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70-428: The internal synthesizer features one voltage-controlled oscillator with simultaneous saw, square/pulse-width and sub-octave square waveforms. Additionally there is a 24dB Low Pass filter, an LFO and a single ADSR envelope generator. In terms of circuitry, it is nearly identical to the earlier SH-101 synthesizers but lacks the noise generator, choice of LFO shapes and modulation/pitch bend controls. However, unlike

140-515: A light-emitting diode LED1 as shown in Figure 5. The LED voltage drop ( V D ) is now used to derive the constant voltage and also has the additional advantage of tracking (compensating) V BE changes due to temperature. R 2 is calculated as and R 1 as Temperature changes will change the output current delivered by the circuit of Figure 4 because V BE is sensitive to temperature. Temperature dependence can be compensated using

210-542: A 4.7 kΩ resistor will provide an approximately constant current of 1 mA ± 5% to a load resistance in the range of 50 to 450 Ω. A Van de Graaff generator is an example of such a high voltage current source. It behaves as an almost constant current source because of its very high output voltage coupled with its very high output resistance and so it supplies the same few microamperes at any output voltage up to hundreds of thousands of volts (or even tens of megavolts ) for large laboratory versions. In these circuits

280-433: A JFET in the circuits listed below for similar functionality. An example: bootstrapped current source. The simple resistor passive current source is ideal only when the voltage across it is zero; so voltage compensation by applying parallel negative feedback might be considered to improve the source. Operational amplifiers with feedback effectively work to minimise the voltage across their inputs. This results in making

350-438: A calibrated Becquerel (decays per second) amount of Ra offer a constant number of charge carriers per second for conduction, which determines the maximum current the tube can pass over a voltage range from 25 to 500 V. Most sources of electrical energy ( mains electricity , a battery , etc.) are best modeled as voltage sources , however some (notably solar cells ) are better modeled using current sources. Sometimes it

420-460: A certain minimum value, the JFET enters saturation where current is approximately constant. This configuration is known as a constant-current diode , as it behaves much like a dual to the constant voltage diode ( Zener diode ) used in simple voltage sources. Due to the large variability in saturation current of JFETs, it is common to also include a source resistor (shown in the adjacent image) which allows

490-400: A circuit, it is called an independent current source. Conversely, if the current through an ideal current source is determined by some other voltage or current in a circuit, it is called a dependent or controlled current source . Symbols for these sources are shown in Figure 2. The internal resistance of an ideal current source is infinite. An independent current source with zero current

560-438: A constant current. A dependent current source delivers a current which is proportional to some other voltage or current in the circuit. An ideal current source generates a current that is independent of the voltage changes across it. An ideal current source is a mathematical model, which real devices can approach very closely. If the current through an ideal current source can be specified independently of any other variable in

630-485: A constant input current or voltage and common source ( common cathode ) driven by a constant voltage naturally behave as current sources (or sinks) because the output impedance of these devices is naturally high. The output part of the simple current mirror is an example of such a current source widely used in integrated circuits . The common base , common gate and common grid configurations can serve as constant current sources as well. A JFET can be made to act as

700-407: A constant resistor; so, a constant current I OUT = V R / R = V IN / R flows through the resistor and respectively through the load. If the input voltage varies, this arrangement will act as a voltage-to-current converter (voltage-controlled current source, VCCS); it can be thought as a reversed (by means of negative feedback) current-to-voltage converter. The resistance R determines

770-499: A control system, the Laplace transforms of the above signals are useful. Tuning range, tuning gain and phase noise are the important characteristics of a VCO. Generally, low phase noise is preferred in a VCO. Tuning gain and noise present in the control signal affect the phase noise; high noise or high tuning gain imply more phase noise. Other important elements that determine the phase noise are sources of flicker noise (1/ f noise) in

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840-472: A current source by tying its gate to its source. The current then flowing is the I DSS of the FET. These can be purchased with this connection already made and in this case the devices are called current regulator diodes or constant current diodes or current limiting diodes (CLD). Alternatively, an enhancement-mode N-channel MOSFET (metal–oxide–semiconductor field-effect transistor) could be used instead of

