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Operational Amplifiers
Published in Michael Olorunfunmi Kolawole, Electronics, 2020
In practical IC implementations, the capacitances available are very small (a few picoFaraday, pF). The question is: How can such small capacitances give rise to such low cutoff frequencies (a few hundred Hz)? The answer is provided by an understanding of the Miller Effect, which basically explains how an impedance sitting across an amplifier is effectively converted into a smaller impedance at the input and roughly an equal impedance at the output of the amplifier: the amount that the effective input impedance is smaller than the actual one across the amplifier is a function of the amplifier’s gain. This explains how an op-amp, with a huge gain and a very small internal capacitance, can act like it has a huge capacitance at its input (a smaller impedance corresponds to a larger capacitance). In essence, the Miller Effect effectively takes impedance across an amplifier and replaces it with smaller impedance at the input and a roughly equivalent one at the output. It should be noted that this effect works only for amplifiers that share a common terminal between the input and output (e.g. ground). The effect works for op-amps with negative and positive gains. However, for op-amps with positive gains greater than one (A > 1), the effect gives rise to negative impedances at the input (meaning, reverse polarity of current that flows for an applied voltage, but Ohm’s Law still applies). The output impedance is still positive.
Compensation and Stability
Published in Douglas Self, Audio Power Amplifier Design, 2013
The Miller effect describes how connecting a capacitance between the input and output of an inverting amplifier makes it behave like a much larger capacitor; the greater the amplifier gain, the greater the capacitance shown.6 It is very useful for amplifier compensation because a relatively low dominant pole frequency can be implemented while keeping the currents flowing in the capacitor small. Figure 13.4 shows how it works in the usual three-stage amplifier. Ccompen is the added compensation capacitor, in parallel with Cbc, the existing collector-base capacitance in the VAS transistor. Cbc varies with the voltage on the VAS transistor and this can cause serious distortion if not dealt with in some way. This issue is dealt with in detail in Chapter 7.
Single-Stage Transistor Amplifiers
Published in Nassir H. Sabah, Electronics, 2017
A MOSFET and a BJT can be combined in a cascode configuration, as in Figure 8.4.16, where an active current source is used as a load. The input transistor is the MOSFET, so as to take advantage of its practically infinite input resistance. Using a BJT for the second transistor, rather than a CG MOSFET, gives higher gain, wider bandwidth, and a large output resistance. Q1 sees the low input resistance of the CB transistor Q2, which is smaller than that of a MOSFET Q2. The Miller effect is therefore reduced and the bandwidth improved.
Highly-matched sub-ADC cells for pipeline analogue-to-digital converters
Published in International Journal of Electronics, 2019
In multi-stage comparators, the last stage (latch) is involved in the loading and fan-out problems. Hence, the pre-amplifier stage is relaxed from the parasitic capacitances of the next circuits (Han et al., 2008; Wong et al., 2008). Note that the capacitive loads on the pre-amplifier stage directly affects the comparison speed. Since that all the stages are concatenated in single-stage comparators, loading issues significantly restrict the pre-amplifiers speed. For example, assume the case that static inverter gates are connected to the comparator’s outputs, as shown in Figure 8(a). These are used for the sake of improving the driving ability. During the pre-amplification, VOUT+ and VOUT-, are around the output CM level. Hence, both the NMOS and PMOS devices, MINV-N and MINV-P, operate in saturation region. Considering the Miller effect, the gate-drain capacitances of M1,2 are multiplied by the gain of the static inverter gate, according to (24):