Open Loop Characteristics

An op-amp in open loop mode is inherently unstable and has relatively narrow bandwidth compared to the circuits with feed-back you will build later.

• The output voltage of an op-amp cannot lie outside the range of its DC power supply voltages
  • Too large an AC input into a high gain amplifier will make the output “rail”, i.e., hit the limits on its output and get clipped.
    • e.g. an op amp powered from ±15 V cannot produce an AC output with peak-to-peak voltage greater than 30V.
• Given their huge open loop gain, even tiny amounts of noise leaking into an op amp can have large effects on their output.
  • Always use decoupling/bypass capacitors at the power supply pins of each op-amp in every circuit to prevent high frequency noise leaking into the op-amp output. Most schematics do not explicitly indicate these capacitors (or even the power connections), but you should always use them; \(\sim 0.1-1 \mathrm{\mu F}\) are reasonable values. (see Figure 1)

Create the circuit in the figure below with a LF356 op amp and \(\sim 0.1-1 \mathrm{\mu F}\) decoupling capacitors.

Open loop op amp circuit. Note the decoupling capacitors from the DC power inputs to Common.

To help ensure correct wiring, it is recommended that you always use the same colour. For example

• Any black wire is connected to Common.
• Any green wire is connected to V+ (DC Power Supply Output 2).
• Any blue wire is connected to V— (DC Power Supply Output 3).
• Any red wire is an Input or Output
• Power with the ±15 volt tracking outputs from your DC supply.
• I recommend using the Wav Gen option (instead of Channel 1) for the Trigger.

Non-inverting amplifier

A non-inverting amplifier with output voltage \(V_{out} = \left(1 + \frac{R_2}{R_1}\right)V_{in}\). The DC power decoupling capacitors are not shown, but they still must exist.

Create feedback for your Op Amp by connecting the inverting input (-) to a resistive voltage divider acting on the output, as shown above. Select the two resistors \(R_1\) and \(R_2\) to give a gain of \(\sim 100\).

• The decoupling capacitors are not shown, but they still must exist.
• Be careful. It is easy to:
  • Make incorrect connections, e.g. insert a wire into the wrong row.
  • Create unintentional connections, e.g. when two resistor leads in different rows accidentally touch.

Inverting amplifier

The previous non-inverting amplifier cannot have a gain less than 1, but an inverting amplifier can.

An inverting amplifier with output voltage \(V_{out} = -\frac{R_4}{R_3} V_{in}\). Note that this diagram follows the common convention that the DC power connections are not shown, but they obviously still exist.

Construct an inverting amplifier as shown in the above figure, choosing \(R_3\) and \(R_4\) to give a gain of \(\sim 100\).

Input impedance

One of the most important difference between non-inverting and inverting op-am circuits is their very different input impedances.

Input impedance is simply the ratio of input voltage to input current, i.e. \(Z_{in}=|V_{in}|/|I_{in}|\). An amplifier with very large input impedance is insensitive to the impedance of whatever input circuit is providing the input signal to the amplifier. (Here the input circuit is the wave generator and associated connections.) Since the amplifier input impedance is the load impedance for the input circuit, \(|Z_{in}|\) must be large enough so that the input circuit signal is not degraded because the the input circuit cannot provide enough power (\(P_{in}=V_{in}^2/|Z_{in}|\)).

It can be hard to directly measure the input impedance of a high gain op amp circuit, so we will use simulations to get a feeling for the different impedances of op amp circuits.

• Instead of assuming an ideal op amp, the “real” simulations are based on a LM741 op amp.
• \(V_{in}\) and \(I_{in}\) in these circuits are simply the voltage and current of the AC supply.

Design tasks

More complex op amps have many capabilities. For example, here is a summing amplifier Falstad circuit that allows you to scale and add signals. (There is also a Falstad example.)

An inverting summing amplifier with output voltage \(V_{out} = -\frac{R_4}{R_{3}a} V_A - \frac{R_4}{R_{3b}} V_B\).

Remember to check out the Falstad circuit examples, in case they have something close to what you want.

  A) Using a single op-amp and some resistors, design and test a differential amplifier circuit whose output is equal to about 1.8 times the difference \(V_A-V_B\) between two arbitrary (within reason) input voltages, \(V_A\) and \(V_B\).

• Use the Demo Probe Comp signal (~1kHz square wave, ~2.5Vpp) as \(V_A\) and a Wav Gen signal for \(V_B\).
  • You can use an alligator clip to connect to Demo Probe Comp

  B) Design and test an amplifying bandpass filter circuit that will amplify signals between very roughly 400Hz and 9kHz by a factor of 100, but attenuates any signals outside this band. You can only use a single op-amp, but multiple resistors or capacitors.

• The range is very rough, e.g. anywhere in the range 300-500 Hz for the low cutoff, 5-15 kHz for the high cutoff, and 75-150 in gain is fine.
  • Remember that when designing filters, the Falstad filter simulator is usually more useful than the circuit simulator.
• Pay careful attention to the loading of filter sections by the input impedance of the amplifiers. The order of filters matters.
• We only care about the amplitude, i.e. it doesn’t matter if the output is inverted or not.
• If the bandpass or gain isn’t as expected, one can change the capacitors and resistors to improve the circuit.