EE 321 Lab 1 Fall 2005EE321 – Lab 1Amplifiers, Biasing, and AC CouplingThe purpose of this lab is to measure the characteristics of an amplifier, and to use thecharacteristics to add a bias circuit at the input.An amplifier can be represented in many different ways. Figure 1 shows a model for a voltageamplifier. The textbook shows similar models for current, transconductance and transresistanceamplifiers. Figure 2 shows a simple block diagram of a generic amplifier. In this lab we will measurethe characteristics of the simple voltage amplifier: the input resistance Ri, the output resistanceRoand the voltage gain Avo.ViAvo ViRoRiVoBAFigure 1.A BViVoFigure 2.We will use an op-amp in the c ircuit in Figure 3 to build a real amplifier, and measure thecorresponding paramete rs of the voltage amplifier model. (Use a 741 or similar op-amp.) As inFigures 1 and 2, the input to our amplifier is at A and the output at B. If we can find the threevalues Ri, Ro, and Avowe can use the mo del to accurately predict how the amplifier will work in acircuit. (From your knowledge of op amps, you should be able to determine Ri, Roand Avofrominspection of Figure 3. Do so as part of the prelab, and compare these values to the values youmeasure below.)1 k ΩΩ−15V+15V741Ω100 k20 kABFigure 3.1. Build the circuit in Figure 3, and verify that it works. If you cannot remember the pinout ofa 741 op-amp, look at its datasheet. (How did you verify its operation?)1EE 321 Lab 1 Fall 20052. Measure the voltage gain Avoof the amplifier in the following two ways:(a) Connect the input of the amplifier to a variable voltage source as shown in Figure 4.Measure and plot VBvs. VAusing a DVM for a sufficient number of voltages so thatyour plot is a nice curve. Be sure that your voltage range is sufficiently large that theamplifier saturates. The slope of the linear portion of the curve is the voltage gain Avo.Compare this value to the predicted one for the circuit of Figure 3.(b) Use the function generator and scope to trace the transfer function as shown in Figure 5.Store and copy the X-Y plot. Carefully mark the scale s. Compare your curve fromPart 2(a) with the X-Y plot. What are the saturation voltages for the output?+15V−15V5k10k5kΩΩΩFigure 4.FunctionGeneratorcircuitScope in X−Y ModeX YFigure 5.3. Measure the input resistance Riof the amplifier model circuit:• Using the variable voltage source of Part 2(a) put a tes t resistor be tween the voltagesource and the input (A).• Use a (non-zero) input voltage such that the output is NOT saturated.• Measure the voltage from your variable source, and the voltage at point A of youramplifier.• Compute Ri. How you do this should be easy if you look at the model of Figure 1.• If the test resistor is not within a factor of 2 of Ri, change it to one about equal to Riand make a more accurate determination of Ri. Measure the test resistor with an ohmmeter to determine its value accurately.• Compare Rito the value you would expect from the circuit of Figure 3.4. Measure the output resistance Ro.• Shorting the output to ground to measure iscmay damage an actual amplifier, so ingeneral, you should not short the output to try to measure Ro.• Set the input voltage so the the output voltage is about 5 V.• Place a load resistance from B to ground, and note the decrease of the output voltage.Choose a test resistor of an appropriate size. (What is an appropriate size? Why?)• Draw the output half of the circuit including the test resistor, marking measured voltages.Compute Ro.2EE 321 Lab 1 Fall 2005• Compare Roto the value you would expect from the circuit of Figure 3.5. In analog electronics you often are interested in only the AC (changing) part of a signal, andmay have to block the DC (constant offset) part. We will do that for this circuit.(a) Connect a 5 kHz, 1V p-p sine wave to the input. Display and sketch the input andoutput.(b) Use the function generator to add a 0.1 V offset to the input. What is the effect at theoutput?(c) Add a DC blocking capacitor as shown in Figure 6. (What size of capacitor should youuse? Why?) Note that RSin Figure 6 is the output resistance of the function generator,which is 50 Ω. How is the output affected. Why?(d) What effect does varying the input offset have on the output voltage?RSFunctionGeneratorBCAFigure 6.6. Certain analog circuits need the AC signal to have a specific DC offset to function properly.The process by which the DC offset is added is called biasing the circuit. For exampletransistor amplifiers usually need a bias voltage to work properly. For this part we want theoutput to have a DC offset of 2 V.• Using the values of Ri, Roand Avodetermined for the model, calculate the appropriatesize for R1needed to add an offset voltage of 2 V.• Add the bias resistor R1shown in Figure 8 to change the output operating point to 2 V.• With a sine input, sketch the output. What is the effect of the biasing on the output?RSFunctionGeneratorCBA−15VR1Figure 7.3EE 321 Lab 1 Fall 2005PRE-LAB1. Look up the data sheet for the LM741 and add the pin numbers to complete the schematicof Figure 3.2. Determine the gain, the input resistance, and the output resistance of Figure 3.3. Calculate the parameters Ri, Roand Gmof the equivalent transconductance amplifier (Ta-ble 1.1 of Sedra & Smith). Draw the transconductance model of the circuit.4. Determine the approximate size of the capacitor C needed for Part 5.5. Determine the value of R1needed for Part 6.6. What is the importance of the input resistance of a circuit? When might you want a circuitwith a low input resistance? When might you want a circuit with a high input