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university:courses:electronics:electronics-lab-ce-loop-gain [23 Mar 2017 16:04] – [Hardware Setup:] Doug Merceruniversity:courses:electronics:electronics-lab-ce-loop-gain [26 Jan 2021 15:58] (current) – [Directions:] Doug Mercer
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 **To Review:** One method to measure the loop gain in negative feedback systems is the voltage injection method. The following shows how the voltage injection method can be applied in practice and what has to be considered to achieve correct results.  **To Review:** One method to measure the loop gain in negative feedback systems is the voltage injection method. The following shows how the voltage injection method can be applied in practice and what has to be considered to achieve correct results. 
  
-Using a suitable injection transformer ( the Analog Parts kit contains a HPH1-1400L ) we can inject a test voltage at an appropriate injection-point in the feedback loop of the system. Then the response of the loop can be measured using a network analyzer like the Analog Discovery. +Using a suitable injection transformer ( the ADALP2000 Analog Parts kit contains a HPH1-1400L ) we can inject a test voltage at an appropriate injection-point in the feedback loop of the system. Then the response of the loop can be measured using a network analyzer like the ADALM2000. 
  
 Figure 2 shows a setup using the voltage injection method to measure the loop gain of a feedback system. A low value resistor is inserted in the feedback loop at the injection-point. The injection transformer secondary winding is connected across the injection resistor to apply the test voltage. This allows the injection of a test voltage without changing the DC-bias operating point of the system.  Figure 2 shows a setup using the voltage injection method to measure the loop gain of a feedback system. A low value resistor is inserted in the feedback loop at the injection-point. The injection transformer secondary winding is connected across the injection resistor to apply the test voltage. This allows the injection of a test voltage without changing the DC-bias operating point of the system. 
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 ====Directions:==== ====Directions:====
  
-Build the measurement setup as shown in figure 5 below. If you are using the HPH1-1400L transformer for T<sub>1</sub> you should connect three of the 6 windings in series for the primary and the remaining three windings in series for the secondary ( see this [[university:courses:electronics:comms-lab-transformers|activity on transformers]] for more details ).+Build the measurement setup as shown in figure 5 below. If you are using the HPH1-1400L transformer for T<sub>1</sub> you should connect three of the 6 windings in series for the primary and the remaining three windings in series for the secondary ( see this [[university:courses:alm1k:alm-lab-transformers|Transformers]] for more details ).
  
 There are three resistors, R<sub>C</sub>, R<sub>B</sub>, R<sub>E</sub> and one capacitor, C<sub>M</sub>, that determine the loop gain of this circuit. You will be running frequency sweeps with the network analyzer testing various combinations of component values. There are three resistors, R<sub>C</sub>, R<sub>B</sub>, R<sub>E</sub> and one capacitor, C<sub>M</sub>, that determine the loop gain of this circuit. You will be running frequency sweeps with the network analyzer testing various combinations of component values.
  
-Voltage divider R<sub>2</sub> and R<sub>3</sub> serves two purposes. First the 10 Ω R<sub>2</sub> matches the impedance of the resistor inserted in the feedback loop, R<sub>1</sub>. The AWG in the Analog Discovery cannot directly drive the 10 Ω resistor so the 100 Ω R<sub>3</sub> increases the load resistance to a value high enough for the AWG to safely drive. The attenuation of the divider also allows us to set the amplitude of the AWG high enough to provide a low noise signal while still injecting a small signal into the loop.+Voltage divider R<sub>2</sub> and R<sub>3</sub> serves two purposes. First the 10 Ω R<sub>2</sub> matches the impedance of the resistor inserted in the feedback loop, R<sub>1</sub>. The AWG in the ADALM2000 cannot directly drive the 10 Ω resistor so the 100 Ω R<sub>3</sub> increases the load resistance to a value high enough for the AWG to safely drive. The attenuation of the divider also allows us to set the amplitude of the AWG high enough to provide a low noise signal while still injecting a small signal into the loop.
  
 {{ :university:courses:electronics:acelg_f5.png?600 |}} {{ :university:courses:electronics:acelg_f5.png?600 |}}
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 The green squares indicate where to connect the ADALM2000 AWG, scope channels and power supplies. Be sure to turn on the power supplies only after you double check your wiring. The green squares indicate where to connect the ADALM2000 AWG, scope channels and power supplies. Be sure to turn on the power supplies only after you double check your wiring.
  
-Open the voltage supply control window to turn on and off the fixed +5 volt power supply. Open the Network Analyzer Instrument and set the sweep to start at 10 KHz and stop at 5 MHz. The Max gain should be set to 0.1X. Set the Amplitude to 1.75 V and the Offset to zero volts. Under the Bode scale set the magnitude top to 40 dB and range to 80 dB. Set the phase top to 180º and range to 360º. Under scope channels click on use channel 1 as reference. Set the number of steps to 500.+Open the voltage supply control window to turn on and off the fixed +5 volt power supply. Open the Network Analyzer Instrument and set the sweep to start at 10 KHz and stop at 5 MHz. The Max gain should be set to 0.1X. Set the Amplitude to 3.5 V peak-to-peak and the Offset to zero volts. Under the Bode scale set the magnitude top to 40 dB and range to 80 dB. Set the phase top to 180º and range to 360º. Under scope channels click on use channel 1 as reference. Set the number of steps to 500.
  
 ====Procedure:==== ====Procedure:====
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 Why did the unity gain frequency change for the case of the with R<sub>B</sub> equal to 4.7 KΩ vs. 2.2 KΩ?\\ Why did the unity gain frequency change for the case of the with R<sub>B</sub> equal to 4.7 KΩ vs. 2.2 KΩ?\\
 Why did the unity gain frequency change for the case with R<sub>E</sub> inserted? Why did the unity gain frequency change for the case with R<sub>E</sub> inserted?
 +
 +<WRAP round download>
 +**Resources:**
 +  * LTSpice files: [[downgit>education_tools/tree/master/m2k/ltspice/ce_loop_gain_ltspice | ce_loop_gain_ltspice]]
 +</WRAP>
  
 **For Further Reading:** **For Further Reading:**
university/courses/electronics/electronics-lab-ce-loop-gain.txt · Last modified: 26 Jan 2021 15:58 by Doug Mercer

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