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| university:courses:electronics:electronics-lab-12 [04 Sep 2017 15:39] – [Measuring Common Mode gain] Doug Mercer | university:courses:electronics:electronics-lab-12 [25 Jun 2020 22:07] (current) – external edit 127.0.0.1 |
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| ====== Activity 12. BJT Differential pair ====== | ====== Activity: BJT Differential pair ====== |
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| ===== Objective: ===== | ===== Objective: ===== |
| ===== Hardware Setup: ===== | ===== Hardware Setup: ===== |
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| The first waveform generator should be configured for a 200 Hz Triangle wave with 4 volts amplitude and 0 offset. The second generator should be configured also for a 200 Hz Triangle wave with 4 volts amplitude and 0 volts offset but with 180 degree phase. The resistor dividers will reduce the signal amplitude seen at the bases of Q<sub>1</sub>and Q<sub>2</sub>to slightly less than 200 mV. Channel one of the scope should be connected with 1+ to the output of the first generator, W1 and 1- connected to W2. Channel 2 should be connected to display 2+ and 2- and set to 1V per division. | {{:university:courses:electronics:diff_pair_tail_res-bb.png|}} |
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| | <WRAP centeralign> Figure 3 Differential pair with tail resistor Breadboard Circuit </WRAP> |
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| | The first waveform generator should be configured for a 200 Hz Triangle wave with 4 volts amplitude peak-to-peak and 0 offset. The second generator should be configured also for a 200 Hz Triangle wave with 4 volts amplitude peak-to-peak and 0 volts offset but with 180 degree phase. The resistor dividers will reduce the signal amplitude seen at the bases of Q<sub>1</sub>and Q<sub>2</sub>to slightly less than 200 mV. Channel one of the scope should be connected with 1+ to the output of the first generator, W1 and 1- connected to W2. Channel 2 should be connected to display 2+ and 2- and set to 1V per division. |
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| ===== Procedure: ===== | ===== Procedure: ===== |
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| The following data should be taken: the X axis is the output of the arbitrary waveform generator and the Y axis is scope channel 2 using both the 2+ and 2- inputs. By changing the value of R<sub>3</sub>, you should explore the effects of the level of the tail current on the gain of the circuit (as seen in the slope of the line as it passed through the origin) and the linear input range and the shape of the nonlinear (tanh) fall off in the gain as the circuit saturates. With minor additions to the basic circuit, such as emitter degeneration resistors, you should also explore techniques to extend and linearize the range of the input swing and the effects on circuit gain. | The following data should be taken: the X axis is the output of the arbitrary waveform generator and the Y axis is scope channel 2 using both the 2+ and 2- inputs. By changing the value of R<sub>3</sub>, you should explore the effects of the level of the tail current on the gain of the circuit (as seen in the slope of the line as it passed through the origin) and the linear input range and the shape of the nonlinear (tanh) fall off in the gain as the circuit saturates. With minor additions to the basic circuit, such as emitter degeneration resistors, you should also explore techniques to extend and linearize the range of the input swing and the effects on circuit gain. |
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| | <WRAP centeralign>{{:university:courses:electronics:diff_pair_tail_res-wav.png?500|}}</WRAP> |
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| | <WRAP centeralign> Figure 4 Differential pair with tail resistor XY plot </WRAP> |
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| ====== Using a current source as the tail current.====== | ====== Using a current source as the tail current.====== |
| {{ :university:courses:electronics:a12_f3.png?500 |}} | {{ :university:courses:electronics:a12_f3.png?500 |}} |
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| <WRAP centeralign> Figure 3 Diff pair with tail current source </WRAP> | <WRAP centeralign> Figure 5 Diff pair with tail current source </WRAP> |
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| | ===== Hardware Setup: ===== |
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| | {{:university:courses:electronics:diff_pair_tail_cs-bb.png|}} |
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| | <WRAP centeralign> Figure 6 Differential pair with tail current source Breadboard Circuit </WRAP> |
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| | ===== Procedure: ===== |
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| | Same procedure as for the tail resistor. |
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| | <WRAP centeralign>{{:university:courses:electronics:diff_pair_tail_cs-wav.png?500|}}</WRAP> |
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| | <WRAP centeralign> Figure 7 Differential pair with tail current source XY plot </WRAP> |
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| ====== Measuring Common Mode gain ====== | ====== Measuring Common Mode gain ====== |
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| | {{ :university:courses:electronics:a12_f4.png?500 |}} |
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| | <WRAP centeralign> Figure 8 Common Mode Gain configuration </WRAP> |
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| Common mode rejection is a key aspect of the differential amplifier. CMR can be measured by connecting the base of both transistors Q<sub>1</sub> and Q<sub>2</sub> to the same input source. The plot below shows the differential output for both the resistively biased and current source biased differential pair as the common mode voltage from W1 is swept from +2.9V to -4.5V around ground. The maximum positive swing on the input is limited to the point where the base voltage of the transistors exceed the collector voltage and the transistors saturate. This can be checked by observing the collector voltage of the transistors single ended with respect to ground (i.e. grounding the 2- scope input.) | Common mode rejection is a key aspect of the differential amplifier. CMR can be measured by connecting the base of both transistors Q<sub>1</sub> and Q<sub>2</sub> to the same input source. The plot below shows the differential output for both the resistively biased and current source biased differential pair as the common mode voltage from W1 is swept from +2.9V to -4.5V around ground. The maximum positive swing on the input is limited to the point where the base voltage of the transistors exceed the collector voltage and the transistors saturate. This can be checked by observing the collector voltage of the transistors single ended with respect to ground (i.e. grounding the 2- scope input.) |
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| {{ :university:courses:electronics:a12_f4.png?500 |}} | ===== Hardware Setup: ===== |
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| | {{:university:courses:electronics:common_mode_gain_bjt-bb.png|}} |
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| | <WRAP centeralign> Figure 8 Common Mode Gain Breadboard Circuit </WRAP> |
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| | ===== Procedure: ===== |
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| | {{:university:courses:electronics:common_mode_gain_bjt-wav.png|}} |
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| | <WRAP centeralign> Figure 9 Common Mode Gain Waveform </WRAP> |
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| <WRAP centeralign> Figure 4 Common Mode Gain configuration </WRAP> | |
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| ===== Questions: ===== | ===== Questions: ===== |
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| Repeat the common mode gain measurements on the circuit shown in figure 3 with the tail current source. How has the result changed and why. | Repeat the common mode gain measurements on the circuit shown in figure 3 with the tail current source. How has the result changed and why. |
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| | <WRAP round download> |
| | **Resources:** |
| | * Fritzing files: [[downgit>education_tools/tree/master/m2k/fritzing/bjt_diff_pair_bb | bjt_diff_pair_bb ]] |
| | * LTspice files: [[downgit>education_tools/tree/master/m2k/ltspice/bjt_diff_pair_ltspice | bjt_diff_pair_ltspice ]] |
| | </WRAP> |
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| **Return to Lab Activity [[university:courses:electronics:labs|Table of Contents]]** | **Return to Lab Activity [[university:courses:electronics:labs|Table of Contents]]** |
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