EE 321L Analog Electronics Laboratory Fall 2010Lab 10BJT Two-stage amplifiersPre-Lab1. Determine Rrefin step 1.2. Estimate the load resistance at which the current mirror breaks down, in step 2.3. Design the circuit in step 3. Think about how the input and output resistances interact,and how R1, R2, and RE1might affect the lower knee frequency of the amplifier.4. Related to step 4, what is the output resistance of an ideal current mirror?5. Estimate the input a nd output resistances in step 6 .6. Estimate the knee frequency in step 7. Which resistance should you use in ω0=1RC?In this lab you will design a current mirror and use it to bias a two-stage amplifier.BJT current mirrorIn this section you will build a BJT current mirror.+15 V−15 VIQ1Q2IrefRref1. Build a 1 mA current mirror from two NPN BJT transistors. Use ±15 V power supplies.Adjust the reference resistor such that you get 1 mA through a 10 kΩ load resistor. Nor-mally a current mirror is built from matched components on the same semiconductordie which ensures that it will operate at the current expected. Since the transistors youwill use are not matched you will likely need to adjust the resistor to get the outputcurrent to be what you want. If you have difficulty you can also add a small resistanceto the emitter of Q2and perhaps Q1(If you expect to run out of 3904s, then use 2222sfor the current mirror).1EE 321L Analog Electronics Laboratory Fall 20102. Atta ch a large range of load resistors to the input (the collector of Q2) includingresistances beyond the failure point of the current mirror. Plot the current versusthe load resistance, possibly on a logarithmic resistance scale. Is the current more orless constant as a function of load r esistance in the regulation region? What causesthe (hopefully small) variation of current with load resistance? What is the outputresistance of the current mirror? Compare the measured failure point to what youexpect from theory.NOTE: since the two BJTs are not exactly matched it takes a little effort to match thecurrents, something which happens automatically on a single silicon die. Can you thinkof another way to create a current source, with just one BJT? Hint: it is a differentuse of a familiar circuit. If we fail at making a useful current source from two BJTswe will revert to this circuit. What would be the disadvantage of a single-BJT currentsource on a silicon die?BJT two-stage amplifierIn this section you will use the current mirror to bias one of the amplifiers in a two-stageamplifier.3. Build a two-stage amplifier consisting of the cascade of two common-emitter withemitter resistance stages, and total gain 25 distributed evenly between the two stages.Use ±15 V power supplies. The first stage should be built from a NPN transistor andbiased by the current source built above. The second stage should be built f r om aPNP transistor and biased with resistors. The base of the second stage is biased by thecollector of the first stage. The input a nd output of the amplifier should both be biasedat 0 V. That means that the output should be at ground when no input is attachedand when the input is attached to ground. Here are some hints:(a) Begin by selecting the bias current for t he second stage, then the collector resistorto produce the desired output voltage, then the emitter resistor to give the desiredgain. This will set the input bias voltage for the second stage.(b) The first stage is biased by the current mirror, so select a collector resistor to setthe output bias voltage. Select a emitter resistor to get the desired gain. Bypassthe current source with a large capacitor. Select base bias resistors (usually calledR1and R2) to set the desired base voltage.(c) Verify that input and output resistances are appropriate and result in at mostsmall gain loss. If that is not the case modify the circuit.4. Measure the small-signal g ain (at a few frequency in the 1 kHz-10 kHz range to makesure you are not on the attenuation slope of the HP filter) and compare with a theo-retical calculation based on your design.2EE 321L Analog Electronics Laboratory Fall 20105. Plot the input-output characteristic in XY mode including saturation levels. Does theamplifier saturate where you expect it?6. Measure input and output resistances and compare with a theoretical prediction.7. This amplifier will function as a high-pass filter. Why? Measure the knee-frequencyand compare it to a theoretical prediction. (If you picked C too large in step 3, youmay reduce it here to make it easier). The knee-frequency is where the gain is reducedby a factor o