widlar current source

Another feature of the current mirror is a relatively low input resistance which helps to keep the input current constant regardless of drive conditions. A Widlar current source is a modification of the basic two-transistor current mirror that incorporates an emitter degeneration resistor for only the output transistor, enabling the current source to generate low currents using only moderate resistor values. But there is a modification in the output transistor. The voltage drop VL across the load (suppose that it is a varying resistor R) is harmful as it enervates the excitation voltage V thus decreasing the current I. In the video, the programmed voltage across Q2’s collector is reflecting across the Q3 collector. As we learned in an earlier chapter, the inclusion of emitter degeneration resistors (RE1 and With the two resistors being equal, IOUT would be the mirror image of IIN. Implementing the block diagram of the current mirror shown in figure 11.1 follows directly from these voltage / current converter stages from Chapter 4, if we connect the output of the I to V converter in figure 11.1.1(b) to the input of the V to I converter in figure 11.1.2. The transistor Viewpoint 1. It's all started with a challenge between two engineers, George R. Wilson and Barrie Gilbert, to make an improved current mirror circuit overnight. transistor, in part because the currents in the two transistors are very different. VCB is the collector-to-base voltage. According to Thomas Lee, Fairchild also wanted to have Widlar on board, and breached professional ethicsby recruiting a key employee of their customer. results in the correct output current, the compliance voltage, is VOUT = VCV = To best understand this important circuit building block and how it makes use of this relationship we need to deconstruct the circuit into input and output sections and examine each in turn. current IOUT in Q2 will most closely match IIN when the collector voltage of example, if the two transistors are designed with different emitter areas. We would like a simple configuration where the active element, a single transistor, serves as the desired current-to-voltage converter. current IOUT caused by the finite beta of Q1 and Q2. The implementation of the current mirror circuit may seem simple but there is a lot going on. Remember that two equal size transistors at the same temperature with the same VGS (or VBE for a BJT) have the same drain current. large, and increase in R2 tends to reduce this current further, causing a correlated increase in

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. It also has a very high output impedance due to the negative feedback through

A test current I/x is applied at the output, and the output resistance is then RO = It is equal to the This classic “diode” connection results in 100% parallel negative feedback (figure 11.2). The collector voltage of T1 is stabilized by the amount equal to the VBE4. 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. 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. The simple two transistor implementation of the current mirror is based on the fundamental relationship that two equal size transistors at the same temperature with the same VGS for a MOS or VBE for a BJT have the same drain or collector current.

A bipolar transistor can be driven by a voltage or by a current. The R2 is connected in series with the T2 emitter and ground. Q1 back to the base of Q3. A Wilson current mirror or Wilson current source, named after George Wilson, is an improved mirror circuit ideally be infinite. the value of VGS. The voltage drop across the emitter resistor RE2 (IOUT x RE2) adds to the ADALM2000 Lab Activity 7m, NMOS Zero Gain Amplifier. Equation 5 . the Collector-Base voltage, VCB, of Q1 is no longer zero.

the output. Now that we understand the concept of the zero gain amplifier, the objective is to investigate its use to

The design can have a different kind of circuit structure to balance the output impedance.

Wilson current mirror circuit uses three active devices that accept the current across its input and provide the exact copy or mirrored copy of the current to its output. In other words, the mirror has a finite output resistance given by the A zero gain amplifier made using an enhancement mode NMOS 2N7000 transistor was simulated where the small Thus For a given drain current, if Figure 11.3, Voltage to Current Converter. The final formula can be drawn from the above formula if the finite β is overruled and if we change the IC1 as IIN and IC2 as IOUT.

Suppose we want to create a 100uA output current from a 300uA input current as in the simulation plot of Given the effect of the finite output resistance (Early effect ) the output Moin SPM, Co-Founder and COO of Agnikul Shares How They are Building India’s First Private Smallsat Rocket, Build a Simple Arduino RC Boat that can be Controlled Wirelessly using 433 MHz RF Modules, Interfacing Gravity Infrared CO2 Sensor with Arduino to Measure Carbon Dioxide in PPM, ESP32-CAM Face Recognition Door Lock System, Automatic Surface Disinfecting Robot using Ultraviolet Lights. One aspect of design which

Let’s see the Widlar circuit construction and simulations in the below image. The Widlar circuit may be used with bipolar transistors, MOS transistors, and even vacuum tubes. Figure 11.1.2 Linear Voltage to Current converter (from Chapter 4). (VGS,VDG=0), so we find: f (VGS, 0) = IIN, implicitly determining ADALM1000 Lab Activity 8m, NMOS Stabilized current source, ADALM2000 Lab Activity 8, BJT Stabilized current source Subscribe below to receive most popular news, articles and DIY projects from Circuit Digest, The CR01005 chip resistor features a three-layer termination process with a nickel barrier, SRP0310/0315/0410/0510/0610 shielded power inductors have a metal alloy powder core and flat wire, The TBU-RS055-300-WH is an integrated dual-channel TBU overcurrent and TVS overvoltage protector, The model CRxxxxA AEC-Q200 compliant chip resistor series is available in eight different footprints, AVHT high-temperature varistors offer great circuit-board layout flexibility for designers, The Model SF-0603HIA-M/SF-1206HIA-M series utilize Bourns' popular multilayer ceramic design, SRP4018FA shielded power inductors are designed to meet high current density requirements, The SM41126EL Chip LAN 10/100 Base-T transformer module is ideal for use in LAN interfaces.

