A voltagecontrolled resistor vcr may be defined as a threeterminal variable resistor where the resistance val ue between two of the terminals. That is, the load current drainsource current of a fet is controlled by the voltage applied on the gate pin. A voltagecontrolled resistor vcr is a threeterminal active device with one input port and two output ports. Ba6110fs standard ics voltage controlled operational amplifier ba6110fs the ba6110fs is a lownoise, lowoffset programmable operational amplifier. The field effect transistor as a voltage controlled resistor. A voltage controlled resistor vcr is a threeterminal active device with one input port and two output ports. In the ohmic region which is quite narrow, in terms of output voltage the linearity is poor, and it also depends on. Controlling a dcdc converter by using the power mosfet as a. If there is no tolerance band, then find the side that has. For a jfet to be effective as a linearly responding resistor, why its important to limit vds to a value that is small compared with vgs,off and to keep vgs below vgs,off. As far as im aware theres a region of the fets response in which it acts as a voltage controlled resistor i just need to figure out how to it up and scale it so i can feed in a couple of. Jan 16, 2010 hi everyone, from the figure, its shown that jfet is configured as voltage controlled resistor. Dejhan, voltage controlled floating resistor using ddcc where the transconductance parameter kp pcoxwl for pmos transistor, p is the mobility of the carrier of pmos transistor, cox is the gate capacitance per unit area, w is the effective channel width, l is the effective channel length. Lib library file contains subcircuit models for voltagecontrolled reactances and admittances.
The field effect transistor as a voltage controlled resistor we consider the use of a nchannel fet as a voltage controlled resistor where the resistance between the drain and source is controlled by the gate. Technology voltagecontrolled resistor device based on. In local oscillator applications, the vco frequency must be able to be varied over the rx or tx range quickly. It wont be very easy to tune it to behave like a proper resistor. A voltagecontrolled resistor bela geczy ltspice is one of the worlds leading circuit simulation tools. Power mosfet as a voltage controlled resistor physics forums. You should really be more specific about how youre planning to use the potentiometer. The resistor needs to behave like a resistor up to frequencies of about 1khz, but of course more is better. The fet operating as a voltagecontrolled resistor vcr works in this region. This is a true variable resistor controlled by a true analogue voltage, no need to use mcus or any other complicated circuitry. The voltagecontrolled resistors, find applications in the area of analog signal processing such as filters,, dividers, amplifiers, and oscillators, etc.
Transistor models and the feedback amplifier wes hayward, w7zoi. It performs nonlinear dc and transient analyses, fourier analysis, and ac analysis linearized. I like this solution since it requires no research on my part into transistor. A mosfet l 1 mm, t ox 10 nm, v t 1 v and m n 300 cm 2 vsec is to be used as 50 w terminating resistor when applying a gatesource.
These can be used to make voltage controlled resistors and capacitors. Hey all, im on the hunt for some sort of analog voltage controlled potentiometer. The output frequency is determined by this master oscillator and an internal. For frequencies greater than 20 hz, c1s impedance can be considered as an ac short circuit. Voltage controlled oscillators tuning a voltage controlled oscillator vco is an oscillator whose frequency can be varied by a voltage or current. Snosbw2f november 1999revised november 2015 lm700 dual. A common request from spice users is a voltagecontrolled resistor. Mosfet as a voltage controlled resistor electrical. Voltage controlled oscillator agc the vcr2n4n7n jfet voltage controlled resistors have an ac drainsource resistance that is controlled by a dc bias voltage vgs applied to their high impedance gate terminal. With two opposingpolarity, voltagesensitive control ports, they offer widerange exponential control of gain and attenuation with low signal distortion.
The basic circuit forms a voltage divider as shown in figures 3, 4, 5, and 6. One of the simplest uses of a voltage controlled resistor is an electronically controlled attenuator or volume control. Gnucap is a general purpose mixed analog and digital circuit simulator. A common request from spice users is a voltage controlled resistor. A voltagecontrolled resistor vcr is a threeterminal device that varies the. D continuous voltagecontrolled resistance d high offisolation d high input impedance. The relative noise voltage, expressed in vv is independent of the applied dcvoltage u non linearity a 3 the harmonic index and the voltage coefficient of resistors are a criteria for the nonlinearity of the current voltage characteristic. Vcrs are most often built with fieldeffect transistors fets. Junction field effect transistor jfet the single channel junction fieldeffect transistor jfet is probably the simplest transistor available. As v gs approaches the pinchoff voltage, rds rapidly increases.