910-448: A given temperature, it follows that V R2 is constant and hence I E is also constant. Due to transistor action, emitter current, I E , is very nearly equal to the collector current, I C , of the transistor (which in turn, is the current through the load). Thus, the load current is constant (neglecting the output resistance of the transistor due to the Early effect ) and

980-407: A load with zero voltage drop across its terminals (a short circuit, an uncharged capacitor, a charged inductor, a virtual ground circuit, etc.) The current delivered to a load with nonzero voltage (drop) across its terminals (a linear or nonlinear resistor with a finite resistance, a charged capacitor, an uncharged inductor, a voltage source, etc.) will always be different. It is given by the ratio of

1050-452: A long distance, since the frequency will not drift or be affected by noise. Oscillators in this application may have sine or square wave outputs. Where the oscillator drives equipment that may generate radio-frequency interference, adding a varying voltage to its control input, called dithering , can disperse the interference spectrum to make it less objectionable (see spread spectrum clock ). Current source A current source

1120-780: A stable single-frequency clock. A digitally controlled oscillator based on a frequency synthesizer may serve as a digital alternative to analog voltage controlled oscillator circuits. VCOs are used in function generators , phase-locked loops including frequency synthesizers used in communication equipment and the production of electronic music , to generate variable tones in synthesizers . Function generators are low-frequency oscillators which feature multiple waveforms, typically sine, square, and triangle waves. Monolithic function generators are voltage-controlled. Analog phase-locked loops typically contain VCOs. High-frequency VCOs are usually used in phase-locked loops for radio receivers. Phase noise

1190-448: A tape recorder. The MC-303 was built in 1996 and is a digital successor of the MC-202. In 1997, Defective Records Software released MC-202 Hack , a software application that enables programming of the MC-202's sequencer on computer. It works by creating audio that is routed into the MC-202's cassette input port. It allows for MIDI files to be converted to MC-202 sequences. This eliminates

1260-462: A timing signal to synchronize operations in digital circuits. VCXO clock generators are used in many areas such as digital TV, modems, transmitters and computers. Design parameters for a VCXO clock generator are tuning voltage range, center frequency, frequency tuning range and the timing jitter of the output signal. Jitter is a form of phase noise that must be minimised in applications such as radio receivers, transmitters and measuring equipment. When

1330-415: A voltage input for fine control. The temperature is selected to be the turnover temperature : the temperature where small changes do not affect the resonance. The control voltage can be used to occasionally adjust the reference frequency to a NIST source. Sophisticated designs may also adjust the control voltage over time to compensate for crystal aging. A clock generator is an oscillator that provides

1400-413: A voltage-controlled crystal oscillator can be varied a few tens of parts per million (ppm) over a control voltage range of typically 0 to 3 volts, because the high Q factor of the crystals allows frequency control over only a small range of frequencies. A temperature-compensated VCXO ( TCVCXO ) incorporates components that partially correct the dependence on temperature of the resonant frequency of

1470-525: A wider selection of clock frequencies is needed the VCXO output can be passed through digital divider circuits to obtain lower frequencies or be fed to a phase-locked loop (PLL). ICs containing both a VCXO (for external crystal) and a PLL are available. A typical application is to provide clock frequencies in a range from 12 kHz to 96 kHz to an audio digital-to-analog converter . A frequency synthesizer generates precise and adjustable frequencies based on

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1540-459: Is a special type of VCO designed to be very linear in frequency control over a wide range of input control voltages. VCOs can be generally categorized into two groups based on the type of waveform produced. A voltage-controlled capacitor is one method of making an LC oscillator vary its frequency in response to a control voltage. Any reverse-biased semiconductor diode displays a measure of voltage-dependent capacitance and can be used to change

1610-432: Is an electronic circuit that delivers or absorbs an electric current which is independent of the voltage across it. A current source is the dual of a voltage source . The term current sink is sometimes used for sources fed from a negative voltage supply. Figure 1 shows the schematic symbol for an ideal current source driving a resistive load . There are two types. An independent current source (or sink) delivers

