1. Lab goals

In this lab you will construct a simple 3-stage operational amplifier from discrete components and begin measuring the DC and low-frequency performance of your unit. It will provide a real-world example for our study of the major opamp specifications which are useful in selecting an appropriate part during the design process.

2. Procedure

alt
Figure 1. Discrete opamp schematic

Construct the opamp of Discrete opamp schematic on a small section of breadboard. The capacitor Cc helps to stabilize this amplifier, but you can greatly help the situation by minimizing the length of jumper wires in the construction.

  • Be sure to allow yourself easy access for replacing capacitor Cc and for attaching meters to nodes ina, inb, and out.

  • Use the physically small ceramic capacitor types for Cc.

  • Add a large capacitor (1 to 10 μF) between the Vcc and Vee nodes to help reduce the effect of the long wires connecting to the power supply.

  • For each of the 3 npn transistors: use the “diode check” mode on the multimeters to measure VBE. Select the transistors with the closest values as Q1 and Q2. Since VBE is sensitive to temperature changes, it is best to minimize touching the transistors until you’ve measured them (use pliers).

  • Do a similar procedure to select your Q3 and Q4 pair.

alt
Figure 2. Compact construction example

See Compact construction example for an example of pre-bending transistor leads and building the circuit in the same general arrangement as the schematic. This makes troubleshooting easier since the geometry is similar and reduces the parasitic inductances and the electric and magnetic coupling between nodes and loops.

alt
Figure 3. Vertical
resistor

Several of the resistors are bent to be in a vertical position. Bend and trim your resistor leads as shown in Vertical
resistor
. The right lead in the figure makes for a convenient loop for attaching probes.

alt
Figure 4. Amplifier connections
  • Attach the +5V and -5V power supply connections to your amplifier.

  • Add external wires to connect your opamp as show in the first circuit of Amplifier connections — an amplifier with voltage gain of 1 with a 0 V input. These wires are OK to be long.

  • Measure the internal node voltages of your amplifier (nodes 1, 2, 3, 4, and also out2) with respect to GND. Display the output node on an oscilloscope to verify that the amplfier is not oscillating then use the benchtop multimeters for the DC measurements.

  • Compare these voltages to your predicted values from the circuit analysis done during class time. Also estimate the transistor collector currents. (the analysis conditions were: Vina = Vinb = Vout = 0 V)

2.1. Construct all circuits of Amplifier connections

Measure the value of your 100 kΩ resistor to 3 digits using the multimeter. Use this same unit for all parts of Amplifier connections

  • For each of the configurations, use the benchtop multimeter to measure the following DC voltages:

    • VoutX

    • Vina (+ input)

    • Vinb (- input)

    • VR5, the voltage across resistor R5

If you are using your AD2 instead of lab equipment

To make precise mV-level DC measurements, you must first calibrate your AD2 (again since it has been awhile since the last time).

Then, as a next step, use an AD2 input channel or the nice DMMs in the lab. First, connect your probes to the same node in your circuit (such as out2), change the vertical scale to show the resulting, should-be-zero-but-isn’t, noisy signal, and measure its DC value (average not DC RMS) in Waveforms. This offset value is effectively added to every measurement you make when using this same vertical scale and offset settings. ┬áThis number must then be subtracted from your other measurements to get the true node voltage.

For example: ┬áThe short-circuit value measured to be +2.8 mV. Using the same channel and scale settings, your meter (or Waveforms) says node out2 is -5.3 mV. The true node voltage is (-5.3 mV - 2.8 mV) = -8.1 mV. Not compensating for the measurement instrument’s offset would give you a 35% error! (-8.1 - (-5.3))/-8.1 * 100 = 35.

2.2. Make an opamp-based amplifier!

Can you make an amplfier with a voltage gain of +10 V/V?

Use your opamp instead of a chip.

The feedback network’s resistance as seen by the opamp’s output should be greater than 5× 10 kΩ since the current sinking performance is limited by the internal 10 kΩ R4.

Demonstrate this by amplifying a 1 kHz input signal which is centered around 0 V.

3. References