ECE 2212

Note that I am distributing both Experiment 8 (BJT Current Sources) and Experiment 9 (Emitter-Coupled Pair).  Your Experiment 8           (1 December)  report is due on Thursday, 8 December.  Experiment 9 (8 December) results are to be included in your notebook.  No report is required.   I will use the laboratory period, Thursday, 15 December, to individually review and evaluate your laboratory notebooks.

EXPERIMENT 8

1 December 2005

BJT CURRENT SOURCES

PURPOSE

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

Ψ     BJTs in an NPN IC Array

Ψ     Simple Current Source

Ψ     Widlar Current Source

COMPONENTS

Ψ     LM3046 transistor array.  The data sheet will be distributed in class.

Ψ     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

Ψ                 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.

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

Ψ                 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 RREF.

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=IX 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 Ix = 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

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

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ECE 2212

EXPERIMENT 9

28 April 2005

 EMITTER-COUPLED PAIR

Note that results of this experiment should be included in your notebook to assist you in studying for the Final Exam.    I will be meeting with each one of you individually during our 15 December   laboratory period  to review your notebook.  It should be up-to-date and complete through this experiment.  Note that I am not collecting Experiment 9.

PURPOSE

The purpose of this experiment is to characterize the DC transfer characteristics and AC gain characteristics of the emitter-coupled pair.

COMPONENTS

Ψ                LM 3046 Transistor Array

Ψ                Three 20 kW resistors of which 2 should be reasonably well matched

Ψ                4.7 kW resistor

Ψ                47 W resistor

PRELAB

Use Figure 9-1  for guidance tp prepare a detailed circuit diagram.   Include  pinouts for  the LM 3046 npn array. The data sheet was distributed in support of the Current Source Experiment.  From your circuit diagram and circuit specifications, calculate the expected important  Q-point values,  and Adm, Acm, and the CMRR in dB.

DC MEASUREMENTS

Refer to the diagram and data sheet of the LM 3046/CA3046 BJT array.

Set up the circuit in Figure 9-1  using Q1 and Q2 for the emitter-coupled pair. Q3 and Q4 form a simple current source.  Ground both the inputs of Q1 and Q2. Measure the all Q-point voltages and currents using the DMM.  Use the oscilloscope to also check for excessive noise which may translate as a noisy dc voltage measurement.   Pay particular attention to VOD. Since the transistors and resistors are reasonably well matched, you would expect VOD = 0. If VOD is larger than a few tens of mV, check your circuit and/or match the collector resistors better.  Lead dress and length is also important.  Be neat!  Compare your Q-point values with the expected and PSPICE simulations.  In addition to using the DMM, look for excessive noise using the scope even though you are measuring the dc voltage matching.

Figure 9-1.

 

TRANSFER CHARACTERISTICS

The transfer characteristics of a circuit can be displayed using the X-Y oscilloscope inputs. The amplitude of the input must be large enough to drive the input through the entire desired range of operation. You are particularly interested in the VOD versus VID characteristic. Use a low frequency sinusoid or triangular wave as the input. From a practical viewpoint, if the input signals are noisy because of low amplitudes, you may choose to use an input voltage divider to provide "cleaner" waveforms. Consider implementing the 100:1 voltage divider input drive circuits, Figure 9-2,  although it doesn’t have to be 100:1. Keep track of the divider ratio you finally use to scale your measurement correctly. Also observe that because the oscilloscope does not have a floating input (i.e., one side of each oscilloscope input is connected to ground), you will have to measure either VO1 or VO2 and scale the final results accordingly by a factor of 2 and also do not forget the sign (180°phase) differences for each of the outputs.

Show that the slope of the transfer characteristic will be equal to |Adm/2|. Compare your results to a SPICE simulation.

 

Figure 9-2

DIFFERENTIAL-MODE OPERATION

Set up your input signals, use 1 kHz, so that the output is reasonably linear. You will need some level of voltage division as shown in the figure.   The figure illustrates a 100:1 divider but the actual value is not critical.  Use the oscilloscope and DMM to measure the differential-mode voltage gain. Compare your results to your calculations and a PSPICE simulation.  Include  the effect of a non-infinite Early voltage to improve your analysis and simulation accuracy.

COMMON-MODE OPERATION

Set up your input signals, again use 1 kHz., so that the output is reasonably linear. You will not need an input voltage divider, Figure 9-3,  because the common-mode voltage gain is sufficiently low that you will have to increase the input level significantly above what you used in the differential-mode measurement.   That is do not use the 100:1 input divider.  Prepare a circuit diagram illustrating how you are measuring the common-mode voltage gain..  Measure the common-mode voltage gain and compute the measured CMRR and convert to dB. Compare your results to your calculations and SPICE simulations.

Figure 9-3

MURPHY’S LAW

Murphy’s Law is far more universal than even Ohm’s Law or even  E= mc2.  No engineer should be without this detailed treatise.   This version is specialized for electrical and computer engineering.