Today we will complete the firstcut audio amplifier and measure the vi curve for a diode.
Two OpAmp Amplifier with Complementary Transistor Driver
Schematic of Two OpAmp Differential Driver Circuit with Complementary Output
Schematic of Diode vi Curve Test Circuit
Report Instructions for Two OpAmp Amplifier with Complementary Transistor Driver
Report Instructions for Diode vi Curve Experiment
Tasks for this experiment:
Beginning with the Complementary Transistor Driver circuit form Laboratory 2, add the two opamp driver circuit to make up a complete circuit as shown below as Figure 1. Replace the capacitor and speaker at the output with a capacitor and a load for today's laboratory. If you used 2N3904/2N3906 instead of 2N4401/2N4403, leave your transistors as placed in the circuit for this laboratory. You may be given an LM358, a dual opamp, instead of an LM324, a quad opamp. If you get an LM324, ask the Instructor how to hook up the unused opamps.
Measurements of voltage that assure that the circuit is working are given below in the report instructions. Make these measurements after your circuit is working and record them for your report.
Set up your signal generator for a sine wave output
Amplitude 1 millivolt peak
Frequency 1,000 Hz
Apply the amplifier and signal generator output to the oscilloscope. The amplifier output is the voltage across the load. Adjust the signal generator output as necessary to provide as large a s possible signal at the output without visible distortion. Measure the voltage gain of the circuit.
Save the plots for your lab report
Repeat for a 1 millivolt peak triangular wave at 1,000 Hz.
Repeat for a 1 millivolt peak square wave at 1,000 Hz.
Change the frequency of the signal generator to find the high and low frequency limits over which the output amplitude remains approximately constant. Write down the frequencies for which the amplitude of the output is 0.7 times the amplitude at 1000 Hz. These are the 3 dB points in the frequency response.
Note that this circuit is a circuit involving two opamps, added to the circuit built for Laboratory 1. Leave the circuit for Laboratory 3 on the wireless prototype board because you will be using it when we discuss biasing of threeterminal devices and for other experiments.
Figure
1. Completed
FirstCut Audio Amplifier
Construct the circuit consisting of a resistor in series with a diode as shown in Figure 2. While the resistor is not connected to the power supply, measure its resistance with your multimeter to the highest accuracy possible with the instrument. Record this resistance for use in finding currents, and for your report.
Adjust your power supply for voltages between 0 V and 6 V and measure the voltage and current through the diode. Measure the current by connecting your multimeter across the resistor and recording the voltage drop, then divide by the resistance that you measured. Note that the ground of the multimeter must be floating, not connected to bench ground, for this measurement. Use, as a minimum, the voltages shown in Table 1.
Table 1. Voltages to be Applied to Diode vi Test Circuit. Measurements to be Taken are Diode Voltage and Current.
Power
Supply voltage 
Diode
Voltage 
Diode
Current 
0.1 Volts 


0.2 Volts 


0.3 Volts 


0.4 Volts 


0.5 Volts 


0.6 Volts 


0.7 Volts 


0.8 Volts 


0.9 Volts 


1.0 Volts 


1.5 Volts 


2.0 Volts 


2.5 Volts 


3.0 Volts 


3.5 Volts 


4.0 Volts 


4.5 Volts 


5.0 Volts 


5.5 Volts 


6.0 Volts 


This is a simple circuit for measuring the voltage and current through a diode, and varying it over input currents up to about 5 milliamperes. Make sure that your diode is inserted with the cathode to ground. Check by applying about 2 Volts to the circuit and checking the output voltage; if the output voltage is about 2 volts, reverse the diode.
Figure
2 Circuit for Measuring Diode vi Curve
Use the instructions given on this link. The lab report is due one week from today. Submit your report as a file by email to no_spam_jkbeard@jameskbeard.com.
Measure the DC voltage at the voltage divider at the top left of the circuit. This voltage divider is a and a resistor in series, with a capacitor bypassing the resistor. This voltage should be about 2.02 Volts. This is your circuit reference voltage.
Measure the DC voltage at the inverting inputs to both opamps. Both should be the same as that of the voltage divider. In your report, explain why this is so.
Measure the voltage at the output of the left opamp in the circuit. It should be slightly more than the circuit reference voltage. Explain why in your report.
The first opamp we define here is the leftmost in the schematic. This opamp is a noninverting amplifier for the input at the lower of the two input terminals. Find the gain of this noninverting amplifier as a function of the two resistors and on the first opamp, up to the coupling capacitor.
The second opamp we define here is the rightmost in the schematic. To the signal from the first opamp, this is an inverting amplifier. Find the gain of this inverting amplifier as a function of the two resistors and in this opamp circuit.
The first opamp circuit is a noninverting amplifier for the upper of the two input terminals. Find th gain of this noninverting amplifier as a function of the two resistors and in this opamp circuit.
Show that if and , then the output of the second opamp is a gain factor times the difference between the voltages at the input terminals. Find this gain factor in terms of and . Give the relationship between the values of the resistors , , and that is required for the two opamp circuits together to be a differential amplifier.
Note that we are using 5% tolerance for the resistors , , and , so we cannot expect for the differential gain to be exact. The difference between the gains is called the common mode gain. Measure the common mode gain by connecting the two input terminals together and applying a 1000 Hz sine wave voltage to them with the signal generator. Apply up to 2.5 volts peak to get a measurable signal on the output, across the load. Note that you measured the gain of the circuit as part of the experiment; this is the differential gain. Compare the common mode gain with the differential gain. Give the difference as a voltage ratio and as a figure in dB.
Plot the diode vi curve from the data you took in the experiment.
Compare the vi curve with a theoretical curve using the equation
where
Will changing the values of and give you better agreement? What are these values? Note that more data at the point where the current begins to flow can be helpful here. Also, the vi curve equation can be manipulated to give a form that may be better in understanding the roles of and in the vi curve.