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ELEC 351L Electronics II Laboratory Spring 2002 Lab 8: BJT Differential Amplifier Measurements Introduction One of the main advantages of the differential amplifier is that it amplifies the differential voltage from a desired signal source, such as a sensor or low-level radio circuit, but attenuates the undesired common-mode signals that are induced on the cable or wires that carry the signal to the amplifier. In order to take advantage of the common-mode rejection capability, however, care must be taken in applying the desired signal to the input of the amplifier. It is also often important to be aware of the small-signal input and output resistances of the amplifier. In this lab experiment you will examine some of the practical aspects of diff amp design. Theoretical Background The basic diff amp circuit that was used in the previous experiment is shown in Figure 1. In the configuration shown, the amplifier takes two “single-ended” input voltages v1 and v2, and it VCC R C1 R C2 vOUT1 vOUT2 Q1 I ref R ref v1 Q2 + _ + _ v2 Io QA QB RA RB VEE Figure 1. Difference amplifier with resistor loads. 1 produces two “single-ended” output voltages vo1 and vo2. A “single-ended” voltage is one that is referenced to ground. For example, we have represented the input voltage v1 in Figure 1 using a voltage source between the base of Q1 and ground, as shown in the circuit fragment on the lefthand side of Figure 2. However, we could have represented the input voltage using the alternative approach shown on the right-hand side of Figure 2. In the latter case, it is understood that voltage v1 is referenced to ground; that is, v1 is the node voltage measured between input terminal #1 and ground. Input terminal #1 is called a single-ended input because it can be represented in a schematic diagram using a single circle (a “single end”) with a voltage label (v1) next to it. Q1 v1 v1 Q 1 + _ Figure 2. Two alternative methods to represent single-ended input voltages. Although the single-ended representation shown in Figure 1 is convenient for the purpose of describing how a diff amp works, it is not the usual configuration. More typically, a single input voltage is applied to the diff amp between the two input terminals; that is, the input voltage is not referenced to ground but rather “floats” above ground. In the diff amp circuit, a voltage applied across the two input terminals is called a differential input voltage. Similarly, the two output voltages vo1 and vo2 labeled in Figure 1 are single-ended output voltages and are referenced to ground. The voltage between the two output terminals can be taken as the output voltage instead of one of the two single-ended output voltages. The voltage measured between the two output terminals is called the differential output voltage. In the laboratory it is somewhat difficult to apply a floating differential voltage to the input of a diff amp or to measure directly the differential output voltage. This is because function generators and oscilloscopes generally have one terminal of their outputs or inputs tied to ground. If one were to attempt to use a function generator to apply a differential voltage to the input of a diff amp, for example, the outer conductor of the cable, which is grounded, would ground one of the inputs of the diff amp. One solution to this problem is to use isolation transformers between the inputs and outputs of diff amps and the single-ended (one terminal grounded) test equipment connected to them. Recall that a transformer is made by winding two or more wires around a common iron or ferrite core. A sinusoidal voltage applied to one of the windings causes a sinusoidal voltage to appear across the other winding(s). The ratio of the input (primary) voltage to the output (secondary) voltage is equal to the ratio of the number of turns on each winding. Isolation transformers typically have turns ratios of 1:1, so whatever voltage is applied across one winding, the same voltage appears across the other winding. This voltage relationship is shown in Figure 3a. 2 1:1 1:1 + + + v v v _ _ _ (a) + + 0.5v _ v + 0.5v _ _ (b) Figure 3. Isolation transformers with 1:1 turns ratios. The circles indicate winding connection points. (a) Basic 1:1 isolation transformer. (b) Isolation transformer with center tap on one of the windings. Some isolation transformers have “taps,” or connection points, at intermediate locations between the two ends of a given winding. A transformer with a center tap (half-way between the two ends) on one winding is depicted in Figure 3b. Note that in a 1:1 transformer the voltage between one end of a winding and a center tap is half that applied to the other winding. This is because the number of turns between an end and a center tap is half the number of turns in the other full winding. Since