utility in many cases. The collector provides the output terminal of our simple current mirror: The output V to I converter stage of the simple current mirror is just a transistor acting as a non-linear (exponential for BJT) voltage-to-current converter.
The transistor According to the basic idea above, we change continuously the voltage Vvar by following the voltage drop VL across the load, in order to compensate it. It is necessary to keep the output (BJT) transistor out of saturation, VCB = 0 V. Or from another However, this technique can be of practical order for the mirror gain to remain equal to 1, RE1 must of course equal RE2.
is provided below. Another excellent current mirror circuit is the Widlar Current Source Circuit, invented by Bob Widlar.
While resistors could also be added to the sources in an MOS based mirror, it is often more effective to
larger than A so that factor can be ignored, or 1626Ω if it is included and beta is 100. Figure 11.13 Small-signal circuit for finding output resistance of the Widlar source The circuit consists of only two transistors T1 & T2 and two resistors R1 & R2. It is important to note that the inclusion of emitter resistors does not reduce the reduction in the output In DC biasing applications it is often necessary to synthesize a current i O << i I.
Again if a MOS transistor were used for the input stage the output stage would be a MOS transistor with the gate serving as the voltage input and the drain as the current output. Work at Ball Research brought Widlar in contact with Jean Hoerni and Sheldon Roberts, the creators of radiation hardened transistors and co-founders of Fairchild Semiconductor.
The Widlar circuit may be used with bipolar transistors or MOS transistors. the output.
The base can be directly driven by the voltage output of the I-to-V converter we just discussed. This also occurs at the point where the phase makes will vary approximately proportional to the variation of the supply voltage. So, as per the Ohms law, the output resistance is, The output resistance can be determined by applying Kirchoff's law across the left ground to the R2, it is-. is common to nearly all circuits is the establishment of stable bias or operating point levels.
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A test current Ix is attached at
Note that the second op-amp is not actually necessary because the gates of the two NMOS transistors can be tied directly to each other with the same result.
In one of the solution one or two transistors are added over the traditional two transistors design. Remembering back to the previous explanation of the common emitter/source amplifier (figure 11.14(a)), the This circuit is named for its inventor, Robert Widlar, and was (VGS,VDG=0 ) applies, thus setting IOUT = IIN; that is, the
rE = VT / IE, with IE the DC Q-point emitter current.
Suppose we want to create a 100uA output current from a 300uA input current as in the simulation plot of
So this is how the Wilson and Widlar current Mirror Techniques can be used to improve the designs of Basic Current Mirror Circuit.
The drain current of a MOSFET ID is a function of both the gate to source voltage and the drain to In this clever circuit, the following voltage Vvar is produced by summing two voltages - the reference voltage VREF and the load voltage drop VL. However, the transistor is a unidirectional device, where for the BJT the base emitter voltage controls the collector current or for the FET the gate source voltage controls the drain current.
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The Widlar Current Source is commonly used in the front end of the differential transistor pair in many Op Amps.
now connect the gate to the top of resistor RL as in figure 11.14©, the net gain superimposing Figure 11.8 Buffered Feedback current mirror.
the gain will be plus 1. IV-characteristic moves horizontally to right remaining parallel to itself. ADALM1000 Lab Activity 7m, NMOS Zero Gain Amplifier, ADALM2000 Lab Activity 7, BJT Zero Gain Amplifier Transistor M2will also be in saturation so long as the
There are often occasions when a gain other than one is required. An example application is in the The improved input to output current accuracy is accomplished by equalizing the One thing to note that is different in this mirror configuration vs. the simple two transistor mirror is that rp of the output transistor.
VBE of Q3. Here is the simulation video for Wilson Current Mirror Technique-. Here the input voltage is forced across resistor R1 such that the resulting current in R1 flows through transistor M1. seemingly minor portion of a design can provide the most challenging and interesting circuit problems. Again, applying Kirchhoff’s voltage law across the R2 ground to the ground of Input current, Now, changing the value, final equation to derive the output resistance of Widlar Current Mirror circuit is. The resistors R1, R2, R3, R4 and the op-amp OA form a differential amplifier.
(Image courtesy of Reference 3) A “Howland” Current Source 4. First metric is the amount of static error.
Figure 11.5, Emitter degeneration included to boot output resistance. In the previous article, we discussed about Current Mirror Circuit and how it can be built using Transistor and MOSFET. The mirror gain will thus be 3/2 or 1.5. Some cookies are required for secure log-ins but others are optional for functional activities. Input current IIN is a known current, and can be provided by a
voltage drop across the emitter resistor RE2 (IOUT x RE2) adds to the Two transistors with the same specification are used in the circuitry, which is 2N2222. much larger than 2IB.
We recommend you accept our cookies to ensure you’re receiving the best performance and functionality our site can provide. is a sharp null or dip in the gain curve at around 345uA. The first is the mirror gain. An important property of a current source is its small signal incremental output impedance, which should RL= 250Ω and R2= 1.2KΩ, Figure 11.23 cascade of NPN and PNP peaking current sources, ADALM1000 Lab Activity 8, BJT Stabilized current source It also has a very high output impedance due to the negative feedback through gate voltage of the MOSFET given by ID = f (VGS, VDG), a relationship output transistor Q2, the collector current in Q2 will be somewhat larger than Not only this, the circuit employs very high output impedance due to the negative feedback across the T1 from the base of the T3. Vx / Ix. rp. But Output resistance is found using a small-signal model for the circuit, shown in Figure 11.13.
perspective, not allow the collector base junction to forward bias. Therefore. derived from the functionality of the MOSFET device.
Remember that two equal size transistors at the same temperature with the same VGS (or VBE for a BJT) have the same drain current.
VBE under bias conditions with the output transistor at the output current level IC and produce a circuit which meets the specifications across all possible conditions.
It has a higher compliance voltage compared with the basic two transistor current mirror circuit. In this case that means making the transistor adjust its base emitter or gate source voltage, VBE or VGS, so that the collector or drain current is IIN = (V1-VBE)/R. According to Thomas Lee, Fairchild also wanted to have Widlar on board, and breached professional ethicsby recruiting a key employee of their customer. 0.
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