1680-403: Is an advantage of this circuit solution. They are implemented as a voltage follower with series negative feedback driven by a constant input voltage source (i.e., a negative feedback voltage stabilizer ). The voltage follower is loaded by a constant (current sensing) resistor acting as a simple current-to-voltage converter connected in the feedback loop. The external load of this current source

1750-412: Is assumed to be the same as the collector or required load current, provided h FE is sufficiently large). Resistance R 1 is calculated as where K = 1.2 to 2 (so that R R1 is low enough to ensure adequate I B ), and h FE,min is the lowest acceptable current gain for the particular transistor type being used. The Zener diode can be replaced by any other diode; e.g.,

1820-495: Is at least somewhat dependent on temperature. This effect is mitigated to a large extent, but not completely, by corresponding voltage drops for the diode, D1, in Figure 6, and the LED, LED1, in Figure 5. If the power dissipation in the active device of the CCS is not small and/or insufficient emitter degeneration is used, this can become a non-trivial issue. Imagine in Figure 5, at power up, that

1890-421: Is connected somewhere in the path of the current supplying the current sensing resistor but out of the feedback loop. The voltage follower adjusts its output current I OUT flowing through the load so that to make the voltage drop V R = I OUT R across the current sensing resistor R equal to the constant input voltage V IN . Thus the voltage stabilizer keeps up a constant voltage drop across

1960-409: Is identical to an ideal open circuit . The voltage across an ideal current source is completely determined by the circuit it is connected to. When connected to a short circuit , there is zero voltage and thus zero power delivered. When connected to a load resistance , the current source manages the voltage in such a way as to keep the current constant; so in an ideal current source the voltage across

2030-405: Is imperfect. In bipolar transistors, as the junction temperature increases the V be drop (voltage drop from base to emitter) decreases. In the two previous circuits, a decrease in V be will cause an increase in voltage across the emitter resistor, which in turn will cause an increase in collector current drawn through the load. The end result is that the amount of 'constant' current supplied

2100-466: Is never exactly equal to V BE and hence it only suppresses the change in V BE rather than nulling it out.) R 1 is calculated as (the compensating diode's forward voltage drop, V D , appears in the equation and is typically 0.65 V for silicon devices. ) Note that this only works well if DZ1 is a reference diode or another stable voltage source. Together with 'normal' Zener diodes especially with lower Zener voltages (<5V)

2170-408: Is only useful when a current source is operating within its compliance voltage. The simplest non-ideal current source consists of a voltage source in series with a resistor. The amount of current available from such a source is given by the ratio of the voltage across the voltage source to the resistance of the resistor ( Ohm's law ; I = V / R ). This value of current will only be delivered to

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2240-407: Is the most important specification in this application. Audio-frequency VCOs are used in analog music synthesizers. For these, sweep range, linearity, and distortion are often the most important specifications. Audio-frequency VCOs for use in musical contexts were largely superseded in the 1980s by their digital counterparts, digitally controlled oscillators (DCOs), due to their output stability in

2310-486: Is used to control the internal synthesiser. The sequencer is programmed much like Roland's early digital MC-4 and MC-8 Microcomposer sequencers, whereby notes are entered with pitch, length and gate length. Additionally, each note in the sequence can have an accent and slide, which is similar to the TB-303 and allows for so called acid sequences. The SH-101 lacks the ability to programme accents. The sequences are lost if

2380-468: The V BE drop. The circuit is actually a buffered non-inverting amplifier driven by a constant input voltage. It keeps up this constant voltage across the constant sense resistor. As a result, the current flowing through the load is constant as well; it is exactly the Zener voltage divided by the sense resistor. The load can be connected either in the emitter (Figure 7) or in the collector (Figure 4) but in both

2450-456: The LED has 1 V across it driving the base of the transistor. At room temperature there is about 0.6 V drop across the V be junction and hence 0.4 V across the emitter resistor, giving an approximate collector (load) current of 0.4/R e amps. Now imagine that the power dissipation in the transistor causes it to heat up. This causes the V be drop (which was 0.6 V at room temperature) to drop to, say, 0.2 V. Now

