lab3 - ECE 315 Lab 3 SIMPLE MOS CURRENT MIRRORS AS ACTIVE...

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S IMPLE MOS C URRENT M IRRORS AS A CTIVE L OADS 1 Objectives In this lab, you will examine the characteristics of a simple circuit, called a current mirror , which is made from two matched MOS transistors. Such circuits produce a copy of their input currents. By simple extensions of the basic mirror idea, we can produce many replicas of the same input current or we can scale an input current by a fixed gain. We use current mirrors as building blocks in designing more complex analog integrated circuits. One of their main functions is to set the large-signal Q point for small-signal operations. 2 Prelab Your responses to these questions should not involve any significant algebraic manipulations or any of the MOS transistor model equations. 1. Consider the pair of matched n MOS transistors, shown in Fig. 1a. Suppose that the Early effect is negligible and that V 1 and V 2 are sufficiently far above ground to keep both transistors saturated. What relationship holds between I 1 and I 2 regardless of the value of V Gn ? What could you say about I 1 and I 2 if the Early effect were significant? (a) (b) (c) Figure 1: Development of a simple n MOS current mirror. 2. Suppose we connect a p MOS transistor to the n MOS transistor on the left, as shown in Fig. 1b. What must be true of the currents I 1 and I 3 for this circuit ? Now the left side of the circuits is complete, i.e., there is a path from V DD to GND (or a loop if you add the power source V DD ). 3. Suppose that we have initially set V Gp so that the saturation current of the p MOS transistor is much larger than that of the n MOS transistors. Including the Early effect, explain how V 1 adjusts itself to arrange for this relationship to hold between I 1 and I 3 . You can think that there is a capacitor at the V 1 node. What would happen to V 1 if we were to adjust the value of V Gp so that the saturation current of the p MOS transistor is much less than that of the n MOS ones? 4. Suppose that we connect V 1 to V Gn , as shown in Fig. 1c, which corresponds to diode connecting the n MOS transistor on the left. A pair of n MOS transistors configured like this is called a simple nMOS current mirror . In this circuit, we allow V Gn to adjust itself in much the same way as V 1 adjusted itself in the circuit of Fig. 1b. In this case, what happens to I 1 and I 2 as V Gn adjusts itself? 1
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lab3 - ECE 315 Lab 3 SIMPLE MOS CURRENT MIRRORS AS ACTIVE...

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