It was a challenge to reveal the idea behind the popular bjt current mirror. The basic current mirror can also be implemented using mosfet transistors, as shown in figure 2. In the video, the programmed voltage across q2s collector is reflecting across the q3 collector. Cascode current mirror wilson current mirror wideswing current mirror 4. One important factor when using multiple outputs from a current mirror.
Because you are not logged in, you will not be able to save or copy this circuit. The voltage gain can be made to attain higher value by using active load like current source. Current mirrors are basic building blocks of analog design. In this example we will construct a current mirror, where we would like to mirror the current four times the current source. Bjt load resistor is usually connected as a constantcurrent source with a very high resistance load output resistance of the current source higher load resistance, higher. Structure and characteristics of the bjt wilson current mirror. Chapter 16 analog integrated circuits chapter outline 16. A current mirror that gives a 10x higher output current is still a current mirror.
It should be noted that the wilson current mirror is not a constantcurrent source. In the final section, we saw some examples of how current mirrors are incorporated into analog integrated circuits. The collector current fluctuation contributes noises across the output. Using the output resistance equation for the wilson current mirror, we have 03 03 0 3 03 3 3 03 1. Simple current mirror bjt and mosfet base current corrected simple current mirror widlar current source wilson current mirror bjt and mosfet cascoded current mirror bjt and mosfet for best performance, transistors should be matched, temperature should be.
Thus, the drain current can be expressed as, figure 4. However, the transistor is a unidirectional device, where for the bjt the base emitter. Current mirror circuit2, wilson current mirror circuit, current mirror circuit using bjt, current transfer ratio, current transfer gain, current mirror circuit with base compensation, playlists. If the supply voltage, v cc changes, the output current will change. The gate source voltage is set by the diode geometry and the injected input or reference. Ee105 fall 2014 microelectronic devices and circuits. Wilson current mirror analysis electrical engineering stack.
Bjt current mirrors mateo aboy professor eric laithwaite. Analogue electronicscurrent mirrorswilson wikibooks. To describe the operation of a bipolar junction transistor bjt. The simple two transistor implementation of the current mirror is based on the. Actually the name current mirror is a bit strange, current copier would be a better description.
The drain current of mosfet depends on its gate voltage v g and drain voltage v d. There are many good resources online and in textbooks on current mirrors. Chapter 7 building blocks of integrated circuit amplifiers. The three t2 collectors may be joined to give 3i in. Build a mirror using pnp transistors and use the output impedance of the simple mirror to determine the early voltage for the pnp transistor. The proposed implementation yields an increase of the output impedance by a factor of about g m r o compared with that of the super wilson current mirror, thus offering a. Maybe, there is nothing so simple containing only three transistors and, at the same time, so incomprehensible and misunderstood circuit as wilson current mirror. Concept of current mirror circuit analog electronics youtube. For example, any version of the wilson current mirror is limited to a single output, because the diodeconnected reference transistor in the mirror is. The problem with using this feature directly is that. A wilson current mirror is a threeterminal circuit fig. The proposed implementation yields an increase of the output impedance by a factor of about g m r o compared with that of the super. Vincent chang current mirror with multiple outputs pnp current mirrors npn current mirrors s e v v i c i s.
Basic current mirror 86 wilson mirror 89 active loads 90 levelshifting circuits 92 the amplified diode 93 a simple operational amplifier 95. A comparative study of various current mirror configurations. Suppose that the reference current, i ref, is supplied by a resistor to the supply voltage. Another excellent current mirror circuit is the widlar current source circuit, invented by bob widlar the circuit is exactly the same as the basic current mirror circuit using two bjt transistors. Also, it can be seen that all internal nodes have low impedance except the output node.
Eecs140 analog circuit design lectures on current sources simple source cont. Wilson current mirror analysis electrical engineering. Practical example of wilson current mirror circuit. Lectures on current sources university of california. To define the terms current gain, cutoff and saturation applied to a bjt. Wilson current mirror circuit creates noise across the output. A small signal input voltage v in is applied to the gate and output is taken at drain v o. This transistor, shown as tr3 in the diagram keeps the collector of tr1 at a voltage equivalent to two diode drops below the rail voltage vcc. A small amount of the current flowing through r provides base current for q3, enabling q3 to conduct current.
