Op Amp Amplifiers: More Applications based on Negative Feedback (Part 2)
|
|
Time to read 6 min
Table of contents
Voltage Buffer (Unity Gain Amplifier) Build a Voltage Buffer on Breadboard [LM358 as an example] Build an Inverting Amplifier Setting up an Inverting Amplifier using Single Rail Supply Setting up an Inverting Amplifier using Dual Rail Supply Non-inverting Amplifier with Input Signal Contains both DC and AC
In Part 1, we explored the fundamentals of operational amplifiers and saw how negative feedback shapes their behavior. In this second part, we move beyond the basics and look at practical applications built on the same principle.
Voltage Buffer (Unity Gain Amplifier)
The first and perhaps simplest example is the voltage buffer, also known as a unity gain amplifier. In this configuration, the Op Amp’s output is directly connected to its inverting input, while the signal is applied to the non-inverting input. The result is an amplifier with a gain of exactly one: the output voltage matches the input voltage.
Why use a Voltage Buffer?
If you wonder what’s the point of using an Op Amp with gain of 1 at all? One of the most important reasons is impedance transformation . A voltage buffer takes a signal from a high-impedance source and presents it to the next stage as a low-impedance output. This means it can protect delicate signal sources from being loaded down, while still delivering the same voltage to a heavier load.
For example, imagine a sensor that can only supply a tiny current without its output voltage dropping. Connecting it directly to a low-resistance load would cause signal loss. Placing a voltage buffer in between allows the sensor to “see” almost no load at all, while the buffer provides the current needed to drive the load.
The circuit shows an LM358 Op Amp configured as a voltage buffer, with a variable load connected at the output. By changing the load resistance RL and measuring the output voltage and current, we can see how well the buffer maintains its output (even when driving heavier loads that draws a few or tens of mAs current).
Build a Voltage Buffer on Breadboard [LM358 as an example]
Now let’s build this circuit on a breadboard. For many students encountering operational amplifiers for the first time, it’s common to confuse the triangular Op Amp symbol in a schematic with the physical device. In reality, the Op Amp is packaged as an integrated circuit (IC), typically in an 8-pin dual in-line package (DIP) like the one shown here.
Taking the LM358 Op Amp as an example, if you look up its datasheet you’ll find a pinout diagram showing the exact function of each pin, from the non-inverting and inverting inputs, to the outputs, and the power supply pins. The diagrams here compare the LM358 and another common dual Op Amp, the TL072.
Build an Inverting Amplifier
In the previous article, we derived the gain formula for an inverting amplifier. From a calculation perspective, the result is straightforward to obtain. If you are not familiar with the derivation, you may want to revisit Part 1 for a quick review.
Setting up an Inverting Amplifier using Single Rail Supply
Our focus here is to physically build the circuit on board. The first thing to pay attention to is power supply configuration, namely VCC and VEE for the Op Amp. This is because an inverting amplifier typically has a gain greater than 1 and reverses the signal’s polarity. In other words, if the input of the Op Amp is a positive voltage, the output will be negative.
If the VEE (negative supply) pin is tied to ground (0V), the Op Amp will not be able to produce the required negative output voltage. Instead, the output will hit the lowest voltage it can generate, a phenomenon known as clipping. The right-hand diagram illustrates how the output waveform becomes cut off when the supply voltage does not cover the full range required by the signal.
💡 Tip: Using an Inverting Amplifier with a Single-Rail Supply
When powering an Op Amp from a single supply (e.g., 0 V and +5 V), the output cannot swing below ground. To handle signals that would otherwise require negative output, you can bias the non-inverting input to a mid-supply reference voltage (e.g., 2.5 V for a 5 V supply). This shifts the entire signal range upward, preventing clipping and keeping the output within the supply limits. Remember to AC-couple the input signal if you don’t want the bias to affect its DC level.
Setting up an Inverting Amplifier using Dual Rail Supply
Setting a Dual-Rail power supply requires:
- Either A single power supply that provides both positive and negative outputs with a common ground (often found in bench power supplies), or
Two independent supplies connected in series, with the middle junction serving as the circuit ground. One supply will then provide the positive rail, and the other will provide the negative rail.
Therefore, another option is to use a dual rail supply, for example, ±5 V or ±12. This allows the Op Amp's output to swing both positive and negative relative to ground. This is ideal for an inverting amplifier, since the output polarity is opposite to the input and may need to go below 0 V.
With dual supplies, there is no need to shift the signal’s DC level or create a virtual ground. The Op Amp can process AC signals centered at 0 V directly, and the output will faithfully reproduce the inverted waveform without clipping — as long as the peak signal voltage stays within the supply rails.
Non-inverting Amplifier with Input Signal Contains both DC and AC
For non-inverting amplifier, we will look at a different case when the signal at Op Amp's non-inverting input contains both a DC component and an AC component. For example, consider an input of:
vin(t) = 1.5 + sin(1000t)
This means the signal is centered at a constant 1.5V (DC offset) and has a small sinusoidal variation of ±1V around that point. Such signals often appear in sensor outputs, where a steady baseline voltage carries a small, time-varying measurement on top.
For this kind of mixed signal, you can approach the analysis in two ways. The first is the straightforward mathematical route, where you directly apply the gain formula to the full expression of the input. A more “engineering” approach is to use the superposition theorem.
In this method, you treat the DC component and the AC component as two independent sources. You then analyze the circuit twice, once with only the DC source active (AC source set to zero), and once with only the AC source active (DC source set to zero). Finally, you sum the two results algebraically to obtain the complete output signal.
Superposition works here because, with negative feedback, the op amp operates linearly. In this region, the total response equals the sum of each source’s individual response.
Key Takeaways from This Article
A voltage buffer (unity gain amplifier) is useful for impedance transformation, isolating high-impedance sources from low-impedance loads.
Negative feedback keeps the Op Amp in its linear region, enabling predictable gain and making superposition analysis possible.
Dual-rail supplies allow op amps to handle signals swinging above and below ground without bias shifting or clipping.
Mixed DC + AC inputs can be analyzed efficiently by separating components and applying the superposition theorem.
Some FAQs on Op Amp Basics
Why use a voltage buffer if the gain is only 1?
Because it isolates stages of a circuit. It has high input impedance and low output impedance, so it prevents loading effects while maintaining the same voltage level.
What causes clipping in an op amp circuit?
Clipping happens when the output tries to go beyond the supply voltage limits. In single-rail setups, this often occurs if a negative output is needed but the negative supply is tied to ground.
When should I choose dual-rail over single-rail supply?
Dual-rail supplies are preferred when your signal needs to swing both above and below ground, such as in inverting amplifiers or AC-coupled audio stages.
Why is superposition valid for op amp circuits with negative feedback?
Negative feedback keeps the op amp in a linear operating region, so the output for multiple sources is the sum of the outputs for each source individually.
How do I handle a signal that has both DC and AC components?
You can analyze it using superposition — treat the DC and AC parts separately, calculate each output, and then add them together for the total response.
Daniel Cao
Daniel Cao is the founder of EIM Technology, where he creates hands-on, beginner-friendly electronics education kits that blend practical hardware with clear, structured learning. With a background in engineering and a passion for teaching, he focuses on making complex concepts accessible to learners from all disciplines.
Related Articles
Related Products
Most of the op amp content in this blog series is adapted from the book Fundamental Analog Circuits and Semiconductors, included in this learning kit. If you want a systematic way to study analog circuits with hands-on experiments, this kit (complete with the book and hardware) is an excellent choice.