Current-Ratio Temperature Compensation in Bipolar Relaxation Oscillators

Current-Ratio Temperature Compensation in Bipolar Relaxation OscillatorsM.H. Splithof1, A.J.M. van Tuijl1, S.L.J. Gierkink2, E.A.M. Klumperink21Philips Semiconductors, Nijmegen, The Netherlands

2MESA Research Institute, University of Twente, Enschede, The Netherlands

Mike.Splithof@nym.sc.philips.com, Ed.vanTuijl@nym.sc.philips.comS.L.J.Gierkink@el.utwente.nl, E.A.M.Klumperink@el.utwente.nl

AbstractA new temperature compensation strategy forbipolar relaxation oscillators is presented. Thecompensation scheme is suitable for both Schmitt-trigger oscillators and emitter-coupled multivibratorsand requires a minimum amount of circuitry for thecompensation. The feasibility of this compensationscheme is demonstrated by the results of a test chip.Without any trimming and with a total currentconsumption of 2.5mA, a temperature coefficient ofapproximately -90ppm/°C has been achieved at

frequencies up to 2.5MHz.

1.IntroductionBipolar relaxation oscillators are frequently used inmonolithic voltage controlled oscillator designs. Theseoscillators display excellent control linearity, thoughtemperature compensation has always been animportant issue. Among the bipolar relaxationoscillators, two major subclasses can be distinguished,which are formed by Schmitt-trigger oscillators andmultivibrators. Various temperature compensationtechniques have been reported [1],[2], though theseschemes always focus on either Schmitt-triggeroscillators or multivibrators.The current-ratio temperature compensation strategy,which will be presented in this paper, offers goodperspectives for both Schmitt-trigger oscillators andmultivibrators. The key in the approach is the fact thatthe currents in all branches of the oscillator core aredefined by the circuit topology only. Utilisingtranslinear loops, the hysteresis voltage is then definedby a precise reference voltage, thereby eliminating thefundamental origin of temperature dependence.

2.Temperature compensation inSchmitt-trigger oscillators

2.1.Schmitt-trigger oscillatorsA well-known bipolar Schmitt-trigger oscillator is

depicted in Figure 1. Its operation is briefly explained asfollows. At any time, either T1 or T2 is conducting,

causing the output voltage across the resistor R1 to be

either low or high. This output voltage controls acurrent switch such that the timing capacitor C is eithercharged or discharged, resulting in a triangular voltagewaveform across the timing capacitor. Due to thepositive feedback, the Schmitt-trigger circuit willremain in a stable state until a critical voltage level atthe trigger input is reached, at which time the circuitwill change state. Subsequently, the current switch willbe operated, such that the timing capacitor will bealternately charged and discharged by a current IC, the

frequency of oscillation being defined by the controlcurrent IC, the capacitor C and the hysteresis voltage of

the Schmitt-trigger.

Schmitt-trigger

+_I0

VIN

+VCC

R1

T2T1

+

_VOUT

IC

2ICC

Figure 1. Basic Schmitt-trigger oscillatorThe hysteresis voltage of the bipolar Schmitt-triggerstrongly depends on temperature [1], which implies thatthe frequency of oscillation has to be compensated fortemperature dependence.

2.2.The current-ratio Schmitt-triggerThe current-ratio Schmitt-trigger [3] is based uponthe basic bipolar Schmitt-trigger and is shown in Figure2. In this circuit, temperature compensation of thehysteresis voltage is achieved by an adaptation of thecircuit topology. T1 and T2 can be recognised to be a

differential pair with unity gain positive feedback. Theresistive collector load of the bipolar Schmitt-trigger has

利用电路的延迟been replaced by a VLO-cell, composed of T5 through T8,and a VHI-cell, which consists of T3 and T4.+_+_2I02I02I0I0VHIVLOVIN+VCCVLO-cell+_VHI-cellI0T2T1T4T3T8T7T6T5VOUTFigure 2. The current-ratio Schmitt-triggerThe circuit operation will now be explained. First,suppose that VOUT is high. Since transistor T5 is off, thecollector load of transistor T1 equals the small signalimpedance of the VHI-cell.(1)4e3eHIrrZ+=Assume that T1 carries a current (1-λ)I0. Then, IC2,IC3 and IC4 can also be expressed in terms of I0 and λ:(2)01CI)1(I⋅λ−= 02CI)1(I⋅λ+=03CII⋅λ= 04CI)2(I⋅λ−=The regeneration process will start when the loop gainreaches unity. This occurs when the collector loadresistance equals the sum of the emitter load resistances:(3)4e3e2e1errrr+=+Assuming equal junction temperatures for alltransistors, substitution of the collector currents andsolving for the current ratio λ yields:(4)212111111HI=λ=λ⇒λ−+λ=λ++λ−After the Schmitt-trigger has changed state, thecollector load of T1 equals the small signal impedanceof the VLO-cell. Now, regenerative switching will occurwhen:(5)5e2e1errr=+Continuing the calculation yields the current ratio atthe lower switching instant: λLO= -1+√2.Summarising: the current ratios at the regenerationinstant are uniquely defined by the circuit topology andindependent of temperature or the absolute values of thecollector currents.Now, let us take a closer look at the circuit when theinput voltage VIN approaches VHI. A translinear loop canbe recognised from VIN to VHI.