In this region, the gain and phase components are both fairly constant. The first region is the low-frequency region, highlighted in red. To better analyze the characteristics of open-loop gain, let’s divide Figure 2 into three distinct regions, as shown in Figure 3. Both gain and phase curves are represented on the same set of axes, with the gain curve labeled G and the phase curve labeled φ.įigure 2 Typical open-loop gain/phase plot In practice, real-world limitations and choices of modern op amp design cause most products to exhibit an Aol response with predictable gain and phase components, as shown in Figure 2. Aol amplifies Ve to generate the ideal output voltage (Vo), which flows through Zo before appearing at the output pin (Vout).Īs implied earlier, Aol is not simply an ideal gain block that behaves the same at all frequencies. In this model, a differential input signal (Ve) develops across the op amp’s input resistance (Rdiff). įigure 1 Simplified op amp small-signal model (open-loop) Let’s return to our simplified small-signal open-loop model of an op amp from part 1, shown in Figure 1. To reiterate a point from the first installment in this series, Aol interacts with the open-loop small-signal output impedance (Zo) across frequency to create the op amp’s overall AC response. In essence, Aol is what makes an amplifier an amplifier. It affects nearly all aspects of small-signal or linear operation, including gain-bandwidth product, stability response, rise and fall time, settling time, and even input offset voltage.
OP AMP OFFSET VOLTAGE SPICE MODEL HOW TO
Here in part 2, I’ll explain how to verify the parameters of an op amp that define its small-signal bandwidth or frequency response while the op amp is in its linear operating region: open-loop gain/phase (Aol), closed-loop gain (Acl) and small-signal step response.Īol is the gain stage that acts on the differential signal applied to the op amp input pins and is undoubtedly the most fundamental parameter of any op amp.
OP AMP OFFSET VOLTAGE SPICE MODEL SERIES
The first installment of this article series discussed the need to verify SPICE model accuracy and how to measure the open- and closed-loop small-signal AC output impedance of operational amplifier (op amp) models.
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