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Lab 1 BJT Differential Amplifier October 9, 2014 Michelle Acosta Executive Summary: The purpose of Lab 1 was to analyze the BJT Differential Amplifier. The differential amplifier is unique in that it requires two input signals and two outputs. Named appropriately, the differential amplifier amplifies the difference between the two input signals. In this case, the circuit analyzed also has a current mirror as a load. The idea of the current mirror is to ideally produce the same amount of current as the differential amplifier by using two Figure 1: The BJT Differential Amplifier before the load resistances and input voltages are altered to produce DOM and SEOM. The left part of the circuit contains the current mirror to provide a constant current. more transistors. The lab consisted of hand calculations, simulation, and physical circuit analysis. All three stages of the lab analyzed the circuit in DC, Differential-Output Mode, and Single-Ended Output Mode. Results are listed in the Summary Table. Ideally the circuit would produce a differential gain of 20V/V in Differential-Output Mode and a gain of around 80V/V in Single-Ended Output Mode. Lab Procedure and Measurement Discussion: The lab was completed over the course of four weeks. Hand calculations and simulation were completed the first week in order to have a basis for the non-ideal case of circuit simulation. The building and testing of the circuit was conducted for three class periods. This gave ample time to test the circuit in DC, DOM, and SEOM. Beginning with the PSPICE simulation, shown in Figure 2, quick results were obtained in order to set the basis for the analysis of the amplifier. The DC analysis was simplest in that once the circuit was on, the DC values were calculated. 2 (a) (b) Figure 2(a) shows the differential amplifier in DOM differential mode. This mode is used to calculate the differential gain (Ad), differential input resistance (Rid), and differential output resistance (Rod). Figure 2(b) shows the differential amplifier in DOM common mode. This mode is used to calculate the common mode gain (Acm), common input resistance (Ricm), common output resistance (Rocm), and CMRR. The AC analysis revolved around the Midband gain. The sketch of the ideal amplifier bode plot is shown in Figure 3. The frequency at which the midband gain is allows the neglecting of capacitances. In addition to simplifying the analysis, the midband gain also provides a constant, high gain. Being that this frequency is the most consistent, this is often what Figure 3: The frequency response of a CS amplifier highlighting the area of the midband gain. It can be seen that all capacitances can be neglected at that frequency. (Sedra-Smith 323) transistors are rated at. Removing RL2 and applying an AC voltage at the input could analyze the circuit analyzed in DOM. There were two different analyses conducted. Differential mode required RL2 to be infinite, vb2 to be grounded, and vb1 to be connected to an AC source. 2 PSPICE was used to calculate the differential gain (Ad), differential input resistance (Rid), and differential output resistance (Rod). In addition to differential mode, common mode was also analyzed. In Common Mode DOM, both Q1 and Q2 are connected to the AC source. PSPICE was used again to calculate the common mode gain (Acm), common input resistance (Ricm), common output resistance (Rocm), and Common Mode Rejection Ratio (CMRR). All values for DOM are noted in the Summary Table. Because DOM has no reference to ground, the difference is measured on the two input voltages vb1 and vb2. It was observed that the circuit was very sensitive making the DOM mode attractive. There was no need to take measurements in common mode because the common mode gain (Acm) was zero, making the CMRR undefined. (a) (b) Figure 4(a) shows the differential amplifier in SEOM differential mode. This mode is used to calculate the differential gain (Ad), differential input resistance (Rid), and differential output resistance (Rod). Figure 4(b) shows the differential amplifier in SEOM common mode. This mode is used to calculate the common mode gain (Acm), common input resistance (Ricm), common output resistance (Rocm), and CMRR. SEOM was analyzed in a very similar way except that RL1 was made infinite instead of RL2. Again, SEOM was analyzed in both Common Mode and Differential Mode. All values for SEOM are noted in the Summary Table. Hand calculations were also done in order to further analyze the circuit and give another set of values to compare with. 3 Finally, measurements were done in lab. The circuit was built without R L1 and RL2. In order to have all necessary transistors, part LM3046 was used. Procedures for DC were the same as those of the hand calculations and simulation. During the AC analysis, a 10 to 1 attenuator was used at the input. In DOM, measurements were conducted for Ad, Rid, and Rod. For SEOM, measurements for Ad, Rod, Acm, and CMMR were found. All values for lab measurements are noted in the Summary Table. The difference measured in SEOM is between the wire and ground. This allowed for easier analysis with the oscilloscope. Although this was true, CMRR could now be defined because there was a reference to ground, giving a common mode gain. The CMRR is used to reject the unwanted signals. This is very important in a differential amplifier and it requires a high CMRR. This was true in the circuit analyzed. The differential gain was large and the common mode gain was small, giving a large rejection ratio, indicating that the unwanted signals were rejected. The amplifiers studied previous to this lab all took an input signal and amplified it to obtain a certain gain. What happened in this lab was that two input signals were taken and the difference between the two signals was amplified. In addition to the amplifier, there was also a current mirror. The ideal current mirror will do exactly as the name implies, create a current identical to that of the rest of the circuit. The current outputted by the current mirror, IREF, will ideally be equal to Io even with a load. In comparison to the In order to further understand the capabilities of the Differential Amplifier and the Current Mirror, the amplifier was analyzed in DC, Differential-Output mode, and SingleEnded-Output mode. During the DC analysis, all of the transistors were matched. Because Q 1 and Q2 were matched and there was no load resistance, it made sense that the current I o would be split evenly between the two sides. Q3 and Q4 were matched as well and created a current that was identical to the one going through the other two transistors. It can be seen in Figure 1 that the current through Q1 and Q2 are matched and that the current in the current mirror is equal to the current at the emitter. In AC the Differential-Output mode and the Single-Ended-Output mode were analyzed in both differential and common modes. For both circuits, an input attenuator 4 was used to bring the voltage down before it was amplified again. The attenuator consisted of several resistors. For DOM in differential mode, vb2 was grounded in order to shut Q2 off and a large voltage was applied to vb1 in order to keep Q1 on. In common mode, both Q1 and Q2 were connected to the source. Comparing the measured values with the hand calculations and simulation values, all made sense. The reason Rod and Rocm were so large for the hand calculations was because the load resistance was initially neglected. If it had been included, Rod would have been much closer to that of the measured value. In SEOM, the same attenuator was used to bring down the voltage without distortion at vb1. In differential mode, vb2 was grounded and in common mode, both Q1 and Q2 were both connected to the source. There were no major discrepancies with the values in the summary table except for the differential gain. The reason the value was so low was not because of an error with the circuit, but instead, an error with calculation. Once correctly calculated, the measured differential gain was much closer to the expected value. Conclusion: Both the BJT Differential Amplifier and the current mirror were analyzed during the lab. The circuit was analyzed in both DC and AC to further understand the behaviors of the circuits under different conditions. In AC, the circuit was analyzed in both Differential-Output mode and Single-Ended-Output mode. The difference between the two depended on where the load resistances were connected and made infinite. Changing the modes in AC allowed for different differential and common mode gains. 5