  Figure 1   A schematic representation of an operational amplifier is shown. For an ideal opamp, Rin = infinity, Rout = 0, and the gain, k = infinity. Figure 2   The inverting opamp circuit is shown in (a). The wiring diagram is shown in (b). Figure 3   The experimental setup is shown. A signal generator provides an input signal. Two power supplies were used, one for +Vcc and the other for -Vcc. Figure 4   Oscilloscope trace shows a 0.6 peak input voltage. The output saturates at Vcc (5V). R1 is 1K and R2 is 10k. The gain is -10 until the amplifier saturates at +/- 5V. The input and the output are 180 degrees out of phase. Figure 5   The noninverting opamp circuit is shown.

## Opamp Lab

Ideal Opamp

A schematic representation of an operational amplifier (opamp) is shown in Figure 1. In most situations an opamp can be considered ideal. An ideal opamp has an infinite input resistance, Rin, zero output resistance, Rout, and an infinite gain, k.

Since Rout is zero, the voltage on the output pin, Vout is equal to the difference in the voltages on the input pins. (1)
And since the gain k is typically greater than 105 the difference between the voltages on the input pins is approximately zero. (2)

Ideal opamp rules

1. The voltage on the inverting input, V- is equal to the voltage on the noninverting input, V+.

2. No current flows into or out of the input terminals.
Rule 1 follows from Equation 2. Rule 2 follows from the assumption that Rin is infinite. It is reinforced by Rule 1, since if there is no voltage across R in no current will flow.

Inverting Opamp Circuit

The inverting opamp circuit is shown in Figure 2. The node equation for the inverting input, node A, is, (3)
No current flows into the opamp from node A. Also since the voltage at the inverting input is equal to the voltage at the noninverting input, the voltage at node A is zero. Equation 3 becomes, (4)
The gain is, (5)

Noninverting Opamp Circuit

The noninverting opamp circuit is shown in Figure 5. Since both opamp input terminals are at the same voltage, the voltage at node A is Vin. Since no current flows into the opamp's negative input terminal, the same current flows in both R1 and R2. Therefore, (6) (7)

1. Wire up the inverting opamp circuit shown in Figure 2. Set Vcc to 5V and -Vcc to -5V.

2. Use a voltmeter to measure the DC voltages applied to the opamp chip.

3. Apply a 1 KHz, 0.1 V (peak) signal to the inverting opamp circuit. (Set the DC signal generator output to zero. Observe the output voltage using an oscilloscope to ensure that the opamp is not saturating.)

4. Design an experiment to measure the output impedance of the inverting circuit.

5. Repeat 1-4 for the nonverting circuit shown in Figure 5.

Questions

1. What is the gain of the inverting circuit?
2. What is the gain of the noninverting circuit?
3. What is the input resistance of the inverting circuit?
4. What is the input resistance of the noninverting circuit?
5. What is the maximum output voltage?
6. What is the output resistance of the inverting circuit? (The noninverting circuit will have a similar output resistance.)