ECE 2212

EXPERIMENT 8

22 April 2004

BJT CURRENT SOURCES

PURPOSE

The purpose of this experiment is to measure and compare the properties of

Ø BJTs in an NPN IC Array

Ø Simple Current Source

Ø Widlar Current Source

COMPONENTS

Ø LM3046 transistor array (We will also use this array in the next experiment, Differential Amplifier. Keep track of your chip) The data sheet will be distributed in class.

Ø Resistors and potentiometers as required

PRELAB

Compute the values of the resistors you will need to evaluate the simple and Widlar current sources at the indicated current levels.

GENERAL INFORMATION

Ø In IC biasing networks, it is essential that transistors be well matched and parameter variations track with temperature. In this experiment you will evaluate the properties of a BJT IC array and apply the devices to several current sources/sinks and mirrors. Figure 1 is a pinout of the LM3046 Transistor Array. Observe that you MUST connect Pin 13, the IC substrate to the most negative point in the circuit.

Ø The first tasks are to characterize the npn BJTs and select those that are most identical.

Ø The only reason there is a fixed 10 kW resistor in the circuit is to protect the BJT against inadvertent applying a high voltage across the Base-Emitter junction as you adjust the potentiometer. You do not want to apply 15 volts to the base of Q1 because the chip becomes toast!!! Effectively, the series combination of the 10 kW resistor and the potentiometer is the R_REF.

Figure 1 LM3046 NPN BJT ARRAY

SIMPLE CURRENT SOURCE

Figure 2 is a schematic diagram of a simple current source.

Measure and compare the characteristics of the BJTs you will use on the array and ascertain the degree of b and I-V characteristics match. Measure at 1 ma and at 50 mA collector currents in the forward active region. Use the parameter analyzer.

Connect the collector of Q2, (VC2) to a 5-volt DC supply. Place a DMM in series with the Q2 collector lead to measure current. Set IC2=I_X to 1 mA. Compare this value to the reference current. Measure all key currents and voltages. Construct the I-V output characteristic by changing VC2. Obtain the output resistance from the slope. Compare these results with the parameter analyzer result, i.e. the slope of the output characteristic and your extracted Early voltage.

WIDLAR CURRENT SOURCE

Figure 3 is a schematic diagram of a Widlar current source.

Figure 3 WIDLAR CURRENT SOURCE

For a reference current of 1 mA, compute the value of RE required to obtain I_x = 50 mA. Note that VCC = 15 volts. Now connect the collector of Q2 (VC2) to a 5-volt DC supply. Place a DMM in series with the Q2 collector lead to measure current. Set IC2 to 50 mA. Measure all key currents and voltages. Construct the I-V output characteristic. Obtain the output resistance. Compare these results with the simple current source results. You will have to measure carefully because the slope will be close to flat as you would expect.

SPICE VERIFICATION

Your report should include a SPICE simulation of the I-V output characteristics for both current sources. Compare with your measured and analytical results.

REPORT INFORMATION

This is a one-week experiment. The report format is one cover page with the abstract and no more than three additional pages.

MURPHY’S LAW

I only provided a very short introduction in Problem Set 10 for an application of Murphy’s Law. Murphy’s Law is far more universal than even Ohm’s Law of E= mc². No engineer should be without this detailed treatise.