In the ohmic region1, the draintosource current, i ds, depends on the gatetosource. Oct 02, 2012 im looking for a way to control a potentiometer, or a resistor, or the gain of an opamp, using a 03. Single voltage controlled cmos grounded resistors and. The gate terminal is formed by joining the ohmic contacts at both the sides.
The currentcontrolled resistor 1 receives at the second input terminal i ctrl the current control signal i b having the current value i b2, which modifies the small signal resistance between the. Lib library file contains subcircuit models for voltage controlled reactances and admittances. Jfet voltagecontrolled resistors datasheet catalog. An important aspect that students miss since it is not explicitly. Pdf voltage controlled resistor using quasifloatinggate. Technically the mosfet can operate as a variable resistor, but there are two main issues. Modeling voltagecontrolled resistors and capacitors in pspice. Voltage controlled bipolar variable resistor page 1. In the ohmic region which is quite narrow, in terms of output voltage the linearity is poor, and it also depends on input voltage. Tunable resistor, mos, video filter, voltage controlled literature survey shows that electronically controlled resistors114 can be replaced instead of resistive components of topologies such as analog filters and oscillator circuits.
Ba6110fs standard ics voltage controlled operational amplifier. This table contains voltage, resistance pairs picked from points on the curve. Lowvoltage, lowpower, chaotic oscillator, derived from a relaxation oscillator article in microelectronics journal june 2000 with 288 reads how we measure reads. Nowadays, negative differential resistance devices and circuits are widely used in oscillators. Voltage controlled oscillator agc the vcr2n4n7n jfet voltage controlled resistors have an ac drainsource resistance that is controlled by a dc bias voltage vgs applied to their high impedance gate. Its nonlinear but that can be accounted for in the software side of things. Circuit designers often require a resistor that consistently alters its resistance based upon an input control voltage. Voltagecontrolled resistor with linearizing diodes. It finds applications in many areas where this property is useful. It is due to the fact that in this region draintosource resistance r ds can be controlled by varying the bias voltage v gs. Im looking for a way to control a potentiometer, or a resistor, or the gain of an opamp, using a 03.
So what ive been trying to do is create a voltage controlled resistor so that i can use control voltage signals such as lfos, envelopes etc to change the delay time. Voltage curve is available, a lookup table can be used in the abm expression. The resistance of the channel region is controlled by changing the physical width. Single voltage controlled cmos grounded resistors and their application. A jfet is a three terminals voltage controlled semiconductor device i. The inputport voltage controls the value of the resistor between the output ports. The drain terminal is the output and the gate terminal is the input, which is connected to a voltage reference. To prevent the resistance from ever going too low you could put a low valve high power resistor in series 0. Note, in this case, that the i bias gaincontrol pins of the two otas are shorted together and fed from a single gaincontrol voltage and current. Pdf voltage controlled resistor using quasifloating. Voltage controlled resistor experimentalists anonymous. The mosfet as a voltage controlled resistor example.
Technology voltagecontrolled resistor device based on phase. In general, this is a valid assumption that reduces the design complexity. The mosfet as a voltage controlled resistor example a mosfet l 1 m m, t ox 10 nm, v t 1 v and m n 300 cm 2 vsec is to be used as 50 w terminating resistor when applying a gatesource voltage, v gs 5 volt. Voltage controlled resistor using quasifloatinggate mosfets article pdf available in maejo international journal of science and technology 71. In the ohmic region1, the draintosource current, i ds, depends on the gatetosource voltage, v. The voltage input is nonlinearly mapped from the voltage values in the table to the resistance values. For a junction fieldeffect transistor jfet under certain operating conditions, the resistance of the drainsource channel is a function of the gatesource voltage alone and the jfet will behave as an almost pure ohmic resistor. The output voltage vo is determined by the reference input voltage and the output loading r. Fet is a device that is usually operated in the constantcurrent portion of its output characteristics.
An important aspect that students miss since it is not explicitly stated is that, on changing the voltage applied on the fets gate pin the resistance offered by the fets channel changes. D continuous voltage controlled resistance d high offisolation d high input impedance. In each of these circuits, the drain and source terminals of the fets q1, q2, q3, and q4. If we refer back to figure 2 in part 1, for a voltage controlled resistor without feedback resistors, we see that for vds 0 volt, the resistance is. Smith, member, ieee, sima dimitrijev, member, ieee, and h.