2520-495: The SH-101, it does include a delay on the LFO. The two units also share a design aesthetic in terms of the control layout, casing, lettering, knobs and slider caps. The MC-202 includes a sequencer that can play back two separate sequences simultaneously. Two sets of CV/Gate connectors on the rear of the unit allow for routing the sequences to external synthesizers. One of the two sequences

2590-482: The advantages of having no off-chip components (expensive) or on-chip inductors (low yields on generic CMOS processes). Commonly used VCO circuits are the Clapp and Colpitts oscillators. The more widely used oscillator of the two is Colpitts and these oscillators are very similar in configuration. A voltage-controlled crystal oscillator ( VCXO ) is used for fine adjustment of the operating frequency. The frequency of

2660-459: The bare emitter follower and the precise op-amp follower above, it keeps up a constant voltage drop (1.25 V) across a constant resistor (1.25 Ω); so, a constant current (1 A) flows through the resistor and the load. The LED is on when the voltage across the load exceeds 1.8 V (the indicator circuit introduces some error). The grounded load is an important advantage of this solution. Nitrogen-filled glass tubes with two electrodes and

2730-428: The base current ( I B ) of NPN transistor (Q1). The constant Zener voltage is applied across the base of Q1 and emitter resistor, R2. Voltage across R 2 ( V R2 ) is given by V Z − V BE , where V BE is the base-emitter drop of Q1. The emitter current of Q1 which is also the current through R2 is given by Since V Z is constant and V BE is also (approximately) constant for

2800-459: The base of Q1, turning it on and causing current to begin to flow through the load into the collector of Q1. This same load current then flows out of Q1's emitter and consequently through R sense to ground. When this current through R sense to ground is sufficient to cause a voltage drop that is equal to the V be drop of Q2, Q2 begins to turn on. As Q2 turns on it pulls more current through its collector resistor, R1, which diverts some of

2870-408: The cases it is floating as in all the circuits above. The transistor is not needed if the required current doesn't exceed the sourcing ability of the op-amp. The article on current mirror discusses another example of these so-called gain-boosted current mirrors. The general negative feedback arrangement can be implemented by an IC voltage regulator ( LM317 voltage regulator on Figure 8). As with

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2940-410: The circuit is considered in details below. A Zener diode , when reverse biased (as shown in the circuit) has a constant voltage drop across it irrespective of the current flowing through it. Thus, as long as the Zener current ( I Z ) is above a certain level (called holding current), the voltage across the Zener diode ( V Z ) will be constant. Resistor, R1, supplies the Zener current and

3010-439: The circuit of Figure 6 that includes a standard diode, D, (of the same semiconductor material as the transistor) in series with the Zener diode as shown in the image on the left. The diode drop ( V D ) tracks the V BE changes due to temperature and thus significantly counteracts temperature dependence of the CCS. Resistance R 2 is now calculated as Since V D = V BE = 0.65 V , (In practice, V D

3080-404: The circuit operates as a constant current source. As long as the temperature remains constant (or doesn't vary much), the load current will be independent of the supply voltage, R1 and the transistor's gain. R2 allows the load current to be set at any desirable value and is calculated by where V BE is typically 0.65 V for a silicon device. ( I R2 is also the emitter current and

3150-671: The circuit, the output power level, and the loaded Q factor of the resonator. (see Leeson's equation ). The low frequency flicker noise affects the phase noise because the flicker noise is heterodyned to the oscillator output frequency due to the non-linear transfer function of active devices. The effect of flicker noise can be reduced with negative feedback that linearizes the transfer function (for example, emitter degeneration ). VCOs generally have lower Q factor compared to similar fixed-frequency oscillators, and so suffer more jitter . The jitter can be made low enough for many applications (such as driving an ASIC), in which case VCOs enjoy

3220-504: The control voltage and the output frequency for a VCO (especially those used at radio frequency ) may not be linear, but over small ranges, the relationship is approximately linear, and linear control theory can be used. A voltage-to-frequency converter (VFC) is a special type of VCO designed to be very linear over a wide range of input voltages. Modeling for VCOs is often not concerned with the amplitude or shape (sinewave, triangle wave, sawtooth) but rather its instantaneous phase. In effect,

