Low voltage techniques

Abstract

ConsultantPuerto Vallerta, Mexico THE INTRINSIC OPERATING VOLTAGE limit of bipolar ICs is somewhat greater than the emitter-base voltage of the transistors. To date, this limit has been pushed only with digital circuitry. Techniques for constructing such devices as operational amplifiers, comparators, regulators and voltage references that will work from voltages as low as those supplied by a single nickelcadmium cell, will be offered. comes evident early in the design of low-voltage, micropower circuits where large output currents are required. The standard Darlington connection cannot be used as it raises input voltage unduly. Super-gain transistors severely restrict the maximum operating voltage, limiting the usefulness of an IC design using them as an output device. Figure 1 shows a buffer that operates from little more voltage than an unbuffered transistor. A key to its usefulness is that the emitter-base voltage of Q2 can be made sufficiently lower than that of Q1 to insure proper operation. Both voltages are predictable functions of measured process variables, and the circuit can be reproduced without special manufacturing controls.This circuit has obvious limitations that can be removed by including the refinements shown in Figure 2. A boost circuit is added to increase the bias current to Q3 with output current.This will not only increase the maximum output current for a given standby current, but also the input-voltage change required to produce it. Some linearization of the transfer function is also provided.that they do not unfavorably alter the frequency response of the inverter, at least at frequencies below a few megahertz. Figure 3 shows a class-B amplifier incorporating this design. At first glance, it would seem that the PNP output driver, consisting of three, common-emitter stages would be impossible to frequency compensate when included within a major loop. However, local feedback through Q6 and the inclusion of Q4 and R2 within the loop tends to control the frequency characteristic by making gain about equal to the NPN. Additional circuitry must be added t o control the drive current to the output PNP in saturation. This can be accomplished with minimal increase in saturated operating current.Obtaining high current gain per stage is one problem that be- Considerable experience with such boost circuits has shownIntegrated versions of this output stage have been built to supply 5 lOmA at a total supply voltagc of 1 . l V with 2 200mV saturation voltage. Unloaded, saturation voltage is 2 l0mV and operating current is 50pA. Maximum voltage is limited only by the BVCEO of the transistors (60V -80V).Band-gap references can easily be designed to operate from a 1.3V supply. This is unnecessarily high in that it excludes the rechargable nickel-cadmium cell as a power source. The circuit in Figure 4 will provide a temperature compensated 200mV and will operate from less than 1V. It also has been corrected for the thermal nonlinearity of emitter-base voltage and the temperature drift of implanted resist...

A Sub-1-V Low-Noise Bandgap Voltage Reference

“…This result reveals the collector currents are set by resistors R3a and R3b instead of as in [12] or the reference voltage and as in [15]. Furthermore, the currents and are proportional to in (41) and (44).…”

“…4 [12]. The output voltage of this reference circuit is (13) If is negligible and such that , then (13) reduces to mV (14) where the scale factor is (15) It is clear the circuit in Fig. 4 uses RBVP since (14) has the same form as (12).…”

“…The use of a separate current source causes the currents inside the natural logarithm of (13) to rely on temperature-dependent parameters instead of ratios of resistors as in (1) or ratios of transistor sizes as in (3). Hence, it complicates the calculation of the scale factor in (15).…”

“…Also, the error term will not be removed if changes over temperature and supply. Even if resistor R4 does not completely cancel the error from , it does reduce the constraint that currents , have to be considerably larger than to minimize the error term as in [12] and [15]. Furthermore, this method is easy to implement, and it does not increase the current consumption.…”

A Sub-i V Low-Noise Bandgap Voltage Reference

“…The reference voltage is created by I adding a forward-biased diode voltage (VBE]), which has a 92 negative temperature coefficient (TC), to the scaled difference 1. Reverse bandgap voltage references from a) [6] and b) [8] For obvious reasons, the traditional bandgap voltage reference The proposed sub-I V low-noise bandgap voltage reference cannot function well if the supply voltage is less than VREF. uses the reverse bandgap voltage principle, which is derived by Recently, new voltage references are reported in literature dividing both sides of (1) by the gain factor M to yield to overcome the low supply voltage problem by using a VBEI (ICI Area 2 (2) current-mode reference [2][3][4].…”