Low-voltage low-power design
3.5. LOW POWER
3.5.2 Power-supply voltage conversion
The starting point is the assumption that the principal power source has a rel- atively low voltage, i.e. 1 V. It may then be that a circuit block requires a higher supply voltage in order to function in an optimum way. These mini- mally required node voltages can be found via systematic biasing techniques as presented in [8].
Realizing higher voltages from a fixed lower supply voltage can be done by means of a DC-DC converter. Two different types can be distinguished:
Charge-pump based DC-DC converters; LC-tank based DC-DC converters.
70 CHAPTER 3. LOW-VOLTAGE LOW-POWER DESIGN
An application could be the generation of a higher voltage in order to drive the gate of (MOS)FETs to a voltage which is well enough beyond the drain voltage (which is at most the supply voltage) in order to gain some extra drain voltage swing, [30] .
3.6
Conclusion
This chapter described the influence on the performance of electronics when low- voltage low-power constraints are given. To get a clear insight, these constraints were split into three separate parts:
low voltage; low current; low power.
Low voltage was shown to mainly influence the maximum possible signal
voltages. For a lower supply voltage, the maximum value of the voltage sig- nals reduce. When using the current as the information carrier, an additional degree of freedom is obtained, i.e. the impedance level, to set the maximum current level. Further, it was shown that the nullor implementations are hardly influenced. The minimum required voltage room is only slightly larger than the minimum required voltage room for the constituent devices. For a floating port, the minimum required voltage is a saturation voltage of a current source larger in comparison to the non-floating port, thus a slight preference may be found for the nullor implementation with non-floating ports.
The performance of current sources was shown to reduce for lowering the supply voltages whereas the performance of voltage sources improves or, at least, remains the same.
The devices are hardly influenced either, only the junction capacitances may be slightly larger, resulting in a speed reduction. But, it may be said to be negligible.
Low current was shown to have a predominant influence on the signal power, bandwidth and noise performance of devices. A reduction in the current con- sumption means a reduction in the performance of all three design aspects of the resistor, bipolar transistor and the (MOS)FET. It does not matter whether the voltage or current is used as the information carrying quantity.
Low power is the combination of low voltage and low current. To make opti-
BIBLIOGRAPHY 71
Each part of the circuit should be supplied from a voltage source with the min- imum required value. This is possible with voltage multipliers. However, due to the low input voltage, i.e. 1 V, the efficiency of these multipliers is moderate to low, or an external inductor is required to obtain better efficiency. There- fore, for low-voltage low-power design the use of voltage multipliers is limited to those parts of the circuits with a relatively low-power consumption, i.e. gates of (MOS)FETs.
Due to the low-power constraint, the orthogonalization of a design procedure is hampered. Now first the separate blocks have to be optimized, independently, and second a weighting must be used for the allowed power consumption of the separate blocks.
Thus an overall conclusion is that due to the low-voltage constraint, signal voltages (related via impedance levels to the current signals or directly being the signals) are limited to the supply voltage, which is very trivial. Low current reduces the performance with respect to noise, signal power and bandwidth. Low power combined with low voltage may hamper the orthogonality in the design process but does not introduce any additional performance degradation.
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Chapter 4
Amplifiers
4.1
Introduction
Amplifiers are required in almost every electronic system. In measurement equipment at the beginning of an information processing chain, they are re- quired to make the, very often, relatively weak information signals more robust so that they are more easily processed and transmitted. At the end of the in- formation processing chain the power level of the information signal requires power amplification in order to be able to drive the output transducer. Each of these amplifiers is not allowed to deteriorate the information signal.
In the world of amplifier design two trends can be distinguished, viz. the design of:
general purpose amplifiers (the opamps); dedicated amplifiers.
The main difference between those two types of amplifiers is that the general purpose amplifier has to be applicable for a wide range of load conditions and feedback factors, whereas for dedicated amplifiers the load is relatively well known. Therefore, the opamps are frequency compensated such that, approxi- mately, a first-order frequency behavior is obtained [1], [2], so that the risk of instability due to load variations is reduced to a large extent. In contrast, for dedicated amplifiers the order of the frequency behavior can be larger than one as the load is better known. However, orders beyond three become practically almost impossible as these amplifiers can become instable due to a relatively small additional phase shift.
In this chapter, the amplifier is assumed to be a dedicated amplifier as it is to be used in the oscillator or bandgap reference by which the load and feedback conditions are relatively well known.
76 CHAPTER 4. AMPLIFIERS
4.2
The basic function
The basic function of the amplifier is to accurately change the power or the signal level and/or the dimension of an information signal,
where is the amplified signal and is the transfer of the amplifier. To be able to reach the required accuracy, devices with an accurate transfer have to determine the overall transfer of an amplifier. In chapter 3, the asymptotic- gain model [3] was shown to be the appropriate model to synthesize accurate amplifiers. This model assumes a linear accurate feedback network (resistors, for instance) and an active part, supplying the required power gain which does not need to be accurately specified. When the loop gain of the amplifier is large, the transfer of the amplifier is determined by the feedback network and an accurate amplification is obtained.
This model can only be used for linear amplifiers as it is based on the super- position principle. The model assumes that by means of a relatively large loop gain, the non-linearity of the active part is counteracted and a linear approxi- mation can be used, i.e. the small-signal equivalent.
Recently, intrinsic non-linear electronics is gaining more interest and is be- lieved to be a serious candidate to use the high frequency potentials of, for instance, SiGe processes. Due to the intended non-linearity, signals are spread over a relatively large frequency range on the chip making it less sensitive to contamination (cf. FM modulation). One could think of using the very accu- rate exponential relation between the base-emitter voltage and collector current of bipolar transistors, or the gate-source voltage and drain current of weak- inversion MOS transistors. These accurate non-linearities are suppressed in linear electronics by using a lot of loop gain and cause distortion when they pop up in the transfer. For the non-linear electronics these non-linearities do not need to be suppressed any longer, but instead are used favorably. Examples of this non-linear signal processing is found in exponential state space filters [4].
In principle, the amplification factor is frequency independent, it is the im- plementation of the mathematical scaling:
where a is the scaling factor. This scaling is speed independent. Introducing speed limitations are an additional step; adding filtering, for instance.