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| university:courses:electronics:electronics-lab-11m [17 May 2015 15:08] – [Procedure:] Doug Mercer | university:courses:electronics:electronics-lab-11m [07 Feb 2022 15:44] (current) – [Activity: The Source follower (NMOS)] Doug Mercer |
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| ====== Activity 11M. The Source follower (NMOS)====== | ====== Activity: The Source follower (NMOS) - ADALM2000====== |
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| ===== Objective: ===== | ===== Objective: ===== |
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| ===== Materials: ===== | ===== Materials: ===== |
| Analog Discovery Lab hardware\\ | ADALM2000 Active Learning Module\\ |
| Solder-less breadboard\\ | Solder-less breadboard\\ |
| Jumper wires\\ | Jumper wires\\ |
| ===== Hardware Setup: ===== | ===== Hardware Setup: ===== |
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| The waveform generator should be configured for a 1 KHz Sine wave with 2 volt amplitude and 0 offset. The Single ended input of scope channel 2 (2+) is used to measure the voltage at the source. The Scope configured with channel 1+ connected to display the AWG generator output. When measuring the input to output error, channel 2 of the scope should be connected to display 2+ and 2- differential. | {{:university:courses:electronics:mos_src_flw-bb.png|}} |
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| | <WRAP centeralign> Figure 2 Source Follower Breadboard Circuit </WRAP> |
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| | The waveform generator should be configured for a 1 KHz Sine wave with 4 volt amplitude peak-to-peak and 0 offset. The Single ended input of scope channel 2 (2+) is used to measure the voltage at the source. The Scope configured with channel 1+ connected to display the AWG generator output. When measuring the input to output error, channel 2 of the scope should be connected to display 2+ and 2- differential. |
| <WRAP centeralign> Figure 2 Input, output waveforms </WRAP> | |
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| ===== Procedure: ===== | ===== Procedure: ===== |
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| | {{:university:courses:electronics:mos_src_flw-wav.png|}} |
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| | <WRAP centeralign> Figure 3 Input, output waveforms </WRAP> |
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| The incremental Gain (Vout /Vin) of the source follower should ideally be 1 but will always be slightly less than 1. The gain is generally given by the following equation: | The incremental Gain (Vout /Vin) of the source follower should ideally be 1 but will always be slightly less than 1. The gain is generally given by the following equation: |
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| From the equation we can see that in order to obtain a gain close to one we can either increase R<sub>L</sub> or decrease r<sub>s</sub>. We also know that r<sub>s</sub> is a function of I<sub>D</sub> and that as I<sub>D</sub> increases r<sub>s</sub> decreases. Also from the circuit we can see that I<sub>D</sub> is related to R<sub>L</sub> and that as R<sub>L</sub> increases I<sub>D</sub> decreases. These two effects work counter to each other in the simple resistive loaded emitter follower. Thus to optimize the gain of the follower we need to explore ways to either decrease r<sub>s</sub> or increase R<sub>L</sub> without effecting the other. It is important to remember that in MOS transistors I<sub>D</sub> = I<sub>S</sub> ( I<sub>G</sub> = 0 ). | From the equation we can see that in order to obtain a gain close to one we can either increase R<sub>L</sub> or decrease r<sub>s</sub>. We also know that r<sub>s</sub> is a function of I<sub>D</sub> and that as I<sub>D</sub> increases r<sub>s</sub> decreases. Also from the circuit we can see that I<sub>D</sub> is related to R<sub>L</sub> and that as R<sub>L</sub> increases I<sub>D</sub> decreases. These two effects work counter to each other in the simple resistive loaded source follower. Thus to optimize the gain of the follower we need to explore ways to either decrease r<sub>s</sub> or increase R<sub>L</sub> without effecting the other. It is important to remember that in MOS transistors I<sub>D</sub> = I<sub>S</sub> ( I<sub>G</sub> = 0 ). |
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| {{ :university:courses:electronics:a11m_e1.png?100 |}} | {{ :university:courses:electronics:a11m_e1.png?100 |}} |
| {{ :university:courses:electronics:a11m_e2.png?250 |}} | {{ :university:courses:electronics:a11m_e2.png?250 |}} |
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| where K =?<sub>n</sub>C<sub>ox</sub>/2<sub></sub>and ??can be taken as process technology constants. | where K = μ<sub>n</sub>C<sub>ox</sub>/2<sub></sub> and λ can be taken as process technology constants. |
