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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 [24 Apr 2017 08:26] – rename Antoniu Miclaus
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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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 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.
  
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university/courses/electronics/electronics-lab-ce-loop-gain.txt · Last modified: 26 Jan 2021 15:58 by Doug Mercer

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