The high-gain and wide passband of operational amplifier (op amp) ICs makes it possible for these units to work like an oscillator within extensive frequency ranges. The op amp oscillates immediately as soon as a feedback is employed across its noninverting input and tweaked to the appropriate amplitude. The IC becomes particularly important in resistance-capacitance-tuned oscillators, because its total gain becomes perfectly suitable to offset the attenuation of the RC network.
Additionally, the double input of the differential IC enables not only positive feedback (for oscillation), but additionally allows the negative feedback in a few circuits (for enhancing the output waveform).
In this article we learn about seven op amp IC oscillators, with three RC configurations, three LC configurations, and one crystal set up. These are very common op amp oscillator circuits which could be applied with pretty much any high-gain operational-amplifier IC.
Since any op amp can be used in these configurations, the precise pin numbers aren't specified, except from the standard pins like the inverting input, noninverting input, input ground (common), and output.
The voltage supply pinouts are not indicated, considering that these (Vcc and Vee pinouts) can change with the specific IC. Wherever feasible, the maximum functional working details is provided for all these basic configurations.
TWIN-T RC AF OSCILLATOR
The first figure below illustrates the a resistance-capacitance tuned AF op amp oscillator twin-T null network. This is implemented through the RC network of C1-C2-C3-R2-R3-R4, which decides the frequency level of the output.
The op amp IC needs to have a voltage gain of 60 dB. This set up is basically an intensely tuned af amplifier on which a positive feedback is introduced so that it begins oscillating.
The twin-T network is introduced in the negative-feedback loop, and since it works like a null network it helps in eliminating a particular frequency (fr) through the negative feedback. The IC gain gets terminated appropriately at all other frequencies, while fr is conveniently transmitted forward. In this oscillator network, the following formulas can be used for the calculations:
- C1 = C2 = 1/2 C3
- R2 = R3 = 2R4
- fr = 1 / (2πR2C1)
In the above formula,
C's will be in farads,
R's will be in ohms,
and the unit of fr will be in hertz.
Coupling capacitor C4 blocks the DC current, and offers a high capacitance with respect to the capacitors C1, C2, and C3. It consequently doesn't have an effect on fr.
Resistor R1 offers a high resistance, based on the IC's input impedance, and helps to decrease loading of the twin-T network through the IC input circuit.
The inclusion of positive feedback, by means of the signal divider R5-R6, results in this very sharply tuned amplifier to oscillate with the frequency fr. When the potentiometer R6 is adjusted to the level where the circuit merely starts to oscillate, output-signal distortion is found to be minimal.
The op amp oscillator circuit could be made to work like a continuously tunable oscillator (e.g., from 20 Hz to 20 kHz) simply by replacing the resistors R2, R3, R4 with a 3-ganged potentiometer, and selecting capacitors C1, C2, and C3 in trios to adjust the bands.
Output control could be accomplished through a potentiometer which may be configured either to the output or introduced between stages of the IC.
PHASE-SHIFT RC AF OSCILLATOR
The phase-shift form of tuned resistance-capacitance audio frequency oscillator are popular for their extremely low harmonic distortion. In this form of oscillator, RC tuning is achieved through a 180-degree phase-shift network configured within the feedback loop of an inverting amplifier.
The network consequently generates the precise signal phase rotation for the oscillation. Fig. 2 above demonstrates the configuration for a phase-shift-type IC oscillator. In this set up, the phase shift network includes 3 RC legs in cascade pattern which are identical to each other: C1-R2, C2-R3, and C3-R4.
In this network , C1 = C2 = C3, and R2 = R3 = R4. All of these legs brings about a 60 degrees of phase shift. The frequency level where the entire shift gets to 180 degrees can be determined through the below given formula:
fr= 1/ (10.88R2C1)
To be able to offset the built in attenuation of the RC system, the op amp's gain must be around 40 dB. The op amp's noninverting input isn't utilized in this configuration, as it is delivered back to the ground by means of the resistor R1 ( which can be around 1000 ohms).
This op amp oscillator circuit could be turned into a continuously tunable type in the range of 20 Hz to 20 kHz, simply by replacing resistors R2, R3, R4, with a 3-gang potentiometer, and by switching the capacitors C1, C2, and C3 in trios, in order to select the bands.