Circuit ideas for designers voltage controlled resistor. Edn a guide to using fets for voltage controlled circuits. But if it is operated on the region prior to pinchoff that is where v ds is small, say below 100 mv, it will. If the output port of a gyrator is connected across a currentcontrolled twoterminal resistor, i. Inquisitive students, with a little help, can design programmable circuits combining. In each of these circuits, the drain and source terminals of the fets q1, q2, q3, and q4 provide a voltage controlled resistance. A voltage controlled resistor vcr is a threeterminal device that varies the resistance between two terminals relative to voltage in the third terminal. Pdf the paper discusses an application of the field effected transistor fet as a voltage controlled resistance which can be done in undergraduate. For a junction fieldeffect transistor jfet under certain operating conditions, the resistance of the. The ltc6990 is part of the timerblox family of versatile silicon timing devices. A guide to using fets for voltage controlled circuits, part 1 edn. D gain ranging capabilitywide range signal attenuation.
Design of lowvoltage highly linear switchedrmosfetc filters. Although theres no spice component that directly models this device, you can easily put one together. An important aspect of the utility of the mosfet in this application comes. To be able to answer your question, im going to rephrase it a bit. Look for the rest of that circuit, and figure out what the input voltage range is, and just feed it the correct voltage. Controlling a dcdc converter by using the power mosfet as.
Grounded voltage controlled resistors gvcrs can be adjusted via. Modeling voltagecontrolled and temperaturedependent. A guide to using fets for voltage controlled circuits, part 2 edn. And super gnarly sounds when you turn the delay time down too low. These resistors in turn may be used as the control element in more complicated electronic circuits. We can view this effect in a jfet as a voltage controlled. Frequency tx or rx range voltage tuning range linear tuning nonlinear tuning fig.
Jan 26, 2009 to prevent the resistance from ever going too low you could put a low valve high power resistor in series 0. The resistor needs to behave like a resistor up to. Pchannel silicon voltage controlled resistor jfet absolute maximum ratings at ta 5. In such applications the fet is also referred to as a voltage variable resistor or volatile dependent resistor. A single resistor, rset, programs the ltc6990s internal master oscillator frequency.
The hvac unit must be measuring a voltage across the 010kohm resistor. In 1998, linear technology hired mike engelhardt to re. Barry harrison abstract most converter designs assume that a closed power switch has zero volts across it. These can be used to make voltagecontrolled resistors and capacitors. Ideally, something that has a 0 5v input swing with a linear resistance output from 0 10kohms, respectively. Effect of input voltage or control voltage on the slope. The field effect transistor as a voltage controlled resistor we consider the use of a nchannel fet as a voltage controlled resistor where the resistance between the drain and source is controlled by the gatesource voltage. I cant help but think that there would be a better question to illuminate whatever you are wondering about. Jfet as voltagecontrolled resistor all about circuits. Currentcontrolled resistor stmicroelectronics design.
Preferably, there is no dc voltage across the fets drain and. Replicating all the characteristics of a potentiometer will be difficult, and perhaps overkill if you only need a variable. Voltage controlled resistor description this circuit shows an epad mosfet inverter circuit connected as a voltage controlled resistor circuit. Calculate the onresistance of an nmos transistor for v gs 2vand gs 5v if v tn 1vand k n 250 av2. Barry harrison abstract most converter designs assume that a closed power switch has zero. Dejhan, voltagecontrolled floating resistor using ddcc where the transconductance parameter kp pcoxwl for pmos transistor, p is the. The nature of vcrs a voltagecontrolled resistor vcr may be defined as a threeterminal variable resistor where the resistance value between two of the terminals is controlled by a voltage. Oct, 2017 basic voltage controlled resistor vcr circuits.
It performs nonlinear dc and transient analyses, fourier analysis, and ac analysis linearized at an operating point. Gatesource voltage on the n channel mosfet controls how much effective resistance there is between drain and source. D gain ranging capabilitywide range signal attenuation d no circuit interaction d simplified drive. Offering superb linearity over a broad range, this ic is designed so that the forward direction conductivity gm can be changed, making it ideal for applications. Lowvoltage, lowpower, chaotic oscillator, derived from a. The paper presents fgmos based voltagecontrolled resistor operating at supply voltage of 0. Offering superb linearity over a broad range, this ic is.