EE 140 ANALOG INTEGRATED CIRCUITS SPRING 2011 C Nguyen Experiment 2 Discrete BJT Op Amps Part I This is a three week laboratory You are required to write only one lab report for all parts of this experiment 1 0 INTRODUCTION In this lab we will introduce and study the properties of a few circuit blocks commonly used to build operational amplifiers Because we are limited to using discrete components we will not be able to construct a complete op amp This will be done in the op amp design project later in the semester In this lab however we will ask you to analyze and design circuits commonly used to make integrated circuit operational amplifiers and you will use these circuits to build a differential amplifier with both resistive and current mirror biasing Although built with discrete devices this op amp uses a classical topology common to most commercial opamps including the well known 741 The operation of these circuits will depend on the use of matched transistors The CA3083 is a matched NPN transistor array built on a single integrated substrate To ensure that the transistors are properly isolated you must connect pin 5 of the array to the most negative point of the circuit 6 volts Data sheets for the CA3083 and discrete npn and pnp transistors needed in this lab are attached In this lab more than any other so far neatness counts Unless you build your circuits neatly they will not operate Trim your resistor leads if necessary Make sure that you record all the measurements that you make as you proceed and include these measurements in your lab report 2 0 MATERIALS REQUIRED CA3083 NPN Array 2 3904 Transistor 2 3906 Transistor Assorted Resistors and Capacitors CTN 1 19 11 1 EE 140 ANALOG INTEGRATED CIRCUITS SPRING 2011 C Nguyen 3 0 PROCEDURE 3 1 Differential Amplifier Consider the following circuit 12 6V C bypass 10 K 4 7k 10 K 4 7k Vo Q1 Q2 Vin 10 K 4 7k C bypass 6V 12 Figure 1 Assuming that both bases are grounded compute the expected values of IC1 IC2 and IE Also calculate values for the differential and common mode gains of this amplifier Using transistors 1 and 2 in the array construct the circuit in Figure 1 Be sure to connect pin 5 to 6 volts It is also a good idea to bypass both your power supplies with 100 F capacitors This will help reduce any power supply noise to generator to amp 1K 1k 47 47 10 10 Figure 2 CTN 1 19 11 2 EE 140 ANALOG INTEGRATED CIRCUITS SPRING 2011 C Nguyen With both bases grounded measure the bias point of the circuit Using one generator measure the mid band differential note that the output of the amplifier is taken single ended from only one collector and common mode gains of the circuit You may find that a resistive voltage divider such as the one pictured in Figure 2 is helpful in measuring the differential mode gain Be careful when measuring the common mode gain especially when measuring voltages less than 50 mV remember that the common mode voltage gain is smaller than 1 Sometimes voltages of this magnitude are severely corrupted by ground currents from large signals on the board such as the input to the amplifier while making a common mode gain measurement In any case you should not use the input divider for common mode measurements because your common mode signal will likely be a large signal After making these measurements do not disconnect your circuit You will need it later 3 2 Simple Current Mirrors The circuits depicted in Figure 3 are the simple and the Widlar current mirrors Fig 3a shows the simple current mirror circuit Its operation is simple and has been discussed in the lecture Note that Vbe is identical for both transistors Neglecting base current IC3 12 Vbe3Vbe3b R Since Vbe3 Vbe4 assuming that VC4 is large enough to keep Q4 in the active region IC4 IC3 More transistors can be connected in parallel to Q4 and neglecting base currents their IC will be identical to that of Q3 Thus Q3 s collector current is mirrored by Q4 6V 6V 6V 6V 6V 6V Figure 3 CTN 1 19 11 3 EE 140 ANALOG INTEGRATED CIRCUITS SPRING 2011 C Nguyen Current sources are often used for biasing in integrated circuits since large value resistors require large areas to fabricate 1 Construct a simple current mirror circuit Assuming 150 Vbe 7V and neglecting the Early effect select a value of R standard values only to yield an output current of about 1 mA for the simple current mirror in Fig 3a Using transistors 3 and 4 in the array construct the current mirror you designed above Measure Iout for Vout 0 V and 6 V Using this data form an estimate of the Early Voltage Remember this is supposed to be a current source which means it should have a high output resistance Rout To measure the output resistance you can connect different size load resistors to the output and see how the output current voltage changes as the load resistance changes the other side of the load resistor should of course go to the 6V power supply Also note that in order to minimize errors in your measurements you should choose load resistor values that force the output voltage to change from about 5V to about 5V Measure the output resistance of your circuit 2 Construct a Widlar current mirror circuit Design a Widlar current source that produces an output current of 1mA Use the same parameters for the BJT as above Use an emitter resistance value of Re 68 and calculate the value of resistance R needed to produce the 1mA current What is the reference current needed to produce the 1mA output current Using transistors 3 and 4 in the array construct the current mirror you designed above Measure Iout for Vout 0 V and 6 V Using this data form an estimate of the Early Voltage Remember this circuit is a current source which means it should have a high output resistance Rout To measure the output resistance you can connect different size load resistors to the output the other side of the load resistor should of course go to the 6V power supply Also note that in order to minimize errors in your measurements you should choose load resistor values that force the output voltage to change from about 5V to about 5V and see how the output current changes as the load resistance changes Measure the output resistance of your circuit Note the output resistance of this Widlar source should be higher than the standard current mirror Compare the results obtained with these two sources CTN 1 19 11 4 EE 140 ANALOG INTEGRATED CIRCUITS SPRING 2011 C Nguyen 3 3 Differential Pair Amplifier with Current Source Biasing Replace Re in the
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