But, the circuit's attenuation increases as the values of the resistances is reduced, and this situation could lead to the output-signal amplitude going down very fast with frequency, and oscillation may simply stop if R2, R3, and R4 are adjusted to lower values with the high frequencies.
Output control could be provided through a potentiometer which could be either connected to the output of the op amp oscillator circuit or introduced between the stages of the op amp.
WIEN-BRIDGE RC AF OSCILLATOR
We have so far learned that the resistance-capacitance-tuned audio frequency oscillators involve 3 resistances and 3 capacitances for the tuning process. However, the op amp oscillator circuit displayed in Fig. 3 below requires just a couple of resistances (R1 and R2) and a pair of capacitances (C1 and C2) for the same tuning facility.
The C1-C2-R1-R2 network forms the 50 % of a Wien bridge network which, just like the twin-T network in Fig. 3-1, is sensitive to the frequency. The RC configuration is attached inside the positive-feedback link (on the noninverting input terminal). In this configuration, C1= C2, R1 = R2, and the frequency response can be determined using the below given equation:
fr = 1 / (2πR1C1)
In the above equation,
fr will be in hertz,
R1 will be in ohms,
C1 will be in farads
The resistors R3, and R4 does the job of providing the negative feedback (that works like a a signal voltage divider) to the inverting input terminal. This negative feedback helps to reduce the output-signal distortion; but, it should be implemented with the right proportion (by appropriately adjusting R3 and R4). This is important to ensure that it doesn't terminate the positive feedback leading to the elimination of the oscillation.
This form of op amp oscillator circuits could be easily turned into a continuously tunable oscillator in the range of 20 Hz to 1 MHz, simply by replacing the resistors R1, and R2 with a dual-gang potentiometer, and by ensuring that the capacitors C1 and C2 are switched in pairs in order to adjust frequency bands.
The tuning of the oscillator circuit can be implemented over a large frequency range, but, its output-signal amplitude might show a varying tendency with response to the frequency.
Nevertheless this varying tendency of the amplitude could be decreased using an appropriately dimensioned nonlinear resistor, for instance a thermistor, varistor, or double-ended zener diode, hooked up in between point "X" and ground. Controlling the output of the oscillator can be executed through a potentiometer which may be either incorporated into the output or placed between the stages of the IC.
TRANSFORMER-FEEDBACK AF OSCILLATOR
Fig. 4 below displays the a basic af oscillator configuration where a small audio transformer (T1) is used for providing both the feedback and the tuning which forces the op amp to work like an oscillator. Any low-gain op amp IC could work effectively in this configuration.
In this oscillator set up, transformer T1 provides the positive feedback to the non-inverting input of the op amp. T1 transformer can be any form of small transformer. The frequency of oscillation can be calculated through the capacitance C1 and the inductance (L) of the L1 transformer winding. The below given formulas can be used for the Fig. 3-4:
- C1 = 1 / (4π2F2L)
- L = 1 / (4π2F2C1)
- fr = 1 / (2π √LC1)
In the above formula,
fr is measured in hertz,
L is measured in henrys,
C1 is measured in farads
It is crucial to ensure that the connection of the transformer winding with the circuit is phased correctly for sustaining the oscillation. However if you find oscillations not working because of wrong transformer winding connections, you may need only one winding of the transformer to be reversed, that's all.
Resistors R1 and R2 do the job of applying the negative feedback, through a signal-voltage divider, to the inverting input terminal. This negative feedback helps to minimize output-signal distortion; but, this must be applied in a properly balanced manner (by suitably adjusting R1 and R2).
This is important to ensure that it doesn't suppress the positive feedback and as a result destroys the oscillation. The output can be controlled through a potentiometer which may be in two ways either to the output or placed between the IC stages.
TRANSFORMER-FEEDBACK RF OSCILLATOR
The radio-frequency or the RF oscillator circuit indicated below in Fig. 5 is identical to the transformer-feedback af oscillator explained in the previous section, except that in this particular rf oscillator, positive feedback for the oscillation and tuning are supplied through an air-core transformer, L1-L2. Any low-gain IC could work effectively in this circuit.
In this op amp oscillator circuit set up, the noninverting input of the op amp gets the positive feedback by means of the inductors L1-L2 together. (L1 can be the winding of the transformer that consists of higher number of turns, while L2 can approximately have one-quarter of the turns in L1. L2 winding needs to be wound closely to the L2 winding, however this coupling does not need to be tightly wound. The oscillation frequency fr can be calculated through the values of the capacitor C1 and the inductor L1.