the turns ratio in this case is 2:1, the voltage ratio is 2:1 (or 1:0.5). To see why isolation transformers are so useful, consider the transformer in Figure 3a. Suppose that that output of a function generator is connected to the left-hand winding. One end of the winding will be grounded, because one terminal of the generator is grounded. A sinusoidal voltage v will be developed across the left-hand winding, so an identical sinusoidal voltage v will appear across the right-hand winding. However, the secondary voltage “floats” above ground. That is, neither end of the right-hand winding is grounded. Thus, an isolation transformer provides a way to create a sinusoidal voltage across two nodes in a circuit without grounding either node. Tapped isolation transformers can be used to “move” the ground reference point in a circuit as shown in Figure 4. Here, a pair of sinusoidal voltages that are equal and opposite with respect to ground have been created by applying the output of a single-ended function generator to a center-tapped transformer. When the upper end of the right-hand winding is positive with respect to ground, the lower end is negative, and vice versa. This design technique can be very useful when applied to diff amp circuits. + 1:1 0.5v g _ + vg + _ 0.5v g _ Figure 4. Repositioning the ground reference using an isolation transformer. 3 Experimental Procedure Reconstruct the diff amp circuit shown in Figure 1 using the same bias point and power supply voltages as in the previous experiment. Remember to connect the COM terminal of the power supply to the circuit ground. Use two matched pairs of 2N2222 or 2N3904 BJTs for the current mirror and for the amplifying devices. The pin-outs of the 2N2222 and 2N3904 are shown in Figure 5. As before, the quiescent output voltages (VOUT1 and VOUT2) should be around +1.4 V. Verify that the quiescent output voltages and voltage drops across the current mirror emitter resistors are correct before continuing. Lead identification for 2N3904 in TO-92 package Lead identification for 2N2222 in TO-18 package (top view) (top view) E B C E B C Figure 5. Identification of terminals for 2N3904 and 2N2222 npn BJTs. Using a transformer, devise a way to apply a differential sinusoidal voltage to the input of the diff amp without connecting either input terminal to ground. Keep in mind that proper operation of the circuit depends upon the DC bias voltages at the bases of Q1 and Q2 each being equal to 0 V. Your design must satisfy both of these requirements (differential input voltage and zero base bias voltage). Next, devise a way to use a second transformer to allow you to measure the differential output voltage of the amplifier using the oscilloscope without grounding the collector of either Q1 or Q2. Apply a differential sinusoidal voltage vidm at a frequency of 1 or 2 kHz to the input of the amplifier, and measure the resulting differential output voltage vodm. Recall that vidm = v1 – v2, and vodm = vo1 – vo2, where v1 and v2 are the single-ended input voltages to Q1 and Q2, respectively, and where vo1 and vo2 are the single-ended output (collector) voltages of the two BJTs. It may be necessary to use a voltage divider at the output of the function generator in order to avoid clipping. If you detect oscillation in the output waveform (often characterized by a “fuzzy bulge” along all or part of the displayed waveform), try connecting 0.001-F capacitors between the bases of Q1 and Q2 and ground. The capacitors should break the positive feedback path at high frequencies by shunting any high-frequency signals to ground. (What is the reactance of a 0.001-F capacitor at, say, 100 MHz?) Calculate the differential-mode, differential output gain Adm-diff of the amplifier, and compare the result to the theoretical value given by Adm diff vodm g m RC . vidm 4 Recall that you can determine an approximate value for gm using gm IC , VT where the product VT has a value of approximately 0.025 V at room temperature and where IC is the quiescent collector current flowing through either Q1 or Q2. Determine the differential-mode, single-ended output gains Adm-se1 and Adm-se2 of the amplifier. It will be necessary to remove one of the transformers to do this. Compare the values you obtain to the theoretical values Adm se1 vo1 g m RC vidm 2 and Adm se2 vo 2 g m RC . vidm 2 Devise a way to measure the differential output resistance rout-diff, and determine its value for your amplifier. Compare the value you obtain to the theoretical value given by rout diff 2 RC . Based on the value you found for rout-diff, give one reason why this amplifier would make a poor choice for driving an 8- speaker (i.e., what would happen if you were to connect the speaker directly across the output terminals? How much of the signal power would be converted to acoustic power by the speaker?) 5