Fig. 1 Simple BJT (NPN) current mirror and its test circuit

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Archibald Bryan
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1 1 Lab 01: Current Mirrors Total 30 points: 20 points for lab, 5 points for well-organized report, 5 points for immaculate circuit on breadboard Note: There are two parts for this lab. You must answer the boxed questions listed under L1 L8 to get full credit. Lab report must be well-organized. You will be graded on this aspect. Use the template from Labs page. Objective The purpose of this lab is to build BJT based Current Mirror Circuits that provide stable and constant DC current for biasing. 1.0 Basic Current Mirror Fig. 1 Simple BJT (NPN) current mirror and its test circuit In the basic current mirror circuit, for a matched pair for transistors, I O =I REF. Fig. 1 shows a simple NPN based current mirror with its test circuit (see Figs and 7.30). Here V CC =15 V, Rp=50 kω potentiometer. Q 1 and Q 2 should be a matched pair of NPN transistors. Test voltage Vo is used to determine range of output resistance Ro. You can use either stand-alone NPN transistors PN2222 or the array of matched NPN transistors MPQ2222 (NPN array). Note that the array will provide better matching. Please refer to datasheets at the bottom of the Labs page. If the required parameters are not listed for this current, then try to approximate them for the smallest set of I C and I B at a given V CE. Note that β=h FE. You can also look up the parameters in the SPICE model of the transistor. Please see the models at the bottom of the Labs page. 1.1 Design Calculate the total resistance Rp needed at the collector of Q 1 to provide I REF =1mA (see eq. 7.73). What is the value of I O if Vo = 2.5V (I O should be close to 1mA; see eq. 7.74)?

L1: Calculate the total resistance R P, V BE1, I O, and R O. 1.2 Simulation: To simulate your circuit accurately, you must the most accurate SPICE model.

2 2 What value of V BE1 (base-emitter voltage of Q 1 ) did you guess? Or did you look it up in the datasheet? What value of V A did you assume (Hint: you can create custom model and look it up under VAF: see section below)? What is the output resistance, Ro (see eq. 7.75)? L1: Calculate the total resistance R P, V BE1, I O, and R O. 1.2 Simulation: To simulate your circuit accurately, you must the most accurate SPICE model. Otherwise you will encounter errors or discrepancies Creating Transistor Component: Set up the schematic using the SPICE model provided at the bottom of the Labs page. Unzip the file, then open in Word/Wordpad/Notepad then copy the text from.model to the very end. Note that.model statement has opening and closing parentheses. Now create a custom symbol for our transistor (this SPICE model will work for either stand-alone or the array version of 2222) in Multisim. Tools->Component Wizard. Give it a descriptive name such as Central_PN2222 (Central Semi makes the 2222 transistors that we will be using). See the picture below. Click Simulation Only option. Next->Single Section, Number of pins=3. Next->Leave everything as is. Next->Left click under Symbol Pin Name and change the pins as following: 1->B or Base, 2->E or Emitter, 3->C or Collector. Next->Paste the model data that you copied earlier. Now you should see the window on the left. Make sure that Symbol pin names match the Model mode names. If you don t have this step done properly, then your simulation won t work. Next-> Save the symbol/model under User Database. It s good to create a new family. Hopefully, you will save your work and your schematic will still have the symbol when you open it next time. It s best to save schematic on USB or on Cloud. Note that the data Lab PCs erase your data when you logout. Now your component will be listed under User Database when you browse under Place Component (Use the dropdown menu in the first box on the left).

3 1.2.2 Simulation Procedure: Now, simulate the circuit by setting Rp to 3 values: Rp for I REF =1mA (as calculated in section 1.1), 7 kω, 30 kω.

Probe and measure I REF and V BE1. Use Vo = 2.5V. Do these values match the calculated values? I REF must be close to 1mA. V BE1 can vary a little (why?). (b) Keep Rp as is.

