Current mirror
The current mirror provides the electronics is an elementary transistor circuit, with which it is possible to derive from an existing reference current an additional current. The current mirror makes it possible to copy streams and scale, and thus represents a current-controlled current source
Motivation
The current mirror circuit is primarily used as a sub-circuit in analog integrated circuits, such as differential amplifiers, operational amplifiers and comparators, for example, for setting of operating points in the amplifier stages. A shift in the DC level of analog signals is therefore possible.
MOSFET current mirrors are used as electronic loads, while the output impedance of the current mirror is used as the load resistance of the amplifier.
In the circuit technique is often a replica of an existing stream needed. With current mirrors it is possible to generate an identical or related in fixed relation to an existing stream. For the latter variant transistors are used, which are identical or the same shape in its design, but are not coextensive - that can be set on the area ratios of the active areas, gate area or emitter area, the ratio of currents alone on their surfaces.
Are the transistors of the current mirror closely adjacent (same temperature ) and by the same production process (ie on a Chip) developed, can be built precision current mirror - with discrete circuits that was not possible in the past.
The properties of operational amplifiers can be achieved only by the internal use of current mirrors. So - in connection with NPN - PNP transistors - even circuits are realized, which are impossible with electron tubes.
Principle of operation of the current mirror
Current mirrors are controllable current sources with a very high output resistance. They have a low input impedance at the feed point of the reference current and by a resistor connected to the voltage-controlled current sources.
Circuit with bipolar transistors
Bipolar transistors behave when they are not operated saturated, the output side as a power source, they are therefore good for current mirror.
A simple current mirror consists of two transistors, as shown in the picture on the right. Transistor Q1 collector and base terminal are connected. An input current Ie flows through the transistor Q1, a base-emitter voltage UBE sets associated with Ie of the following relationship approximated by the temperature-dependent thermal voltage UT and the copy -dependent reverse current IS
( A portion of the input current Ie flows as a base current in the transistors Q1 and Q2, this percentage has been neglected. )
The transistor Q1 is a base-emitter voltage that is linked solely by the transistor characteristics and the temperature of the input stream is created. The base terminals of the two transistors are connected, so that the same base-emitter voltage of two transistors. The current of the transistor Q2 depends from UBE, it is also:
The properties of the transistors Q1 and Q2 and their temperatures are equal, the dependency of the collector current of the base-emitter voltage is equal to and Q2 of the same collector current flows as indicated by Q1.
Current mirror are therefore often made of the same type, on-chip transistors. Thus they are created by the same production process and have the same temperature properties.
If the base currents into account, yields precisely taken
Thus arises the simple current mirror with a bipolar error which decreases with increasing current amplification factor. Another error is caused by the Early effect, which causes a voltage dependency of the output current by modulating the virtual base width. The corresponding parameter Earlyspannung has manufacturing tolerances on, should be great for good current stability and increases with the physical base width and thus usually with decreasing current gain at. Therefore reduce transistors with high current gain, although the error by the base current, but produce a less high output resistance - the output current varies strongly with the output voltage.
In integrated circuits can be achieved by the use of multi-emitter transistors, a multiplication or reduction of the flow achieved by the multiple emitter terminals are connected in parallel. Is executed, for example, in the above circuit Q2 as multi-emitter transistor having three emitters, a collector current of Q2 by a factor of 3 is larger than the collector current of Q1. If, however, Q1 designed as triple multi-emitter transistor is reduced, the collector current of Q2 to 1/3.
Circuit with MOSFETs
If the simple current mirror constructed with identical MOSFETs, one obtains for the relation between input current Ie and the gate-source voltage UGS:
For output current Ia applies:
(kn represents a baugruppenspezische size dar. Pictured right, is it an NMOS therefore kn). Thus we obtain for the ratio of the two currents:
After all parameters as well as W, L and Uth are for identical transistors equal and UGS is equal by the interconnection, you can shorten these quantities and receives.
Uth the voltage is the threshold voltage at the gate from the channel begins to conduct.
A deviation from this ideal behavior is due to the channel length modulation of the transistors, but this was ignored here. Error arising from the temperature dependence of Uth, among others; Therefore, and due to the required equality of the layer thicknesses and thus the component parameters such current mirrors are preferably constructed monolithically ( on a chip ).
Disadvantage of the MOSFET current mirror to the bipolar current mirror is the voltage dependence of the controlled output current.
Examples
The table below shows typical examples of the current mirror in the bipolar technology and the equivalent MOSFET circuits. The resistors in the bipolar current mirrors are optional, in MOSFET technology, they are uncommon.
The main criteria in choosing a current mirror circuit, the minimum supply voltage, the output resistance and the accuracy requirements of the mirrored current.
To meet the accuracy requirements, current mirrors are constructed symmetrically as possible, usually in the quad layout or common- centroid layout. In MOSFET current mirrors only gates the same length and width are used, the mirror ratio is therefore determined only by the number of transistors. The current mirror is surrounded with so-called dummy gates that have to produce a uniform environment to the current mirror. For applications with high demands on accuracy of the voltage drop across the drain -source path must be as identical as possible and the gate interconnection are held de-energized, otherwise the gate-source voltage of the various transistors will be different, leading to large errors in the mirror ratio.