“The size of MOS transistors has shrunk dramatically since their invention decades ago. Reductions in gate oxide thickness, channel length, and width have driven significant reductions in overall circuit size and power consumption. Due to the reduction in gate oxide thickness, the tolerable supply voltage is reduced, while the reduction in channel length and width reduces the product form factor and accelerates its speed performance. These improvements drive the performance of high frequency CMOS rail-to-rail input/output amplifiers to meet the increasing demands of today’s system designers for a new class of analog circuits that can operate at the same low supply voltages as digital circuits. Work.

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The size of MOS transistors has shrunk dramatically since their invention decades ago. Reductions in gate oxide thickness, channel length, and width have driven significant reductions in overall circuit size and power consumption. Due to the reduction in gate oxide thickness, the tolerable supply voltage is reduced, while the reduction in channel length and width reduces the product form factor and accelerates its speed performance. These improvements drive the performance of high frequency CMOS rail-to-rail input/output amplifiers to meet the increasing demands of today’s system designers for a new class of analog circuits that can operate at the same low supply voltages as digital circuits. Work.

This application note answers some unique questions about a generation of CMOS rail-to-rail amplifiers. The article begins with a general discussion and description of the topologies of traditional voltage-feedback and current-feedback amplifier circuits, as well as common causes of feedback amplifier oscillations. For ease of analysis and discussion, we divide the CMOS rail-to-rail amplifier circuit into four blocks: input, intermediate gain, output, and feedback network stages. The frequency dependent gain and phase shift of each stage will be shown, followed by a complete system simulation with all 4 basic circuit blocks shown and discussed. The second part will demonstrate and discuss the mechanisms, trade-offs, and advantages of three usage scenarios for addressing amplifier oscillations.

Voltage Feedback Amplifier

Figure 1 shows a simplified scheme of the EL5157 – a very popular high bandwidth voltage feedback amplifier. This scheme uses a classical differential input stage to drive a folded Cascode second stage, which converts the input stage’s differential voltage at a high impedance gain node into a current that increases with the amplifier’s high voltage gain accomplish. Essentially, the output impedance of a second-order current source that becomes an output signal at a high-impedance node increases any current disparity created in the signal-pass transistor. The output stage is a push-pull class AB buffer that buffers the high voltage gain into the single-ended output of the amplifier.

Figure 1: Voltage Feedback Amplifier

output sensing

An Inductor is an Electronic component whose impedance is affected by frequency: its impedance is lower at low frequencies and higher at high frequencies. The “ideal” op amp output impedance is zero, but in practice the output impedance of the amplifier is inductive, increasing with frequency just like an inductor. Figure 2 shows the output impedance of the EL5157. A challenge often encountered in applications utilizing op amps is driving a capacitive load. This is challenging because the inductive output of the op amp combines with capacitive loads to create an LC resonant tank topology, where the capacitive load, along with the inductive drive impedance, causes extra when the feedback is closed around the loop. phase lag. The reduction in phase margin has the potential to cause the amplifier to oscillate. While oscillating, the amplifier can get very hot and may even self-destruct. To solve this problem, there are many extraordinary solutions.