The Norton amplifier topology brings back the essence of analog design by using clever circuit techniques to deliver strong performance with minimal components. It is not about a brand name—it’s about a timeless analog philosophy that continues to inspire engineers and hobbyists today. This approach shows why analog circuits remain powerful and relevant, even in our digital age.
In electronics, a Norton amplifier—also known as a current differencing amplifier (CDA)—is a specialized analog circuit that functions as a current-controlled voltage source. Its output voltage is directly proportional to the difference between two input currents, making it ideal for applications requiring precise current-mode signal processing.
Conceptually, it serves as the dual of an operational transconductance amplifier (OTA), offering a complementary approach to analog design and expanding the toolkit for engineers working with current-driven systems.
So, while most amplifier discussions orbit op-amps and voltage feedback, the Norton amplifier offers a subtler, current-mode alternative—elegant in its simplicity and quietly powerful in its departure from the norm. Let us go further.
Norton amplifier’s analog elegance
As shown in the LM2900 IC equivalent circuit below, the internal architecture is refreshingly straightforward. The most striking departure from a conventional op-amp—typically centered around a voltage-mode differential pair—lies in the input stage. Rather than employing the familiar long-tailed pair, this Norton amplifier features a current mirror followed by a common-emitter amplifier.
Figure 1 Equivalent circuit highlights the minimalist internal structure of the LM2900 Norton amplifier IC. Source: Texas Instruments
These devices have been around for decades, and they clearly continue to intrigue analog enthusiasts. Just recently, I picked up a batch of LM3900-HLF ICs from an online seller. The LM3900-HLF appears to be a Chinese-sourced variant of the classic LM3900—a quad Norton operational amplifier recognized for its current-differencing input and quietly unconventional topology. These low-cost quads are now widely used across analog systems, especially in medium-frequency and single-supply AC applications.
Figure 2 Pin connections of the LM3900-HLF support easy adoption in practical circuits. Source: HLF
In my view, the LM2900 and LM3900 series are more than just relics—they are reminders of a time when analog design embraced cleverness over conformity. Their current differencing architecture, once a quiet alternative to voltage-mode orthodoxy, still finds relevance in industrial signal chains where noise rejection, single-supply operation, and low-impedance interfacing matter.
You will not see these chips headlining new designs, but the principles they embody—robust, elegant, and quietly efficient—continue to shape sensor front-ends, motor drives, and telemetry systems. The ICs may have faded, but the technique endures, humming beneath the surface of modern infrastructure.
And, while it’s not as widely romanticized as the LM3900, the LM359 Norton amplifier remains a quietly powerful choice for analog enthusiasts who value speed with elegance. Purpose-built for video and fast analog signal processing, it stepped in with serious bandwidth and slewing muscle. As a dual high-speed Norton amplifier, it handles wideband signals with slew rates up to 60 V/μs and gain-bandwidth products reaching 400 MHz—a clear leap beyond its older cousins.
In industrial and instrumentation circles, LM359’s current-differencing input stage still commands respect for its low input bias, fast settling, and graceful handling of capacitive loads. Its legacy lives on in video distribution, pulse amplification, and high-speed analog comparators—especially in designs that prioritize speed and stability over rail-to-rail swing.
Wrapping up with a whiff of flux
There is not much more to say about Norton amplifiers for now, so we will wrap up this slightly off-the-beaten-path blog post here. As a parting gift, here is a practical LM3900-based circuit—just enough to satisfy those who find joy in the scent of solder smoke.
Figure 3 Bring this LM3900-based triangle/square waveform generator circuit to life and trace its quiet Norton-style elegance. Source: Author
Triangle waveforms are usually generated by an integrator, which receives first a positive DC input voltage, and then a negative DC input voltage. The LM3900 Norton amplifier facilitates this operation within systems powered by a single supply voltage, thanks to the current mirror present at its non-inverting (+) input. This feature enables triangle waveform generation without the need for a negative DC input.
In the above schematic diagram, amplifier IC1D functions as an integrator. It first operates with the current through R1 to generate a negative output voltage slope. When the IC1C amplifier—the Schmitt trigger—switches high, the current through R5 causes the output voltage to rise.
For optimal waveform symmetry, R1 should be set to twice the value of R5 (and here R1=1 MΩ and R5=470 KΩ, which is close enough). Note that the Schmitt circuit also provides a square wave output at the same frequency.
Feeling inspired? Fire up your breadboard, test the circuit, or share your own twist. Whether you are a seasoned tinkerer or just rediscovering the joy of analog, let this be your spark to keep exploring.
Finally, I hope this odd topic sparked some interest. If I have misunderstood anything—or if there is a better way to approach it—please feel free to chime in with corrections or suggestions in the comments. Exploring new ground always comes with the risk of missteps, and I welcome the chance to learn and improve.
T. K. Hareendran is a self-taught electronics enthusiast with a strong passion for innovative circuit design and hands-on technology. He develops both experimental and practical electronic projects, documenting and sharing his work to support fellow tinkerers and learners. Beyond the workbench, he dedicates time to technical writing and hardware evaluations to contribute meaningfully to the maker community.
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