An operational amplifier, or op-amp, is a high-gain electronic amplifier used to process and manipulate analog signals. It is one of the most important building blocks in electronics and is commonly found in devices such as audio systems, sensors, medical equipment, and computers. An op-amp typically has two input terminals — an inverting input and a non-inverting input — and one output terminal. By comparing the voltage difference between its inputs, it can amplify signals and perform mathematical operations such as addition, subtraction, integration, and differentiation.

The operational amplifier was first developed in the 1940s. Early versions were designed using vacuum tubes for use in analog computers during World War II. One of the key contributors was Karl D. Swartzel Jr. at Bell Telephone Laboratories, who created one of the first practical op-amps in 1941. These early amplifiers were called “operational” amplifiers because they were used to perform mathematical operations in analog computation systems.

Originally, op-amps were mainly used in analog computers to solve complex mathematical equations related to engineering, military targeting systems, and scientific research. As transistor technology improved in the 1950s and integrated circuits were introduced in the 1960s, operational amplifiers became much smaller, cheaper, and more reliable.

Today, operational amplifiers are used in a wide range of modern electronic applications. They are commonly found in audio amplifiers, active filters, oscillators, voltage regulators, sensor interfaces, medical instruments, communication systems, and control circuits. Modern op-amps are integrated into tiny semiconductor chips and are essential components in both consumer electronics and industrial systems.

Even though modern operational amplifiers are inexpensive and widely available as integrated circuits, the circuit shown below demonstrates a simple discrete operational amplifier built using only three transistors.

Fig. 1: the basic discrete op-amp

The schematic shown in figure 1 is surprisingly simple, using only a handful of components to form a basic discrete operational amplifier. Despite its simplicity, the circuit performs reasonably well. Using the component values shown, a simulation of the design (presented later in this article) with a +/-15V power supply resulted in a DC offset at the output of only around 380uV, which is quite respectable for such a minimal transistor-based design. Real-world tests showed around 3 to 5mV, which is still respectable. You could replace R1 with a 5k variable resistor to really dial in the DC offset to 0V, if you wanted to, but I don't really see the point.

Is it “audiophile” quality? No — but that is not really the point of the circuit. Its value lies in demonstrating the fundamental principles of op-amp operation using discrete components rather than achieving ultra-high-end audio performance.

One thing to note is that the onsemi BC559C and BC549C transistors used in the design are now considered obsolete and may become increasingly difficult to source over time. At the time of writing this article, a few thousand devices were still available from Mouser Electronics manufactured by Diotec Semiconductor, although there is no guarantee of long-term availability. If these exact parts cannot be obtained, possible substitutes include the BC558 and BC548 transistor series. However, these alternatives generally have slightly higher noise characteristics, which may affect performance in low-noise or sensitive analog applications.

The input stage is formed by Q1 and Q2, which act as a differential pair. The non-inverting input is connected to the base of Q1, while the inverting input is connected to the base of Q2. These two transistors compare the voltage difference between the inputs. Small changes between the two input voltages cause the current flowing through the pair to shift from one transistor to the other.

Resistor R1 provides the tail current for the differential pair by setting the current flowing from the emitters of Q1 and Q2 toward the negative supply rail. Resistor R2 acts as the collector load for the differential stage and helps convert the changing transistor current into a voltage signal.

The voltage developed at the collectors of Q1 and Q2 drives Q3, which acts as the main voltage amplification stage. Q3 is configured as a common-emitter amplifier, providing most of the circuit’s voltage gain. Resistor R3 is the collector load for Q3 and determines the output voltage swing and gain characteristics.

Capacitor C1 provides frequency compensation. In high-gain amplifier circuits, phase shifts at high frequencies can cause oscillation or instability. The capacitor helps stabilize the amplifier by reducing high-frequency gain and improving phase margin, much like the compensation capacitor found inside many integrated operational amplifiers.

When negative feedback is applied externally from the output back to the inverting input, the circuit behaves similarly to a conventional op-amp. The amplifier adjusts its output so that the voltage difference between the inputs becomes extremely small. Depending on the feedback network used, the circuit can function as an inverting amplifier, non-inverting amplifier, buffer, filter, or other analog building block.

While this design is relatively simple and not intended for precision or high-performance audio applications, it serves as an excellent educational example of how operational amplifiers work internally using discrete transistor stages.

Below are some screen grabs from the simulation of the op-amp. Figure 2 shows the testing setup with the op-amp configured as non-inverting with a gain of 3.2 and a 1kHz 1V input sinewave. Total harmonic distortion (THD) in the simulation is a fairly good at 0.0021%!

Fig. 2: Testing scenario

Fig. 3: Output in non-inverting test @ 1kHz 1V input; gain 3.2

Fig. 4: Frequency response curve; gain 3.2 @ 1V input

Fig. 5: Total noise

For such a simple circuit, it does perform surprisingly well with fairly low-noise and THD characteristics. One final thing to be aware of. There's no short-circuit protection employed, so if the output is shorted to ground or a supply rail; Q3 most likely will be destroyed.

TINA-TI simulation file for those that wish to play with this further. TINA-TI v9 is available for free from Texas Instruments' website.