3290-423: The crystal. A smaller range of voltage control then suffices to stabilize the oscillator frequency in applications where temperature varies, such as heat buildup inside a transmitter . Placing the oscillator in a crystal oven at a constant but higher-than-ambient temperature is another way to stabilize oscillator frequency. High stability crystal oscillator references often place the crystal in an oven and use

3360-535: The current source can supply to a load. Over a given load range, it is possible for some types of real current sources to exhibit nearly infinite internal resistance. However, when the current source reaches its compliance voltage, it abruptly stops being a current source. In circuit analysis, a current source having finite internal resistance is modeled by placing the value of that resistance across an ideal current source (the Norton equivalent circuit). However, this model

3430-429: The current to be tuned down to a desired value. In this bipolar junction transistor (BJT) implementation (Figure 4) of the general idea above, a Zener voltage stabilizer (R1 and DZ1) drives an emitter follower (Q1) loaded by a constant emitter resistor (R2) sensing the load current. The external (floating) load of this current source is connected to the collector so that almost the same current flows through it and

3500-404: The current will not depend at all on the voltage across the load). Thus, efficiency is low (due to power loss in the resistor) and it is usually impractical to construct a 'good' current source this way. Nonetheless, it is often the case that such a circuit will provide adequate performance when the specified current and load resistance are small. For example, a 5 V voltage source in series with

3570-558: The diode might even worsen overall temperature dependency. Series negative feedback is also used in the two-transistor current mirror with emitter degeneration . Negative feedback is a basic feature in some current mirrors using multiple transistors, such as the Widlar current source and the Wilson current source . One limitation with the circuits in Figures 5 and 6 is that the thermal compensation

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3640-400: The emitter resistor (they can be thought of as connected in series). The transistor, Q1, adjusts the output (collector) current so as to keep the voltage drop across the constant emitter resistor, R2, almost equal to the relatively constant voltage drop across the Zener diode, DZ1. As a result, the output current is almost constant even if the load resistance and/or voltage vary. The operation of

3710-400: The face of temperature changes during operation. Since the 1990s, musical software has become the dominant sound-generating method. Voltage-to-frequency converters are voltage-controlled oscillators with a highly linear relation between applied voltage and frequency. They are used to convert a slow analog signal (such as from a temperature transducer) to a signal suitable for transmission over

3780-413: The focus is not on the time-domain signal A sin( ωt + θ 0 ) but rather the argument of the sine function (the phase). Consequently, modeling is often done in the phase domain. The instantaneous frequency of a VCO is often modeled as a linear relationship with its instantaneous control voltage. The output phase of the oscillator is the integral of the instantaneous frequency. For analyzing

3850-467: The frequency (such as altering the charging rate of a capacitor by means of a voltage-controlled current source ) are used (see function generator ). The frequency of a ring oscillator is controlled by varying either the supply voltage, the current available to each inverter stage, or the capacitive loading on each stage. VCOs are used in analog applications such as frequency modulation and frequency-shift keying . The functional relationship between

3920-439: The frequency of an oscillator by varying a control voltage applied to the diode. Special-purpose variable-capacitance varactor diodes are available with well-characterized wide-ranging values of capacitance. A varactor is used to change the capacitance (and hence the frequency) of an LC tank. A varactor can also change loading on a crystal resonator and pull its resonant frequency. For low-frequency VCOs, other methods of varying

3990-539: The injected current in the base of Q1, causing Q1 to conduct less current through the load. This creates a negative feedback loop within the circuit, which keeps the voltage at Q1's emitter almost exactly equal to the V be drop of Q2. Since Q2 is dissipating very little power compared to Q1 (since all the load current goes through Q1, not Q2), Q2 will not heat up any significant amount and the reference (current setting) voltage across R sense will remain steady at ≈0.6 V, or one diode drop above ground, regardless of

4060-442: The instantaneous oscillation frequency. Consequently, a VCO can be used for frequency modulation (FM) or phase modulation (PM) by applying a modulating signal to the control input. A VCO is also an integral part of a phase-locked loop . VCOs are used in synthesizers to generate a waveform whose pitch can be adjusted by a voltage determined by a musical keyboard or other input. A voltage-to-frequency converter ( VFC )

