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FET Current Mirrors

Characteristics of MOSFET Current Mirrors

A current mirror using MOSFETs.

We can easily use enhancement mode MOSFETs to form current mirrors, as shown in the circuit to the right. Just as we connected collector to base with bipolar junction transistors, we can connect drain to gate of a MOSFET to cause the device to operate as a forward-biased diode. In this configuration, the gate-source voltage, VGS, is about 1.2V. Note that we cannot use JFETs because the junction would be forward biased in this configuration. Nor can we use depletion-mode MOSFETs here. However, we can use both P-channel and N-channel MOSFETs, just as we can use both NPN and PNP transistors in current mirrors.

Since MOSFETs have no gate current, all of IREF flows through Q1, so with matched FETs, IO = IREF. There are no extra calculations involving gate current, as there are with base current in bipolar junction transistors (BJTs).

All mirror configurations that use BJTs will also work with MOSFETs, and have the same names with either transistor type. Even though there is no issue with gate current, ROUT is still a matter of concern in some applications (for the basic MOSFET, ROUT = rds), so all of these configurations get used when appropriate.



Advantages and Disadvantages

As with all such things, MOSFETs have both advantages and disadvantages over BJTs when used in current mirror circuits. Therefore, both technologies find practical use in many kinds of analog circuits. Also, while it is possible to manufacture both kinds of transistors within a single IC (BiCMOS, for example), the process is more difficult and expensive than for either technology alone.

The main advantage of the MOSFET version is that FETs draw no gate current. This means that no effort need be expended in allowing for small extra currents in various parts of the mirror circuit. Beyond that, it is easy to manufacture MOSFETs on the same IC die with different channel widths or lengths. The width/length ratio controls how much channel current will flow through that channel, so we can make mirrors that provide multiples or sub-multiples of the reference current. (Yes, we can get the same result with BJT current mirrors by controlling the relative emitter areas of the mirror transistors, but it's easier to manage with MOSFETs.)

There are two disadvantages to MOSFET current mirrors as compared to their BJT equivalents: First, VGS for the reference transistor will be approximately 1.2V, where VBE won't be more than 0.8V, even with relatively high mirror currents. The second disadvantage is that it is difficult to match MOSFETs to each other for use as mirrors. This leads to less precise matching and poorer mirroring than is generally achieved with BJTs.



Can We Use Discrete MOSFETS?

If we try building a current mirror with discrete, off-the-shelf BJTs of the same type, the circuit will work. The transistors won't be well-matched, but the circuit will function, although imperfectly. Unfortunately, this is not really true for MOSFETs. Variations between individual MOS transistors, even of the same type, are too great. Of course, you can test for MOSFETs that match well enough, but even then their temperature responses will probably not match. And you'd probably have to go through several hundred MOSFETs to find two that were acceptably close in all parameters. This is far too expensive to be practical.

The classical depiction of an enhancement-mode MOSFET is as a block of doped silicon (P-type for an NMOS transistor), with a couple of N-doped areas for the source and drain connections. The area between them is coated with a thin layer of silicon dioxide (essentially glass), and then the gate area (typically polysilicon) is grown on top of the insulating silicon dioxide. This construction is perfectly fine for individual MOSFETs, and can be used to produce devices that will handle high voltages and/or large amounts of current.

But in most situations we don't need to deal with high voltages or currents, and the variations that don't matter when turning lights or electric motors on or off will matter when we're dealing with low voltages and currents, and need to match behaviors. We can't effectively solve the problem with single, discrete devices.

The far more practical approach is to manufacture ICs containing the MOSFETs you want, with the proper parameters to fit the need. Even then, the degree of difference between two MOSFETs manufactured side by side is considerably greater than for BJTs. During manufacture, factors such as undercutting and oxide creep can cause the channel dimensions to be other than intended, and the problem gets worse as the channel gets longer and/or wider.

To overcome this problem, specialized techniques have been developed when manufacturing ICs with MOSFETs that must be matched or have specific relationships to each other. The main solution is to make every FET on the chip exactly the same size and shape as every other one. This means that the extent of such problems as undercutting is the same for all FETs, and can be compensated for the entire chip as a whole. Then, if a wider channel is needed for one of them, it is obtained by connecting two or more basic FETs in parallel. This is equivalent to connecting multiple BJts in parallel to double or triple the current in one leg of a mirror, but it is done on-chip, as part of the design of the IC.

Even with these techniques, MOSFET current mirrors are by no means perfect, but they do work properly and can certainly hold their own when compared to BJT current mirror circuits.


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