will be grounded for alternating current (AC) (at the oscillation frequency) across the capacitor C4.
In this way we obtain the ends A and B of the coil are 180 ° out of phase (it works as an investor).
The end B is fed to the base of the transistor through C1, causing it (the transistor) changes state, this in turn changes the polarity at the ends of the coil, the process is repeated and thus producing oscillation.
The function of the coil L2 is shock and prevents RF oscillator signal passing to the power supply.
Analyzing the performance of the coil to pass and taking into account that the connection D (central branch) is ground through the capacitor C4, the waveforms at the ends of the coil will be: 
The oscillation frequency of this type of oscillator is given by the formula:
for = 1 / [2π x (LC) 1 / 2].
Notes:
- C3 may be a variable capacitor to adjust the oscillation frequency
- The exponent 1 / 2 equals the square root
The Colpitts oscillator is a type of oscillator is often used in generating high quality and is primarily used for frequencies above 1 MHz. Its stability is higher than the Hartley oscillator.
To achieve this oscillation circuit uses a voltage divider formed by two capacitors, C1 and C2.
From the junction of these capacitors grounded out. Thus the higher terminal voltage of C1 and C2 will lower opposing tensions.
The positive feedback obtained from the lower terminal of C2 and is carried to the base of the transistor through a resistor and a capacitor
L2 coil (choke) is used to prevent the AC signal does not pass the Vcc supply
This oscillator is used for VHF (Very High Frequency) frequencies from 1 Mhz to 30 Mhz.
At these frequencies would be very difficult to use Hartley oscillator because the coils to be used would be very small.
The oscillation frequency of this type of oscillator is given by:
for = 1 / [2π x (LC) 1 / 2]
where :
- C = C1xC2 / [C1 + C2]
- L = L1
Notes:
- R1 can be a variable resistor (potentiometer) to adjust the magnitude of the output signal is fed to the input.
- The exponent 1 / 2 equals the square root.
Mario Dominguez Zambrano
EES Section: 02
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