ECE 1212 – Electronic Circuit Design Laboratory

Notes for Experiment 5

These notes provide references to text, figures and equations in Sedra & Smith that can be useful in completing Experiment 5, and for understanding the circuit conditions required to produce a linear amplifier.

Pre-Lab A:

  1. Make sure you obtain the data sheet from the course website. The most popular datasheet for the CD4007 (Fairchild) is missing much of the material that is useful for amplifier design.
  1. It is assumed that all students are familiar with the physical operation of the nMOSFET, the different modes of operation, and the current-voltage relationships for each mode. This material is covered in Sections 5.1 and 5.2, and in particular the (iD - vGS) characteristic in saturation is discussed in Section 5.2.3.
  1. See Figure 5.13 and the text preceding it. In the saturation region, the slopes of the various curves will be greater than that suggested by Figure 5.13 (zero), but less than that suggested by Figure 5.17. The slope is not zero as a result of channel-length modulation,as discussed in Section 5.2.4. This also results in models for the MOSFET that include a finite output resistance.
  1. This step requires you to approximate by computing the slope of the line connecting two points on the linear portion of a curve, and then to take the reciprocal of the computed slope. This computation is prone to error for at least two reasons:
  • The slope you obtain depends on precisely which points you select on the curve.
  • The slope is very small, therefore the reciprocal is very large, and small errors in computing the slope will translate to large errors in approximating .
  1. From Figure 5.17, if we project lines along the linear portions of the various curves, then all of these lines intersect in a common point, where and . You can check whether this is true or approximately true using your data from Steps 4 and 5.
  1. This calculation also requires you to compute the slope of a line connecting two points, but not to take the reciprocal of this slope. Therefore, you should expect some error in the calculation of , but perhaps less than you will experience in calculating .
  1. The transconductance is discussed in Section 5.5, and is defined in Equation 5.49 as the derivative of the (iD - vGS) characteristic at the bias point. From Equation 5.21 and Figure 5.14, we expect that the drain current will increase as the square ofin saturation. Therefore, the value you obtain for will be significantly smaller when than when .
  1. If you choose VDDtoo large, you risk damaging the MOSFET. If you choose VDD too small, then you will have to choose small resistor values in order to achieve the desired drain current, and it will be harder to achieve the required gain.
  1. Use the curve tracer data from Step 6 for this calculation.
  1. Make sure you fully understand the current mirror circuit, and its role in the amplifier, before completing this step. See Sections5.7.4 and 7.4.1. When you build the circuit, will the drain currents in the two transistors be identical? Consider and ; assuming that the circuit is functioning properly, would you expect them to be the same? Why or why not? What must be true about the two transistors on your CD4007 package in order for the current mirror to function? Could you build a current mirror using two of the 2N2222 BJTs from Lab 4?
  1. You should find that the current mirror functions well for some values of , and poorly for other values. If the drain currents are not approximately equal for any choice of , then you must correct your circuit before proceeding.
  1. Remember that you will need to change for this step, which means that you will use a different range of values for .
  1. It is important to understand the conditions required for the current mirror to perform well, because this will restrict the choice of resistor values used in the amplifier.

Pre-Lab B:

  1. Your DC analysis should produce equations relating the various DC voltages and currents in the circuit, assuming that all of the transistors are biased in the saturation region.
  1. For the AC analysis, use the small-signal model for the nMOSFET shown in Figure 5.37(a). See Section 5.6 for analysis of each of the basic MOSFET amplifier configurations.
  1. Make sure that you understand these formulas, and that in each case the resistance in the denominator is the resistance to which the associated capacitor is connected.
  1. With a reduced drain current, the transconductance will also be reduced, which might indicate that it will be harder to achieve the required gain. However, a small drain current will also mean that will be larger, which will increase the gain. It is up to you to discover which of these factors is more significant, and the implications for designing the amplifier.

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