Analog Circuits Laboratory EXPERIMENT 3: BJT CURRENT SOURCES

Transcription

1 EE171L rev2 1 University of California, Santa Cruz Baskin School of Engineering Electrical Engineering Department Analog Circuits Laboratory EXPERIMENT 3: BJT CURRENT SOURCES 1. DESCRIPTION AND OBJECTIVES This laboratory investigates several basic bipolar junction NPN transistor current mirror circuits that implement current sources universally employed in integrated circuits to provide reference bias currents. These include the basic BJT mirror, base current compensated current mirror and the Wilson Mirror. Attention will be focused on understanding each of these circuit s basic features with respect to how well they implement an ideal current source. You should have a deeper understanding of the differences between ideal voltage and current sources, as well as an appreciation for the non-ideal characteristics of practical current sources. 2. GENERAL DISCUSSION We will be using a type CA3086 (or equiv.) NPN transistor array. This 14-pin DIP device contains five electrically matched BJT s: two are configured as a differential pair, while the remaining three are uncommitted single transistors. The chip is not static sensitive. However, when using it, be certain the common monolithic substrate, pin-13, is always at the lowest DC circuit potential. In this lab just be sure its grounded! Refer to the attached data sheet for pin definitions and other useful information about this device.

2 EE171L rev CIRCUIT-1: BASIC BJT CURRENT MIRROR Construct the circuit shown in fig. 1. Be sure to accurately measure (with the most significant figures possible) and. Also, verify that your 10k pot is really 10k before using it! Figure 1. Basic BJT Current Mirror. sets to about 1 ma. and the 10k potentiometer constitute the load on Q 2. This load configuration will make it easier to quickly obtain necessary experimental measurements. You are to measure (current through R1),, the mirrored current ratio, and determine the Norton and Thevenin equivalent circuits for this mirror. From your data, produce two accurate and well-drawn plots as follows: Graph 1: vs. for the full range possible for (10k pot over its full range). Calculate the slope from a least-squares regression line analysis and use it to estimate. Indicate which data pairs you used and where the voltage compliance range is (use data sheets to estimate where it ends). Graph 2: vs. for the same range of as graph 1. Explain why the current ratio changes.

3 EE171L rev2 3 Experimental Procedure: 1. Adjust the DC power source for 15 [V] before connecting it to your circuit. This is nominal; get it close and measure it precisely. Be sure the substrate, pin 13, is grounded. Although these circuits are simple, put the pin numbers on your experimental schematics to help you keep track of what has been done. 2. Keep a rough sketch of graph 1 in your engineering notes as you proceed. This will give you enough perspective to judge how close and how many data measurements are needed. 3. To help you collect and calculate the right quantities, consider beginning with the following table: V R1 V CE2 V CC2 V R2 I C2 I Take all measurements with as many significant figures as possible. Your calculations should be expressed with proper precision. 4. Don t try and measure current using the current feature on the lab DVM. Current measurements require breaking the circuit and inserting some amount of sampling resistance (contained inside the DVM) that depends on what current scale is being used (lower currents require greater resistance). This will cause unavoidable errors here. Therefore, all currents should be calculated from an appropriate measured voltage using Ohm s law (be sure and log the voltage). and must be found in this manner and will require floating voltage measurements.

4 EE171L rev CIRCUIT-2: Basic Current Mirror with Base Current Compensation Modify circuit 1 by adding the base current compensation transistor Q 3 as shown in fig. 2 below. Repeat the same procedure experimental procedure you followed for the first circuit and report with the same data and graphs. Figure 2. Basic BJT Base Compensated Mirror.

5 EE171L rev CIRCUIT-3: Wilson Current Mirror Modify circuit 2 into a Wilson Mirror as shown below in fig. 3. Repeat the same procedure experimental procedure you followed for the first circuit. Also report with the same data and graphs. Figure 3. Wilson Current Mirror. 6. QUESTIONS 1. Carefully compare the results for these three circuits and discuss how well each one implements an ideal current source. Try and rank them in terms of which is the best to the worst in this regard. 2. If the transistors were not constructed on the same monolithic substrate, how would this affect the performance of each circuit? 3. Calculate the small-signal output resistance from data sheets and compare to the results you experimentally obtained for.