4130-532: The inverting input a virtual ground , with the current running through the feedback, or load, and the passive current source. The input voltage source, the resistor, and the op-amp constitutes an "ideal" current source with value, I OUT = V IN / R . The transimpedance amplifier and an op-amp inverting amplifier are typical implementations of this idea. The floating load is a serious disadvantage of this circuit solution. A typical example are Howland current source and its derivative Deboo integrator. In

4200-458: The last example (Fig. 1), the Howland current source consists of an input voltage source, V IN , a positive resistor, R, a load (the capacitor, C, acting as impedance Z ) and a negative impedance converter INIC ( R 1 = R 2 = R 3 = R and the op-amp). The input voltage source and the resistor R constitute an imperfect current source passing current, I R through the load (Fig. 3 in

4270-421: The need to use the MC-202 keys to enter sequence information. Version 2 of the software (released in 2009) also allows sequences programmed directly on the MC-202 to be converted back into MIDI files. Voltage-controlled oscillator A voltage-controlled oscillator ( VCO ) is an electronic oscillator whose oscillation frequency is controlled by a voltage input. The applied input voltage determines

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4340-427: The output current is not monitored and controlled by means of negative feedback . They are implemented by active electronic components (transistors) having current-stable nonlinear output characteristic when driven by steady input quantity (current or voltage). These circuits behave as dynamic resistors changing their present resistance to compensate current variations. For example, if the load increases its resistance,

4410-420: The source approaches infinity as the load resistance approaches infinity (an open circuit). No physical current source is ideal. For example, no physical current source can operate when applied to an open circuit. There are two characteristics that define a current source in real life. One is its internal resistance and the other is its compliance voltage . The compliance voltage is the maximum voltage that

4480-427: The source). The INIC acts as a second current source passing "helping" current, I −R , through the load. As a result, the total current flowing through the load is constant and the circuit impedance seen by the input source is increased. However the Howland current source isn't widely used because it requires the four resistors to be perfectly matched, and its impedance drops at high frequencies. The grounded load

4550-533: The thermal changes in the V be drop of Q1. The circuit is still sensitive to changes in the ambient temperature in which the device operates as the BE voltage drop in Q2 varies slightly with temperature. The simple transistor current source from Figure 4 can be improved by inserting the base-emitter junction of the transistor in the feedback loop of an op-amp (Figure 7). Now the op-amp increases its output voltage to compensate for

4620-428: The transfer ratio ( transconductance ). Current sources implemented as circuits with series negative feedback have the disadvantage that the voltage drop across the current sensing resistor decreases the maximal voltage across the load (the compliance voltage ). The simplest constant-current source or sink is formed from one component: a JFET with its gate attached to its source. Once the drain-source voltage reaches

4690-454: The transistor decreases its present output resistance (and vice versa ) to keep up a constant total resistance in the circuit. Active current sources have many important applications in electronic circuits . They are often used in place of ohmic resistors in analog integrated circuits (e.g., a differential amplifier ) to generate a current that depends slightly on the voltage across the load. The common emitter configuration driven by

4760-520: The unit is powered down, however a tape interface is provided so that sequences can be stored to and recalled from an audio tape recorder. There are DIN sync inputs and outputs which allow the unit to synchronise playback, either as master or slave, with other DIN sync-equipped instruments such as the TB-303 or the Roland TR-808 . The unit can also generate and sync to frequency-shift keying signals from

4830-435: The voltage across the emitter resistor is 0.8 V, twice what it was before the warmup. This means that the collector (load) current is now twice the design value! This is an extreme example of course, but serves to illustrate the issue. The circuit to the left overcomes the thermal problem (see also, current limiting ). To see how the circuit works, assume the voltage has just been applied at V+. Current runs through R1 to

4900-408: The voltage drop across the resistor (the difference between the exciting voltage and the voltage across the load) to its resistance. For a nearly ideal current source, the value of the resistor should be very large but this implies that, for a specified current, the voltage source must be very large (in the limit as the resistance and the voltage go to infinity, the current source will become ideal and

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