EE 2212

EXPERIMENT 9

10 November 2016

BJT CURRENT SOURCES

Note 1: Report is due Thursday, 17 November.

Note 2: The CA 3046 is the same electrically as the LM 3046. Just a different manufacturer.

Note 3: As usual, do not use the current mode on your DMM because of issues with the internal fuse; measure the voltage drop across the appropriate resistor and employ Ohm’s Law.

PURPOSE

The purpose of this experiment is to build, model and characterize the properties of a:

Ø Basic/Simple Current Source

Ø Widlar Current Source

COMPONENTS

Ø LM3046/CA3046 transistor array. The data sheet is posted on the class WEB page

Ø Resistors and potentiometers as required for the current sources.

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. Figure 9.1 is a pin-out of the LM3046/CA3046 Transistor Array. Observe that you MUST connect Pin 13, the IC substrate, to the most negative point in the circuit or bad things happen to the IC and the resultant fragrance in the lab is unmistakable.

Ø The only reason there is a fixed 10 kW resistor in the circuit of Figure 9.2 is to protect the BJT against inadvertent application of 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 (literally and figuratively)!!! Again, bad things happen to the IC and the resultant fragrance in the lab is unmistakable. Effectively, the series combination of the 10 kW resistor and the potentiometer is the R_REF. Measure this total resistance valueYou could substitute a fixed resistor of the same value for the potentiometer.

Figure 9.1 LM3046/CA3046 NPN BJT ARRAY

SIMPLE CURRENT SOURCE

Figure 9.2 is a schematic diagram of a simple current source.

Connect the collector of Q2, (VC2) to a 6-volt DC supply. Use the DMM to measure the voltage drop, V_R, across the 1 kΩ resistor and realize that IC2 = V_R/1 kΩ. Set IC2=I_X to 1 mA by adjusting the 10 kΩ potentiometer. Compare this value to the reference current. Measure all key currents and voltages. Construct the I-V output characteristic by changing VC2 from 0 to 6 volts. Obtain the output resistance from the slope. Compare to a SPICE simulation. Best approach is to enter your data in an EXCEL spread sheet and let the graphing function do all the “heavy lifting”. Of course, use only data in the “flat” region.

WIDLAR CURRENT SOURCE

Figure 9.3 is a schematic diagram of a Widlar current source.

For a reference current of 1 mA, compute the value of R2 required to obtain I_x = 100 mA ±10%. Note that VCC = 15 volts. Now connect the collector of Q2 (VC2) to a 6-volt DC supply. Use the DMM to measure the voltage drop, V_R, across the 10 kΩ resistor and realize that IC2 = V_R/10 kΩ. You may have to change the value of R2 from the computed value to come within 100 mA ±10% . Measure all key currents and voltages. Sketch the I-V output characteristic from VC2 from 0 to 6 volts.. 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. Again, best approach is to enter your data in an EXCEL spread sheet and let the graphing function do all the “heavy lifting”. Compare to a SPICE simulation.

Not quite a TESLA but getting there

After All, This A Lab. How many of you have seen the cute cat videos?

Enough Said!