EE 2212

EXPERIMENT 8

5 December 2013

THE EMITTER-COUPLED PAIR

Note 1: A written report is not required. We will jointly review your notebook during the 12 December laboratory period with the expectation that this experiment will be included.

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

PURPOSE

The purpose of this experiment is to characterize the properties of a:

Ø The emitter-coupled pair (DC transfer characteristics)

Ø The emitter-coupled pair (AC gain measurements).

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.

Ø Three 20 kW resistors for the collector resistors of which two should be reasonably well matched

Ø 4.7 kW resistor for the input voltage divider

Ø 47 W resistor for the input voltage divider

GENERAL INFORMATION

Ø In IC biasing networks, it is essential that transistors be well matched and parameter variations track with temperature. Figure 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.

Figure 1 LM3046/CA3046 NPN BJT ARRAY

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PRELAB FOR THE EMITTER-COUPLED PAIR

Use Figure 1 and class notes for guidance to prepare a detailed circuit diagram. Include pinouts for the LM3046/CA3046 npn array. From your circuit diagram and circuit specifications, calculate the expected important Q-point values, and A_dm, A_cm, 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 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 V_OD. Since the transistors and resistors are reasonably well matched, you would expect V_OD = 0 or reasonably close. If V_OD 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 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 V_OD versus V_ID 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 2, although it doesn’t have to be 100:1. The signal generators have a 100 mV minimum. By using a 100:1 external divider, you can achieve a relatively noise free signal at the input to the BJT bases. 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 V_O1 or V_O2 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 |A_dm/2|. Compare your results to a SPICE simulation.

Figure 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 divider value is not critical. Use the oscilloscope and DMM to measure the differential-mode voltage gain. Compare your results to your calculations and a SPICE 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 because the Acm is low, Figure 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 3

If you decide to pursue a BSEE degree, you should at least understand the basics of computer engineering. Above and beyond CS1, the following provides an important understanding of computer technology hardware.

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