As the name suggests, a voltage controlled oscillator or a VCO circuit is an oscillator circuit whose output frequency can be controlled or varied through an adjustable control voltage input. This means, if the input control voltage is increased, the output frequency will increase proportionately, and vice versa.
If the control voltage varies continuously up and down, then the output frequency will also correspondingly increase and decrease, in response to the varying amplitude of the control voltage levels.
Just like any other VCO circuit, in the discussed circuit below, the output frequency is determined by the level of the control voltage applied.
- The main advantage of this VCO design is that it comes with a broad control voltage range, which extends right from 0 V up to the maximum positive supply voltage limit. The power supply level could be anything from +3V to +25 V.
- Additionally this voltage controlled oscillator (VCO) is designed to generate both triangular as well as a squarewave output signals.
Having said that, the user has to be careful while working with low voltage inputs, and ensure that the highest output voltage level is at least 1.5 V lower than the supply input.
How the Circuit Works
The circuit functions by using the 'integrator/comparator' theory. Capacitor C1 forms the part of the integrator (designed around opamp A1), and it is charged using a constant current source as determined by the instantaneous level of the applied control voltage.
As a result, the A1 output voltage falls linearly. The comparator output (built using A2) changes state and transistor T1 begins conducting as soon as the comparator's bottom switching threshold is attained. Capacitor C1 atthis point tes discharged triggering the A1 output to increase (again, this rising voltage is linear in nature). The action repeats the moment the A1 output extends to the comparator's upper switching limit, and this switches OFF T1.
The duty cycle of the output voltage controlled frequency is going to be FIFTY PERCENT if the R2 and R3 values are identical (R2 = R3), and when the value of R1 is two times than that of R4 (R1 = 2 x R4).
The relationship between the R9 and R10 values becomes responsible for the level of the triangular DC output voltage. With respect to the R9/R10 values shown in the schematic, the triangular DC level will likely be 1 / 2 the supply voltage. The peak-to-peak output level, can be determined by solving the following equation:
(Vpp) = R5 / (R5 + R6) x Vin
Control Voltage, Supply Voltage Relationship
The characteristics of the proposed voltage controlled oscillator or the VCO using a pair of (common) supply voltages can be witnessed in the following graph.
When both the supply inputs are equal (Vin = Vc) in magnitude the maximum frequency that can be achieved from the circuit could be increased or decreased by correspondingly selecting the C1 value lower or higher. With regards to the opamp's slew rate, the vertical angle of the squarewave signal may decline for the higher frequencies.
- R1, R5, R6 = 100K
- R2, R3, R4 = 47K
- R7, R8, R9, R10 = 10K
- C1 = 47 nF
- T1 = BC547
- A1, A2 = LM358
VCO using IC 555
A neat little voltage controlled oscillator can also be built using the resolute IC 555 and a few other assisting components, as shown below:
The discharge process of capacitor C1 is voltage controlled. This is implemented by the quick charging pulses controlled by the internal discharge transistor of IC2. To understand the working, imagine the pin3 output of timer IC2 is initially low and the discharge transistor pin 7 is switch ON, pulling current through R6 to activate transistor TR3. This particular transistor maintains the transistor TR2 base at the positive supply voltage. This guarantees that it is switched off.
The capacitor C1 may consequently start discharging via R4 with a rate as decided by the TR1 collector current. As the voltage across C1 drops to around 1/3rd supply voltage level, the IC555 output turns high. Simultaneously the discharge transistor along with TR3 also switch OFF, which means that current through R3 now activates TR2, which begins charging C1 via R4. As soon as the voltage across C1 gets to 2/3rd of the supply voltage, IC2 yet again changes state and the cycle keeps repeating. A functioning property of transistors is such that practically all the current going to the emitter side, runs out back from the collector. A tiny amount of current passes out from the base, however normally, this is very small to be considered.
Due to the fact that the emitter voltage follows the applied base voltage, deducting the base/emitter forward drop of around 0.6 volts, this voltage output enables you to control the voltage over the emitter resistor R2 with this circuit. The transistor base voltage as a result controls the collector current, giving rise to a voltage controlled current source.
The control voltage could be applied directly on the transistor TR1 base in order to control the frequency output in this circuit. However, if an op-amp is employed with the transistor as displayed in the next diagram below, then the voltage across R2 may follow accurately the input voltage, and the the op-amp would automatically begin compensating for the emitter-base voltage drop as well as disparities due to temperature changes. The resulting output frequency control range can then begin from zero rather than 0.6 volts.
Calculations required for designing this IC555 based VCO circuit can be as easy as for the simple fixed-frequency circuits, which means just a couple of rules must be followed. Resistor R4 has to be relatively low in values compared to the value of R2. There should always be sufficient voltage across TR 1 for it to work, therefore the input must in no way cross 1 V value below 1/3rd supply voltage. This particular voltage controlled oscillator circuit is vulnerable to variations in the supply voltage, therefore the supply should be strictly regulated. Once the maximum control voltage range is fixed, the period for the discharge element of every single cycle could be worked out using the formula:
t = (C x R x E) / V
Here C repersents the timing capacitor C1, R stands for the emitter resistor R2, E indicates 1/3rd of the supply level and V repersents the voltage across R2.
To th result from the above expression, we have to add the time taken for the charging portion of the cycle:
t = 0.7 x C1 x R4
The two elemnts collectively provide the full period for each cycle hence, as a common practice, their inverse provides the equivalent frequency. With the part values indicated with a 9 V supply input, the circuit produces a frequency response from 0 Hz to approximately 1kHz foin response to an input control voltage between 0 to 2 volts.
The output frequency is composed of positive pulses of 100us width, excellent for generating a wide range of noise over a small loudspeaker using the most basic of output buffer stages, maybe just one BJT stage. Using C1 as a 1 nF capacitor it delivers approximately 9 kHz for exactly the same input voltage range, using proportionately shorter output pulses.
Which Opamps are Suitable
Remember that in case an op-amp is employed as indicated in the above VCO circuit diagram, it has to be of a kind having input and output rated to work right down to the negative supply voltage. The IC CA3140 as indicated is acceptable, some other possible variants could be the CA3130, or one of the opamps from the IC CA3240, LM358 or LM324.
Various other transistors may also work very well instead of the one shown in the diagram, as long as these are assembled with the proper polarity.
The practical use of this IC 555 voltage controlled oscillator circuit may not be extensive, and may be restricted to generally with audio related work, but the design is very cheap, straightforward and pretty stable with its specifications.