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university:courses:electronics:electronics-lab-12 [14 Dec 2012 21:24] – [Questions:] Doug Merceruniversity: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 ======
  
 ===== Objective: ===== ===== Objective: =====
  
 To investigate the simple differential amplifier using NPN transistors. To investigate the simple differential amplifier using NPN transistors.
-First a few notes on hardware limitation issues. The waveform generator in the Analog Discovery Lab system has a high output bandwidth and with that high bandwidth comes wide band noise. The input signal level needed for the measurements in this lab activity is rather small because of the gain of the differential amplifier. If the waveform generator output were used directly the signal to noise ratio of its output is not high enough. The signal to noise ratio can be improved by increasing the signal level and then placing an attenuator and filter ( figure 1 ) between the generator outputs and the circuit inputs. The materials needed for this are:+First a few notes on hardware limitation issues. The waveform generator in the ADALM2000 system has a high output bandwidth and with that high bandwidth comes wide band noise. The input signal level needed for the measurements in this lab activity is rather small because of the gain of the differential amplifier. If the waveform generator output were used directly the signal to noise ratio of its output is not high enough. The signal to noise ratio can be improved by increasing the signal level and then placing an attenuator and filter ( figure 1 ) between the generator outputs and the circuit inputs. The materials needed for this are:
  
 2 - 100Ω resistors\\ 2 - 100Ω resistors\\
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 ===== Materials: ===== ===== Materials: =====
-Analog Discovery Lab hardware\\+ADALM2000 Active Learning Module\\
 Solder-less breadboard\\ Solder-less breadboard\\
 Jumper wires\\ Jumper wires\\
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 ===== Hardware Setup: ===== ===== Hardware Setup: =====
  
-The first waveform generator should be configured for a 200 Hz Triangle wave with 2 volts 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. 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|}}  
 + 
 +<WRAP centeralign> Figure 3 Differential pair with tail resistor Breadboard Circuit </WRAP> 
 + 
 +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.
  
 ===== Procedure: ===== ===== Procedure: =====
  
 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.
 +
 +<WRAP centeralign>{{:university:courses:electronics:diff_pair_tail_res-wav.png?500|}}</WRAP>
 +
 +<WRAP centeralign> Figure 4 Differential pair with tail resistor XY plot </WRAP>
  
 ====== Using a current source as the tail current.====== ====== Using a current source as the tail current.======
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 {{ :university:courses:electronics:a12_f3.png?500 |}}  {{ :university:courses:electronics:a12_f3.png?500 |}} 
  
-<WRAP centeralign> Figure 3 Diff pair with tail current source </WRAP>+<WRAP centeralign> Figure 5 Diff pair with tail current source </WRAP>
  
-====== Measuring Common Mode gain ======+===== Hardware Setup: =====
  
-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 is swept + and - 200mV around ground .+{{:university:courses:electronics:diff_pair_tail_cs-bb.png|}}  
 + 
 +<WRAP centeralign> Figure 6 Differential pair with tail current source Breadboard Circuit </WRAP> 
 + 
 +===== Procedure: ===== 
 + 
 +Same procedure as for the tail resistor. 
 + 
 +<WRAP centeralign>{{:university:courses:electronics:diff_pair_tail_cs-wav.png?500|}}</WRAP> 
 + 
 +<WRAP centeralign> Figure 7 Differential pair with tail current source XY plot </WRAP> 
 + 
 +====== Measuring Common Mode gain ======
  
 {{ :university:courses:electronics:a12_f4.png?500 |}} {{ :university:courses:electronics:a12_f4.png?500 |}}
  
-<WRAP centeralign> Figure 4 Common Mode Gain configuration </WRAP>+<WRAP centeralign> Figure 8 Common Mode Gain configuration </WRAP> 
 + 
 +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.) 
 + 
 +===== Hardware Setup: ===== 
 + 
 +{{:university:courses:electronics:common_mode_gain_bjt-bb.png|}} 
 + 
 +<WRAP centeralign> Figure 8 Common Mode Gain Breadboard Circuit </WRAP> 
 + 
 +===== Procedure: ===== 
 + 
 +{{:university:courses:electronics:common_mode_gain_bjt-wav.png|}} 
 + 
 +<WRAP centeralign> Figure 9 Common Mode Gain Waveform </WRAP> 
  
 ===== 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.
 +
 +<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>
  
 **Return to Lab Activity [[university:courses:electronics:labs|Table of Contents]]** **Return to Lab Activity [[university:courses:electronics:labs|Table of Contents]]**
  
  
university/courses/electronics/electronics-lab-12.1355516674.txt.gz · Last modified: 14 Dec 2012 21:24 by Doug Mercer

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