This frequency fr can be determined through formulas as given below:
- C1 = 1 / (4π2F2L)
- L1 = 1 / (4π2F2C1)
- fr = 1 / (2π √L1C1)
In the above equation, the unit of C1 is picofarads, unit of L1 is in microhenrys, and the unit of fr is in megahertz.
It is crucial to connect the transformer winding with the right phasing in order to start the oscillation. That said, if you find the oscillations not happening, because of a wrong winding connection, you would only require just one winding ends to be reversed, that's all.
R1 and R2 performs the role of applying the negative feedback to the inverting input terminal of the op amp, by forming a signal-voltage divider. This negative feedback helps to stabilizes the working of the op amp oscillator and minimizes the output-signal distortion.
But, this negative feedback needs to be correctly balanced by appropriately adjusting the values of R1 and R2. This is important to ensure that it doesn't terminate the positive feedback and in the course destroys the oscillation.
The variable capacitor C1 can be used for getting a continuous tuning of the oscillator. In order to get a larger range of adjustment, L1 and L2 could be modified as pairs for selecting the frequency bands.
USING AF/RF OSCILLATOR TOGETHER
The Fig. 6 shown below exhibits an oscillator configuration that, which uses plug-in type inductors and capacitors, for producing both audio-frequency or radio-frequency signals through a diverse range of frequency.
In this op amp oscillator set up, the positive and negative feedback are used both together. The tuned circuit built using the L1, C1 parts connected in the negative-feedback loop decides the oscillation frequency of the circuit. Since the L1, C1 works like a wavetrap, this tuned stage of the circuit eliminates its resonant frequency, fr, from the feedback loop.
The op amp consequently behaves like a sharply tuned amplifier, becomes responsible for transmitting the frequency fr. However, its gain gets terminated at all other frequencies. The positive feedback, provided by the resistors R1 and R2 which are configured like a signal voltage divider, subsequently enables the op amp to oscillate at frequency fr.
The parameters in this circuit can be calculated using the following given formulas:
Using a high value capacitance and inductance together enables the generation of audio frequencies, whereas using lower values of capacitance and inductance enables the production of radio frequencies.
The strength at which the oscillation takes place is determined by the the adjustment of positive-feedback potentiometer R2. Therefore the R2 allows controlling the level of output-signal amplitude. The circuit can work using a number of different op amps.
Considering that output-coupling capacitor C2 is required to generate both low and high frequencies, its value must be suitably adjusted to some intermediate value, for example it can be 0.01 µF, unless of course the user is prepared to change this capacitor in addition to L1 and C1.
CRYSTAL-CONTROLLED RF OSCILLATOR
The below given Fig. 7 demonstrates an op amp based a crystal-controlled RF oscillator circuit that can work without the need of any tuning. This configuration can operate with multistage op amp circuits with various sensitivity levels. Having said that,it is advised to make use of medium and high-gain op amps.
In this configuration, the crystal (XTAL) works like an exceptionally high-Q bandpass filter within the op amps's positive-feedback loop. The positive-feedback current transferred through the crystal, builds up a voltage drop around resistor R2, which is utilized on the op amp's noninverting input pin of the IC.
This finally forces the circuity to oscillate with the crystal frequency. Capacitor C1 is utilized simply for blocking the DC content, through a capacitance whose value specifically selected for low reactance at the crystal frequency.
The input ground of the op amp which is common ground is connected directly to the circuit ground, as indicated in the diagram. The circuit can be also seen having a negative-feedback loop, created using the resistors R1 and R3, which constitues a signal voltage divider network.
Negative-feedback current flowing by means of this resistive divider causes a voltage drop to appear across resistor R1, which is supplied to the op amp's inverting input pin.
It may be important to adjust the amplitude of this voltage by appropriately setting up the values of the resistors R1 and R3. This ensures that the positive feedback does not gets terminated, eventually killing the oscillation.
The use of the negative feedback helps to enhance stability of this op amp oscillator circuit and also helps to minimize the output-signal distortion. Nevertheless, this could be totally furnished only when a high harmonic output is needed, for example, in many of the transmitter type circuit applications.