Camenzind_Analog design chips

In this circuit we also have two higher-performance current mirrors: M15 to M18 (see figure 3-24) to get the maximum open-loop gain and M5 to M8 (see figure 3-25) to get the highest possible common-mode rejection (now 98dB, up from 94dB in figure 8-16).

The output stage is capable of supplying 1mA peak and can get within 100mV of the rails with a 5kOhm load. This performance can be increased by making M28 and M29 wider.

Though capable of a much higher current without wasting idle power, this rail-to-rail output has the same weaknesses as the previous one: it is very sensitive to capacitive load and the open-loop gain is

previous circuit).

In the next circuit (figure 8-21) the polarity of the input stage is reversed and the current mirror for the second stage (M11 to M14) is designed to have the highest possible output impedance, resulting in an increased loop gain. Also note that the primary current has been (arbitrarily) reduced to 5uA.

The lower operating current level has only a minor effect on the sizes of most devices; they still need to be large to obtain satisfactory matching, much larger than the process (0.35u, or the higher-voltage portion of a 0.18u process) would allow. The idle current of the output stage is now reduced to 10uA.

Open-loop gain is 107dB at low frequency and with no load. Capacitive loading is still a problem (but much reduced if the minimum closed-loop gain is higher than 1) and, as before, the closed loop gain is a function of load impedance.

The input operating range now extends from about +0.8V to 150mV below the negative rail. If a single supply is used the inputs can function at or below ground level.

Because of the large dimensions used for the input transistors the white noise level is a relatively low 21nV/rtHz. Note, however, that they are run at twice the level of I1.

Fig. 8-21: Op-amp with N-channel input.

Now let's extend the operating range of the input by using both p- channel and n-channel devices. In the circuit of figure 8-22 we are adding an n-channel differential pair w hich takes over when the DC level at the inputs reach about +0.8Volts and the p-channel devices get cur off. There are three voltage regions for the input now: within 0.8 Volts of the negative rail only the p-channel devices are active; from about -0.8 Volts to +0.8V at the inputs, both pairs amplify and within 0.8V of the positive only the N- channel devices amplify.

When both pairs are active, the open-loop gain is at a maximum, reaching 120dB. When the common-mode level is either high or low, the

loop gain drops by 110dBdB. Many schemes have been offered in the literature which hold this gain more constant (e.g. by allowing only one pair to operate at a time), adding another dozen devices. For most applications the benefits of this measure are limited; in fact simultaneous operation of both pairs increases performance (noise, for example drops to 20nV/rtHz, compared to 30nV/rtHz when only one pair is amplifying).

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Fig. 8-22: Op-amp with rail-to-rail inputs and output.

The two input stages work with the second stage (M11 to M18) as folded cascodes. The lower part of the second stage (M15 to M18) is a current mirror derived from M1 to M4, set here at about 10uA; the upper part (M11 to M14) mirrors the current again, so that at M19/M23 the currents cancel if there is no input signal. M19 and M23 set the bias current of the output transistors (M27, M28)

With an input signal one or both input stages change the currents in the second stage, resulting in a net positive or negative current through M19/M23, which is translated into a larger current at the output.

Auto-Zero Op-Amps

Auto-zero or chopper stabilized amplifiers have been around for decades but continue to evolve. In a modern embodiment two amplifiers are used, checking up on each other.

Each amplifier has a "Trim" input, i.e. a single node which changes its offset voltage in both directions.

A built-in oscillator flips the two switches periodically at a rate of a few hundred to a few thousand Herz. In position A the inputs of amplifier 2 are shorted together and its own offset voltage is amplified (with the openloop gain) and corrected by feeding the output to the trim input. The required trim voltage is stored in capacitor Ca.

In the second phase of the oscillator the inputs of amplifier 2 are connected in parallel to those of amplifier 1 and its output now feeds the

which is completely eliminated. Above the switching frequency the behavior of the auto-zero amplifier is identical to a regular op-amp.

Apart from the higher current consumption because of the additional circuitry there is one drawback: switching noise. At the switching frequency there is a noise peak which also causes (intermodulation) distortion. This effect can be ameliorated by changing the switching frequency at random, i.e. creating a spread spectrum.

Distortion in an Op-Amp

An op-amp is basically a non-linear circuit. The input stage, for example, can accommodate only a small differential voltage before it is limited by the input devices, both bipolar and CMOS.

Feedback reduces the distortion caused by these limitations. Increasing the amount of feedback increases the linearity.

If you increase the open-loop gain by a factor of 10 (20dB), distortion drops by a factor of 10, assuming that, by increasing the gain you have not added more distortion.

9 Comparators

To most people a comparator is merely an op-amp without feedback. With the very large open-loop gain the output abruptly traverses the entire available voltage range when one input passes the level of the other.