For example, any version of the wilson current mirror is limited to a single output, because the diodeconnected reference transistor in the mirror is in the ouput circuit. Summary of key concepts voltage source easily synthesized from reference current source using mosfet in diode configuration current source easily synthesized from current source using current mirror circuit. Widlar current mirror design using bjtmemristor circuits. You can edit the widths and the lengths by right click on part and click property editor. Construct the following circuit with the following sizes for the transistors. Both threetransistor and fourtransistor versions are shown in figure 3. It may be used as a wilson current source by applying a constant bias current to the input. Direct scaling of the width also faces difficulties. Note that at the q2 side, more current mirrors needs to be used to provide current ki d2 ki d1. Circuit for pnp current mirror using simple device with dotmodel statement shown figure 1 shows a schematic for a pnp current mirror.
The mirrored current is a precise copy of the input current. When using discrete transistors, you may glue their cases together to do this. The implementation of the current mirror circuit may seem simple but there is a lot going on. The current mirror with ac grounded output for finding closedloop current gain. A diode connected bjt is essentially just a pn junction diode. But there is a modification in the output transistor. Current mirrors usually employ the same length for all of the transistors. The gain of the current mirror can be simply varied from 1. Jan 30, 2017 structure and characteristics of the bjt wilson current mirror. For the love of physics walter lewin may 16, 2011 duration. The left side, which is the same as that shown in fig.
Here the wilson current mirror is simulated using proteus. The simple mos current mirror the simple current mirror can be obtained from the one transistor current source by using a second transistor in diode connection that generates the necessary gatesource voltage of the transistor in the output stage. Current mirror with multiple outputs pnp current mirrors npn current mirrors. Elg35 electronics ii the bjt differential amp with an. Analogue electronicscurrent mirrorswilson wikibooks, open. This is due to the feedback which raises output impedance and directly affect the collector current. It should be noted that the wilson current mirror is not a constant current source. Implementation and applications of current sources and. Multiple current sources and sinks with different magnitudes can be synthesized from a single current source. Pdf applications of current mirrors in analog signal.
Transconductance op amp, circuit symbol, am modulator example application 09. Comparative characteristics of current sources and mirrors. At first glance, the current mirror circuit shown to the right would seem to be backwards. Transistor current mirror circuit electronics notes. An example application of the current mirror is the transconductance op amp. Currentoutput circuit techniques add versatility to your. Consider the single stage cs amplifier with an ideal current source load shown in fig. Circuit ideahow the wilson current mirror equalizes the. A current mirror is a transistor circuit that regulates the current through a load resistance, the regulation point is set by a simple resistor adjustment. Within the circuit, a third transistor is introduced. Widlar current source the basic current mirror requires that the bias current and reference current be equal the wildar current source sets the mirrored current to a value smaller than i ref by using an extra resistor the widlar current source allows you to establish small bias currents a without using large resistor values. Transistor m 1 is operating in the saturation or active mode, and so is m 2. In fact, however, it works quite well, although in a roundabout fashion.
To best understand this important circuit building. Bjt current mirror, bjt current mirror with base current compensation, wilson bjt current mirror, lowlevel bjt current mirror, bjt differential amplifier with current mirror load, mosfet differential amplifier with current mirror load 08. V d bipolar diode exponential diode connected transistor quadratic v t v d i d v ds v gs v t after we reach the point v d v t, the transistor will always be in sat. Transistors in a current mirror circuit must be maintained at the same temperature for precise operation. Vth referenced cmos selfvth referenced cmos selfbias circuit bias circuit. An ideal current source, i, driving a resistor, r, and creating a voltage v. Widlar current mirror design using bjtmemristor circuits amanzhol daribay and irina dolzhikova, electrical and computer engineering department, nazarbayev university, astana, kazakhstan amanzhol. Wilson current mirror bjt based addition of a diodeconnected transistor in series with q2 can reduce the effect of. It may be used as a wilson current source by applying a constant bias current to the input branch as in fig. We thus prefer to employ a unit transistor and create copies by repeating such a device. Current ratioing is achieved by only scaling the width of transistors. In this setup, the output current i out is directly related to i ref, as discussed next.
Current mirror examples, proper scaling, bipolar current mirrors duration. Another variation of the basic current mirror circuit is referred to as the wilson mirror or wilson current mirror. V tn 2 i o i ref lecture20current mirrors 10 mos current mirrors. The ref200 current mirror uses a full wilson architecture for high accuracy. It uses a full wilson type architecture as shown in figure 2, with lasertrimming to ensure high accuracy. By applying a negative feedback simply joining the base and collector the transistor can be reversed and it will begin acting as the opposite. Current mirror circuit2 wilson current mirror analog. Analysis of the modified mos wilson current mirror.
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