Operational amplifiers (op amps) and comparators are fundamental building blocks in electronics, often used for signal conditioning and control. While they may appear similar at a basic level, their internal designs and intended applications differ significantly. This article delves into the internal architecture of the LM324 op-amp and the LM393 comparator, highlighting why using a comparator as an op amp in typical amplifier circuits is generally not advisable.
Let’s examine the schematic diagrams provided in the datasheets for the LM324 (op-amp) and LM393 (comparator) to understand their core differences:
Schematic diagrams comparing the internal circuitry of the LM324 op-amp and LM393 comparator, highlighting the compensation capacitor in the LM324 and the different output stages.The input stages of both the LM324 and LM393 exhibit considerable similarity. However, a crucial distinction emerges when we observe the presence of an internal compensation capacitor within the LM324 op-amp, clearly marked in red in the schematic. This capacitor is intentionally integrated to ensure stability in various negative feedback configurations, including the direct output-to-inverting input connection, preventing unwanted oscillations in typical op-amp circuits.
In the LM324, transistors Q10 and Q11 function as emitter followers, contributing no additional voltage gain and thus not posing extra stability concerns. Transistor Q12 in the LM324, on the other hand, operates similarly to Q7 in the LM393, both designed to provide substantial voltage gain.
The most prominent divergence lies in their output stages. The LM324 employs push-pull emitter followers in its output stage, which are characterized by unity gain. Conversely, the LM393 utilizes a common emitter stage formed by Q8 in its output, which introduces another stage of high voltage gain.
In essence, the LM324 op-amp incorporates frequency compensation to accommodate a wide spectrum of feedback configurations without compromising stability. In stark contrast, the LM393 comparator lacks this compensation. Adding to potential instability, the LM393 even includes an extra voltage gain stage, further differentiating it from typical op-amp designs.
Considering these internal architectural differences, particularly the absence of frequency compensation and the presence of an additional gain stage in the LM393, attempting to implement standard negative feedback configurations—commonly used to create op-amp based amplifiers—with a comparator like the LM393 is highly likely to result in oscillations or significant instability, especially at lower closed-loop gains.
Datasheet specifications further emphasize these distinctions. The LM393 comparator boasts a minimum voltage gain of 50 V per millivolt of input voltage, whereas the LM324 op-amp exhibits a considerably lower gain of only 15 V per millivolt. This substantial difference in gain, as indicated in the datasheets, reinforces the observation from the schematic analysis: the LM393 possesses inherently higher gain than the LM324.
While it’s theoretically possible to configure a comparator for amplifier applications, the LM393 datasheet provides limited guidance or specific techniques to achieve stable amplification. The design characteristics of comparators, optimized for fast switching and not linear amplification, make them a less suitable choice compared to op-amps for general-purpose amplification tasks requiring negative feedback and stability.