However, this linear relationship is not necessarily required.

output current is the same as the input current when VDG=0 for the output transistor, and both VA is the Early voltage Figure 11.12 Plot of the collector current of Q1 and Q2

NPN or PNP transistors will just as well function in this configuration. In figure 11.4.1 we see an example where 2 (N=2) devices are connected together on the input and 3 (M=3) devices are connected together on the output.


Q2 is the same as that of Q1 which is 2XVBE above the common voltage. will vary approximately proportional to the variation of the supply voltage. In Chapter 8 we explored the transistor and you should recall that the BJT device is a current amplifier of sorts (current controlled current source) in that the collector current is β times the base current. Despite the fact that basic current mirror circuit can be constructed using two simple active components, BJTs and MOSFETs or using an amplifier circuit, the output is not perfect, as well as it has certain limitations and dependencies on the external things. Note that the output current is equal to the input current IR1 which in turn is dependent on The resistors R1, R2, R3, R4 and the op-amp OA form a differential amplifier.

This asymmetry in the charging vs. discharging current available for this (V1-VBE1-VBE2)/R1. is common to nearly all circuits is the establishment of stable bias or operating point levels. So this is how the Wilson and Widlar current Mirror Techniques can be used to improve the designs of Basic Current Mirror Circuit. In this simple configuration, the output current The longer the channel the less it is affected by An example application is in the

The additional transistor T4 balances the collector voltage of T1 and T2. This circuit is named for its inventor, Robert Widlar, and was patented in 1967. If we

With this minimum allowable output voltage (see section 11.5.2).

signal AC gain and phase was calculated as the drain current was swept.

utility in many cases. The output current can also be easily calculated if the base-emitter voltage of the transistors is less than 1V. RE1 = 500Ω (introducing about 500mV of degeneration) RB should be 1578Ω if β is enough

In one of the solution one or two transistors are added over the traditional two transistors design. Two assumptions are made here – one is that all transistors have the same current gain which is and second is that collector currents of T1 and T2 is equal, as the T1 and T2 are matched and the same transistor. An error source in this simple BJT based current mirror is that the transistors Q1 and Q2 (figure 11.4) each remove a base current IB from the input current IIN. We may see this idea in the Widlar's bilateral current source. Our data collection is used to improve our products and services. Idea: Keeping a constant current by following voltage (bootstrapping). In figure 11.15 we have an NPN transistor biased into conduction with a collector voltage VC which RL) and is essentially constant with input voltage changes applied from VIN. As can be seen in figure 11.17 there

2. bias current is frequently undesirable.

source. This is to introduce another kind of current mirror, actually a stabilized current source, which has an We may see this idea in the Widlar's bilateral current source. gate voltage of the MOSFET given by ID = f (VGS, VDG), a relationship Read more about our privacy policy. Q1 due to the Early effect.

The current gain (ßQ3) of the emitter follower buffer stage (Q3) greatly reduces the gain error caused by the finite base currents of Q1 and Q2. (Image courtesy of Reference 3) A “Howland” Current Source 4.

The problem with using this feature directly is that β is not a well controlled value from device to device and can vary with changes in temperature.

zero VDG and provided transistors M1 and M2 have good matching of their

The current being 'copied' can be, and often is, a varying signal current. The emitter degeneration resistance introduces local current feedback for transistor Q1 is replaced by its small-signal emitter resistance rE because it is diode connected. It provides a much more accurate However, even in integrated circuits the best design practice is to use identical unit size transistors when making current mirrors. We will be making the following two assumptions. An important feature of the current mirror is a relatively high output resistance which helps to keep the output current constant regardless of load conditions. the mirror. Input current IIN is a known current, and can be provided by a Implementation: Using an op-amp follower as a following voltage source.

For this purpose, we simply connect the collector to the base or gate to drain or “diode connect” the transistor. voltages seen at VBE and VC are plotted vs. the applied voltage at VIN in figure 11.12. There are several options to improve the output of Current Mirror Circuit. A Widlar current source is a modification of the basic two-transistor current mirror that incorporates an emitter degeneration resistor for only the output transistor, enabling the current source to generate low currents using only moderate resistor values.

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