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很多CMOS运放电路设计的书,也是这种互补集电极输出,并且会讲到这些电路的原理,缺陷和补偿。
就是上面这示意图中的方框外上下两个电容,业余很难调得好.
Rail to rail Output的输出结构,一般是基于Dennis Monticelli的电路进行改进
The rail-to-rail output stage also presents challenges to the chip designer when it comes to quiescent biasing and reduction of crossover distortion. An elegant solution is the output stage designed by Dennis Monticelli, described briefly (in the context of low-distortion) in §5.9.2.67 At the broad brush-stroke level, it is a clever circuit that biases the push–pull output transistors to maintain current overlap at crossover, and, better still, with continuing current through both transistors throughout the output swing. Read the basic description in §5.8.3 first, then join us for an exploration of its detailed workings.
This is one cool circuit! We ran a SPICE simulation, to check out its linearity, and to see the overlap of source and sink currents over the full swing. Figure 4x.92 shows the circuit, as entered in IntuSoft’s ICAP/4 SPICE software. First we explored the transfer function, by sweeping the input current and watching the output current and the individual source and sink currents. In Figure 4x.93 the output current looks quite linear, to the eye. Also, you can see (maybe) that both transistors stay in conduction (just barely) over the full cycle. To explore this latter point, we plot in Figure 4x.94 the source (I+, from Q13) and sink (I−, from Q14) currents versus output current Iout,on both coarse and expanded scales; the latter shows the 100 μA–200 μA current in the “wrong” transistor. By comparison, these currents drop fully to zero in a normal class AB push–pull output stage, as shown in the corresponding SPICE simulation of Figure 4x.95.
Seeing this circuit being adopted by many designers of high-performance op-amps in the last 25 years, we continue to be impressed by Monticelli’s creation. He originally developed the circuit to work with silicon-gate MOSFETs, but it works especially well with BJTs. His circuit is perfectly suited for balanced push–pull drive (to the bases of Q13 and Q14, in Fig. 4x.92). It works well at high frequencies – for example, the OPA1611 has an 80 MHz GBW, and it can deliver lots of output drive (the OPA209 is happy delivering 65 mA, which is plenty for a 40 V op-amp)
Many op-amp datasheets don’t reveal the inner workings of their output stage, but sometimes you can recognize a Art of Electronics – The x-Chapters 4x.11.5. Designs by the masters: the Monticelli output stage 341 Monticelli rail-to-rail output stage from a distinctive open loop output impedance versus frequency plot. Many of the TI op-amps with a Monticelli output stage add an output feedback capacitor to an input-stage gain node, which causes Zout to drop to an amazingly low value, e.g., 1 Ω at 100 kHz for the OPA1611, before rising and then falling again; see for example Figure 5.34 in AoE3, or Figure 28 from the OPA1611 datasheet (plotted here as Fig. 4x.96)
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