The purpose of this lab is to measure the characteristics of an amplifier, and to use the characteristics to add a bias circuit at the input. We will represent the same amplifier three different ways. These are a block diagram, a model, and a schematic of the components. Figure 1 shows the block diagram of a generic amplifier. Figure 2 shows the model for a voltage amplifier. The characteristics which we need to measure are R_i, R_o and A_vo.
 

We will use the circuit in Figure 3 as our amplifier. (Use a 741 or similar op-amp; the 741 pin out is shown in Figure 4.) In Figures 1, 2 and 3, the input to our amplifier is at A and the output at B. If we can find the three values R_i, R_o, and A_vo we can use the model to accurately predict how the amplifier will work in a circuit.
 

• Build the amplifier.
• From your knowledge of op amps, you may be able to determine R_i, R_o and A_vo from inspection of Figure 3. If you remember how calculate the values so you can compare them to the values you measure below.

• Connect the variable voltage source of Figure 5 to the input, and measure the voltage at both the input (A) and output (B) with a DVM. Plot the input voltage versus the output voltage. The gain, A_vo, is the slope of this line.

 

 

• This process can be automated. Use the function generator and scope to trace the transfer function as shown in Figure 6. Store and copy the X-Y plot. Carefully mark the scales just as on the first graph.
• Compare your curve with the X-Y plot. What are the saturation voltages for the output?

• To do this add a test resistor between the variable source and the input (A).
• Make sure the output is NOT saturated.
• Measure the voltage of your variable source, and the voltage at point A.
• From a drawing of the input input circuit connected to the amplifier model you can see how R_i effects these voltages. Compute R_i.
• If the test resistor is not within a factor of 2 of R_i, change it to one about equal to R_i and make a more accurate determination of R_i. Why does this make the determination more accurate.
• Measure the test resistor with an ohm meter to determine its value accurately.
• Why can't you use an ohm meter to find R_i?

• Using a short to measure i_sc may damage an actual amplifier, so in general, you should not short the output to try to measure R_o.
• Instead place a load resistance from B to ground, and note the decrease of the output voltage.
• To see how to analyze this, draw the output half of the amplifier model including the test resistor, marking measured voltages.
• Choose a test resistor of an appropriate size. (What is an appropriate size?)
• Compute R_o.

• Input a 5 kHz, 1V p-p sine wave. Display and sketch the input and output.
• Use the function generator to add a 0.1 V offset to the input. What is the effect at the output?
• Add a 0.015 mu capacitor as shown in Figure 7 (R_s is the source resistance of the signal generator). How is the output affected.



 

• Why?
• What effect does varying the input offset have on the output voltage?

• Add the bias resistor R₁ shown in Figure 8 to change the output operating point to about 5V



 

• For an input v_i of 0V the output v_o should be 5.0V .

• The resistance values determined for the model can be used to calculate R₁.
• With a sine input, sketch the output.
• What is the effect of the biasing on the output?