Current Source/Sinks

Transcription

1 Motivation Current Source/Sinks Biasing is a very important step in MOS based analog design. A current sink and current source are two terminal components whose current at any instant of time is independent of the voltage across their terminals. These current sink and source are widely used for Biasing the MOS transistors for a particular functionality. It is therefore essential to learn about these components. The other components which are used for biasing are current mirrors and differential amplifiers. Prerequisites Small Signal and Large Signal Analysis, MOS Physics Learning Outcome Various types of Current Source and Sink. Parameters and Characteristics of various sources and sinks Limitations and Advantages of Current Source and Sinks. How to design a Current SOurce and Sink based on given specifications Suggested Time 7 hours

2 Animation Fig1_currsoursink.swf (For your convenience you can get them inside Self Learning Quadrant) Simple Current Sink The current of a current sink or source flows from the positive n ode, through current sink or source, to the negative node. A current sink typically has the negative node at and the current source has the positive node at. Figure 1(a) shows the MOS implementation of a cuurent sink. The gate is taken to whatever voltage is necessary to create the desired value of current. Voltage divider can be used to provide this voltage. We note that in the non-saturation region the MOS device is not a good current source. In fact, the voltage across the current sink must be larger than in order for the current sink to perform properly. For Figure 1(a) this means that If the gate-source voltage is held constant, then the large-signal characteristics of the MOS transistor are given by the output characteristics like the one shown in Figure 1(b). If the source and bulk both connected to ground, then the small signal output resistance is given by If the source and bulk are not connected to the same potential, the characteristics will not change as long as is a constant.

3 Figure 1(a) Current sink (b) Current-voltage characteristics of (a) Simple current source Figure 2(a) shows an implementation of a current source using a p- channel transistor. Again, the gate is taken to a constant potential as is the source. With the definition of and of the source shown in Figure 2(a), the large signal I-V characteristic is shown in Figure 2(b). The small-signal output resistance of the current source is given by the Equation (2). The source-drain voltage must be larger than for this current source to work properly. This current source only works for the values of given by

4 Figure 2(a) Current Source (b) Current-voltage characteristics of (a) Cascode Current Source/Sink The advantage of the current sink and source of Figures 1(a) and 2(a) is theri simplicity. However, there are two areas in which their performance may need to be improved for certain applications.one improvement is to increase the small-signal output resistance-resulting in a more constant current over the range of values. The second is to reduce the value of, thus allowing a larger range of over which the current sink/source works properly. We shall illustrate methods to improve both areas of performance. Analysis of Cascode Current Source/Sink (a) Increasing Small-signal output resistance First, the small-signal output resistance can be increased using the principle illustrated in Figure 3(a). This principle uses the common-gate configuration to multiply the source resistance r by the approximate voltage gain of the common gate configuration with and infinite load

5 resistance. The exact small-signal output resistance calculated from the small-signal model of Figure 3(b) as can be where and. Figure 3(a) Technique for increasing the output resistance of a resistor r (b)small signal model of circuit in (a) Figure 4(a)Circuit for increasing of a current sink (b) Small signal model of circuit in (a)

6 The above principle is implemented in Figure 4(a), where the output resistance ( ) of the current sink of Figure 1(a) should be increased by the common-gate voltage gain of M2. To verify the principle, the smallsignal output resistance of the cascode current sink of Figure 4(a) will be calculated using the model of Figure 4(b). Since and, summing the currents at the output node gives Since, we can solve for as Typically, is greater than unity so that Equation (6) simplifies to We see that the small-signal output resistance of the current sink of Figure 4(a) is increased by the factor of. (b) Reducing The other performance limitation of the simple current sink/source was the fact that the constant output current could not be obtained for all values of. This was illustrated in Figures 1 and 2. While this problem may not be serious in simple current source/sink, it becomes more severe in the cascode current sink/source configuration that was used to increase small signal output resistance. It therefore becomes necessary to examine methods of reducing the value of. Obviously, can be reduced by increasing the value of W/L and adjusting the gate-source voltage to get the same output current. However, another method that works well for the cascode currentsink/source configuration will be presented. We must introduce an important principle used in biasing MOS devices before showing the method of reducing of the cascode current sink/source. This principle can best be illustrated by considering two

7 MOS devices, M1 and M2. Assume that the applied dc gate-source voltage can be divided into two parts, given as where is that part of which is in excess of the threshold voltage,. This definition allows us to express the minimum value of for which the device will remain in saturation as Thus, can be thought of as the minimum drain-source voltage for which the device remains saturated. In saturation, the drain current can be written as The principle to be illustrated is based on Equation (10). If the currents of two MOS devices are equal (because they are in series), then the following relationship holds: If both MOS transistors are of the same type, then Equation (11) reduces to or The principle above can also be used to define a relationship between the current and W/L ratios. If the gate-source voltages of two similar MOS devices are equal (because they are physically connected), then is equal to. From Equation (10) we can write

8 Consider the cascode current sink of Figure 5(a). Our objective is to use the above principle to reduce the value of. If we ignore the bulk effects on M2 and M4 and assume that M1, M2, M3 and M4 are all matched with identical W/L ratios, then the gate-source voltage of each transistor can be expressed as as shown in Figure 5(a). At the gate of M2 we see that the voltage with respect to the lower power supply is. In order to maintain current-sink/source operation, it will be assumed that M1 and M2 must have at least a voltage of as given in Equation (9). In order to find of Figure 5(a) we can write Since, substituting this value into Equation (15) gives The current-voltage characteristics of Figure 5(a) are illustrated in Figure 5(b), where the value of of Equation (16) is shown. Figure 5(a)StandardCascode current sink (b) Output characteristics of circuit in (a)

9 High Swing Cascode of Equation (16) is dropped across both M1 and M2. The drop across M2 is while the drop across M1 is. From the results of Equation (9), this implies that of Figure 5 could be reduced by and still keep both M1 and M2 in saturation. Figure 6(a) shows how this can be accomplished. The W/L ratio of M4 is made onequarter of the identical W/L ratios of M1 through M3. This causes the gate-source voltage across M4 to be rather than. Consequently, the voltage at the gate of M2 is now. Substituting this value into Equation (15) gives The resulting current-voltage relationship is shown in Figure 6(b). It can be seen that a voltage of is across both M1 and M2, giving the lowest value of and still keeping both M1 and M2 in saturation. Using this approach and increasing the W/L ratios will result in minimum values of. Figure 6(a) High-swing cascode (b)output characteristics of circuit in (a).

10 Improved High-swing cascode A problem exists with the circuit in Figure (6). The of M1 and the of M3 are not equal. Therefore, the current will not be an accurate replica of due to channel length modulation as well as drain-induced threshold shift. If precise mirroring of the current to is desired, a slight modification of the circuit of Figure (6) will minimize this problem. Figure (7) illustrates this fix. An additional transistor, M5, is added in series with M3 so as to force the drain voltages of M3 and M1 to be equal, thus eliminating any error due to channel length modulation and drain-induced threshold shift. Figure 7. Improved high-swing cascode When power dissipation must be kept at a minimum, the circuit in Figure (7) can be modified to eliminate one of the currents. Figure (8) illustrates a self-biased cascode current source that requires only one reference current.

11 Figure 8. Self-biased high-swing cascode current source