3 Simulation Procedure: Now, simulate the circuit by setting Rp to 3 values: Rp for I REF =1mA (as calculated in section 1.1), 7 kω, 30 kω. Note that in the active region, changes in Vo should have minimal/no effect on I O. This little fact is the foundation of the current mirror. (a) Set Rp for I REF =1mA (as calculated in section 1.1). Probe and measure I REF and V BE1. Use Vo = 2.5V. Do these values match the calculated values? I REF must be close to 1mA. V BE1 can vary a little (why?). (b) Keep Rp as is. Set up DC sweep and vary Vo from 0 to 5 V in 500 mv increments. Probe and measure Io. Plot Io vs Vo. Place cursors around Vo = 2.5 V in the active region and calculate slope (=1/Ro) for a small range of Vo. Note that Multisim will give you dy/dx value in the little window with cursor info. Using this slope, find the small-signal output resistance, Ro in the active region. (c) Now repeat the above simulation for R P =7 kω and 30 kω. L2: Simulate and report the output resistance R O (in kω) and I O for Rp as calculated in section 1.1 for I REF =1mA, 7 kω, and 30 kω. You must capture simulation plots. 1.3 Experiment: Use the shortest possible wires (ask for short jumper wires if you don t have any) and clip the terminal wires of your components and make them as short as possible. Make your circuit very neat and organized. Please ask to see an example. You will be graded on this aspect of the experiment. (a) Build the circuit with either stand-alone or the array of transistors (better matching on an array). Use Vo = 2.5V. Adjust Rp so that you obtain I REF =1mA. Note this value of Rp. Does it match the with the calculated and simulated values earlier? (b) Repeat 1.2.2b. Measure and obtain a couple of points for Io around Vo=2.5V (e.g. Vo=2.4 V, Vo=2.6 V). Calculate the Ro=ΔVo/ΔIo. Does this value match the simulated value? Don t forget to check the multi-meter mode before you measure a current or voltage, if it is set incorrectly you will blow a fuse! (c) Now repeat the above experiment for Rp=7 kω and 30 kω. Is there a match between the calculated, simulated and measured values? Small errors are due to the mismatch between the transistors (i.e. emitter-base junction area, etc). How does it affect the current mirror gain (I o /I REF )? Would you expect this match to be better or worse for an integrated circuit (IC) implementation of the basic current mirror? L3: Measure the output resistance R O (in kω) and I O for Rp for I REF =1mA, 7 kω, and 30 kω. L4: In your report, create a table with calculated, simulated and measured values. Part 2 continued the next page

4 4 2.0 Advanced Current Mirror We will now modify the circuit of Fig. 1 and create Widlar Current Source (see section 7.5.5) as shown in Fig. 2. Widlar Current Source is used when a small current is needed. Another advantage over the basic current mirror is that it has a higher output resistance. Remember that an ideal current source should have an infinite resistance. Fig. 2 Advanced BJT (NPN) current mirror: Widlar current source In the Wildar current source circuit, for a matched pair for transistors, I O can be fraction of I REF. This circuit is used to provide a small I O. Repeat the steps in section 1 (design, simulation, and experiment). 2.1 Design Follow the expl 7.6, ex 7.22, and problem Eq (p.544) can be used as the design equation:.. eq (7.87) Assume that I REF =1mA and calculate the value of R E required to give I O =50μA. Calculate the resistance Rp needed at the collector of Q 1 to provide I REF =1mA. What is the output resistance, Ro (see eq.7.88: use same β as before, g m =I C2 /V T =I O /V T, and r o =V A /I C2 =V A /I O )? This value should be very high (>6 MΩ). Compare it with Ro in part1. L5: Calculate the resistances R E, R P, and R O.

5 5 2.2 Simulation: Simulate the circuit by setting Rp to the value found in the previous section 2.1 a) Set up DC sweep and vary Vo from 0 to 5 V in 500 mv increments. Probe and measure Io. Plot Io vs Vo. Place cursors around Vo = 2.5 V in the active region and calculate slope (=1/Ro) for a small range of Vo. Note that Multisim will give you dy/dx value in the little window with cursor info. Using this slope, find the small-signal output resistance, Ro in the active region. L6: Simulate and report the output resistance R O. You must capture simulation plot. 2.3 Experiment: Build the circuit and adjust values as necessary. Use R E close enough to your calculated value. First verify that I REF ~1mA and Io~50uA. (a) Measure and obtain a couple of points for Io around Vo=2.5V (e.g.vo=2.4 V, Vo=2.6 V). Obtain more points if necessary. The current will vary very little so approximate it as best as you can. Calculate the Ro=ΔVo/ΔIo. Does this value match the simulated value? L7: Measure the output resistance R O. Is the output resistance of Widlar current source higher or lower than the basic current mirror? Is this expected? L8: In your report, create a table with calculated, simulated and measured values.

Transistor Characteristics and A simple BJT Current Mirror

Transistor Characteristics and A simple BJT Current Mirror Current-oltage (I-) Characteristics Device Under Test DUT i v T T 1 R X R X T for test Independent variable on horizontal axis Could force current