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| university:courses:electronics:electronics-lab-12m [24 Apr 2017 08:32] – rename Antoniu Miclaus | university:courses:electronics:electronics-lab-12m [25 Jun 2020 22:07] (current) – external edit 127.0.0.1 |
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| ====== Activity 12M. MOS Differential pair ====== | ====== Activity: MOS 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 2 volt amplitude and 0 offset. The second generator should be configured also for a 200 Hz Triangle wave with 2 volts amplitude and 0 volts offset but with 180 degree phase. 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 1 V per division. | {{ :university:courses:electronics:diff_pair_tail_res_mos-bb.png|}} |
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| | <WRAP centeralign> Figure 3 NMOS Differential pair Breadboard Circuit </WRAP> |
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| | The first waveform generator should be configured for a 200 Hz Triangle wave with 4 volt 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. 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 1 V 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>, the student can 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 fall off in the gain as the circuit saturates. With minor additions to the basic circuit, such as source degeneration resistors, the student can 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>, the student can 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 fall off in the gain as the circuit saturates. With minor additions to the basic circuit, such as source degeneration resistors, the student can explore techniques to extend and linearize the range of the input swing and the effects on circuit gain. |
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| | {{ :university:courses:electronics:diff_pair_tail_res_mos-wav.png?500 |}} |
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| | <WRAP centeralign> Figure 4 NMOS Differential pair XY plot </WRAP> |
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| {{ :university:courses:electronics:a12m_f3.png?400 |}} | {{ :university:courses:electronics:a12m_f3.png?400 |}} |
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| <WRAP centeralign> Figure 3 Gain curves </WRAP> | <WRAP centeralign> Figure 5 Gain curves </WRAP> |
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| ====== Using a current source as the tail current.====== | ====== Using a current source as the tail current.====== |
| {{ :university:courses:electronics:a12m_f4.png?500 |}} | {{ :university:courses:electronics:a12m_f4.png?500 |}} |
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| <WRAP centeralign> Figure 4 Diff pair with tail current source </WRAP> | <WRAP centeralign> Figure 6 Diff pair with tail current source </WRAP> |
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| | ===== Hardware Setup: ===== |
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| | {{ :university:courses:electronics:diff_pair_tail_cs_mos-bb.png|}} |
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| | <WRAP centeralign> Figure 7 Diff pair with tail current source Breadboard Circuit </WRAP> |
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| | ===== Procedure: ===== |
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| | {{ :university:courses:electronics:diff_pair_tail_cs_mos-wav.png?500 |}} |
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| | <WRAP centeralign> Figure 8 Diff pair with tail current source XY plot </WRAP> |
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| ====== Measuring Common Mode gain====== | ====== Measuring Common Mode gain====== |
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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 M<sub>1</sub> and M<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 is swept + and - 200mV around ground . | Common mode rejection is a key aspect of the differential amplifier. CMR can be measured by connecting the base of both transistors M<sub>1</sub> and M<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 +4.5V to -4.5V around ground. The gain will be effected the most as the transistors go from the saturation region to the triode (resistive) region as the positive voltage on the gates approaches the drain voltage. This can be monitored by observing the drain voltage single ended with respect to ground (i.e. with the 2- input grounded). The amplitude of the generator should be adjusted until the signal seen at the output just starts to clip/fold over(as you see in your waveform plot). |
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| {{ :university:courses:electronics:a12m_f5.png?500 |}} | {{ :university:courses:electronics:a12m_f5.png?500 |}} |
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| <WRAP centeralign> Figure 5 Measuring Common Mode gain </WRAP> | <WRAP centeralign> Figure 9 Measuring Common Mode gain </WRAP> |
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| | ===== Hardware Setup: ===== |
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| | {{ :university:courses:electronics:common_mode_gain_mos-bb.png|}} |
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| | <WRAP centeralign> Figure 10 Common Mode Gain Breadboard Circuit </WRAP> |
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| | ===== Procedure: ===== |
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| | {{ :university:courses:electronics:common_mode_gain_mos-wav.png |}} |
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| | <WRAP centeralign> Figure 11 Common Mode Gain Waveform </WRAP> |
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| ===== Questions: ===== | ===== Questions: ===== |
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| What is common-mode voltage, and how should a differential amplifier (ideally) respond to it? | What is common-mode voltage, and how should a differential amplifier (ideally) respond to it? |
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| | <WRAP round download> |
| | **Resources:** |
| | * Fritzing files: [[downgit>education_tools/tree/master/m2k/ltspice/mos_diff_pair_bb | mos_diff_pair_bb]] |
| | * LTspice files: [[downgit>education_tools/tree/master/m2k/ltspice/mos_diff_pair_ltspice | mos_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]]** |