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| Looking at the follower in another way, because of the inherent DC shift due to the transistor's V<sub>th</sub>, the difference between input and output should be constant over the intended swing. Due to the simple resistive load R<sub>L</sub>, the drain current I<sub>D</sub> increases and decreases as the output swings up and down. We know that I<sub>D</sub> is a (square law) function of V<sub>GS</sub> In this +2V to -2V swing example the minimum I<sub>D</sub>=2V/2.2KΩ or 0.91 mA to a maximum I<sub>D</sub>= 6V/2.2KΩ or 2.7mA. This results in a significant change in V<sub>GS</sub>. This observation leads us to the first possible improvement in the source follower. | Looking at the follower in another way, because of the inherent DC shift due to the transistor's V<sub>th</sub>, the difference between input and output should be constant over the intended swing. Due to the simple resistive load R<sub>L</sub>, the drain current I<sub>D</sub> increases and decreases as the output swings up and down. We know that I<sub>D</sub> is a (square law) function of V<sub>GS</sub> In this +2V to -2V swing example the minimum I<sub>D</sub>=2V/2.2KΩ or 0.91 mA to a maximum I<sub>D</sub>= 6V/2.2KΩ or 2.7mA. This results in a significant change in V<sub>GS</sub>. This observation leads us to the first possible improvement in the source follower. |
| {{ :university:courses:electronics:a11m_f2.png?500 |}} | {{ :university:courses:electronics:a11m_f2.png?500 |}} |
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| <WRAP centeralign> Figure 3 Improved Source Follower </WRAP> | <WRAP centeralign> Figure 4 Improved Source Follower </WRAP> |
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| | ===== Hardware Setup: ===== |
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| <WRAP centeralign> Figure 4 Input vs output error for resistor and current source load </WRAP> | {{:university:courses:electronics:imp_mos_src_flw-bb.png|}} |
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| | <WRAP centeralign> Figure 5 Improved Source Follower Breadboard Circuit </WRAP> |
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| | ===== Procedure: ===== |
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| | {{:university:courses:electronics:imp_mos_src_flw-wav.png|}} |
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| | <WRAP centeralign> Figure 6 Improved Source Follower Waveform </WRAP> |
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| ====== Source follower output impedance====== | ====== Source follower output impedance====== |
| {{ :university:courses:electronics:a11m_f3.png?500 |}} | {{ :university:courses:electronics:a11m_f3.png?500 |}} |
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| <WRAP centeralign> Figure 5 Output impedance test </WRAP> | <WRAP centeralign> Figure 7 Output impedance test </WRAP> |
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| ===== Hardware Setup: ===== | ===== Hardware Setup: ===== |
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| The waveform generator should be configured for a 1 KHz Sine wave with 1 volt amplitude with the offset set equal to minus the V<sub>GS</sub> of M<sub>1</sub> ( approximately -V ). This injects a +/- 0.1mA (1V/10KΩ) current into M<sub>1</sub>'s source. Scope input 2+ measures the change in voltage seen at the source. | {{:university:courses:electronics:mos_out_imp_test-bb.png|}} |
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| | <WRAP centeralign> Figure 8 Output Impedance Test Breadboard Circuit </WRAP> |
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| | The waveform generator should be configured for a 1 KHz Sine wave with 2 volt amplitude peak-to-peak with the offset set equal to minus the V<sub>GS</sub> of M<sub>1</sub> ( approximately -V ). This injects a +/- 0.1mA (1V/10KΩ) current into M<sub>1</sub>'s source. Scope input 2+ measures the change in voltage seen at the source. |
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| ===== Procedure: ===== | ===== Procedure: ===== |
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| | {{:university:courses:electronics:mos_out_imp_test-wav.png|}} |
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| | <WRAP centeralign> Figure 9 Output Impedance Test Waveform </WRAP> |
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| Plot the measured voltage amplitude seen at the source. The nominal source current in M<sub>1</sub> is (Vn - V<sub>GS</sub>) / 4.7KΩ or 720uA. We can calculate r<sub>s</sub>from this current as ohms. How does this r<sub>s</sub> compare to the from the test data? Change the value of R<sub>1</sub> from 4.7 KΩ to 2.2 KΩ and re-measure the output impedance of the circuit. How has it changed and why? | Plot the measured voltage amplitude seen at the source. The nominal source current in M<sub>1</sub> is (Vn - V<sub>GS</sub>) / 4.7KΩ or 720uA. We can calculate r<sub>s</sub>from this current as ohms. How does this r<sub>s</sub> compare to the from the test data? Change the value of R<sub>1</sub> from 4.7 KΩ to 2.2 KΩ and re-measure the output impedance of the circuit. How has it changed and why? |
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| | <WRAP round download> |
| | **Resources:** |
| | * Fritzing files: [[ https://minhaskamal.github.io/DownGit/#/home?url=https://github.com/analogdevicesinc/education_tools/tree/master/m2k/fritzing/mos_source_follower_bb| mos_source_follower_bb]] |
| | * LTSpice files: [[ https://minhaskamal.github.io/DownGit/#/home?url=https://github.com/analogdevicesinc/education_tools/tree/master/m2k/ltspice/mos_source_follower_ltspice | mos_source_follower_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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