What is an inverting amplifier circuit?

This time, I'd like to start by explaining what an inverting amplifier circuit is!

In an inverting amplifier circuit, the input signal is fed to the inverting input terminal of the operational amplifier via resistor R1. The output voltage Vout is connected to the inverting input terminal via resistor R2, and by feeding the output voltage back into the inverting input, the output is controlled so that the inverting input voltage approaches the non-inverting input (reference voltage), thus establishing negative feedback. On the other hand, the non-inverting input terminal is connected to the reference voltage, which is generally GND (0V).

Inverting amplifier circuit using OP97

When the input voltage of an inverting amplifier is Vin and the output voltage is Vout, the relationship between Vin and Vout is as follows:

The amplification of an inverting amplifier circuit is determined by the values of R1 and R2. Also, as the name"inverting amplifier"suggests, the most important point is that the polarity of the output voltage is reversed.

Let's try building an inverting amplifier circuit on a breadboard!

Now, let's actually assemble the circuit, paying attention to the connection positions and values of the resistors!

This time, we will set the resistance values to [R1=1kΩ, R2=4.7kΩ, load resistance 10kΩ].

Breadboard connection example of an inverting amplifier circuit using OP97

The inverting amplifier circuit that was actually created

Thisis the pinout forthe ADALM2000.

This time, as with the previous evaluation of the unity-gain circuit,

• 1+/1-(Oscilloscope channel 1/​ ​GND of oscilloscope channel 1)

・ 2+/2-(Oscilloscope channel 2/​ ​GND of oscilloscope channel 2)

・GND

・ V+/V-(Positive power supply / Negative power supply)

• W1 (Signal Input 1)

We will use the following pins. Connect ch1 of the oscilloscope to the input side and ch2 to the output side, and observe the waveforms of each.

After connecting the included jumper wires to their respective locations, you can start taking measurements by connecting the ADALM2000 to your PC with a USB cable!

This time too

Input signal frequency: 1kHz (sine wave)

Input signal amplitude: 2Vp-p

Power supply voltage: ± 5V

Measurements are taken under these conditions, and the frequencies and Vp-p values of the input voltage waveform and output voltage waveform are checked, similar to a unity-gain circuit.

Under the conditions R1=1kΩ and R2=4.7kΩ,

Therefore, the amplification factor becomes-4.7 times. In other words, we want to obtain an output voltage waveform with 4.7 times the amplitude of the input signal and the polarity reversed.

Let's start Scopy and begin the setup! To start the setup, select the tab on the left side of the screen.

As with the previous time, we will configure two settings: "Signal Generator" and "Power Supply." The configuration conditions are as follows:

Power supply voltage: ± 5V

Input signal frequency: 1kHz (sine wave)

Input signal amplitude: 2Vp-p

For detailed instructions on how to set up Scopy, please refer to the previous article!

Let's experiment with ADALM2000! Unity Gain Circuit

Actual measurement! Inverting amplifier circuit!

Let's take a look at the waveform. Since it's 4.7 times 2Vp-p, we should get an output voltage waveform of approximately 9.4Vp-p!

Input/output voltage waveforms of an inverting amplifier circuit measured with Scopy.

Hmm, that's strange. What I got was a waveform of approximately 8.6Vp-p with both ends clipped.

LTspice waveform of an inverting amplifier circuit

Surprisingly, LTspice also yielded an output voltage waveform that clipped at approximately 8Vp-p. This suggests that the clipping might be due to the characteristics of the OP97.

In situations like this, check the datasheet! You're bound to find some clues there.

*Since the OP97 component does not exist in LTspice, the evaluation was performed using the substitute component OP07.

It had characteristics that were exactly as you'd expect! This is the key point here. When the OP97 is operated with ± 15V, it seems that it can only swing up to ± 14V as a typical example. In this case, since the input was ± 5V, it's safe to assume that it could only swing up to about ± 4V.

Bonus: Let's try reducing the input signal size!

How can I make the output signal swing to its full potential?

One approach would be to change the resistance value to reduce the amplification. On the other hand, to allow the output voltage waveform to swing fully without changing the circuit, one could set the input signal so that it stays within ± 4V even when multiplied by 4.7​. This time, we'll try the latter approach!

Let's try changing the input signal, which was originally set to 2Vp-p, to 1Vp-p and perform a measurement. Ideally, we should get a waveform that swings between 4.7Vp-p, or ± 2.35V.

Input and output voltage waveforms of an inverting amplifier circuit measured with Scopy (1Vp-p input version)

While it wasn't exactly the ideal value, I was able to obtain a full-swing waveform that was roughly as expected! I encourage you all to try changing the conditions and see how the waveform changes.

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