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U. Glaeser

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The collector-emitter voltage of transistor Q2, VCE.Q2,<br />

is given by<br />

© 2002 by CRC Press LLC<br />

VCE.Q2<br />

= ( VOL<br />

− αVsig)<br />

− ( VBB<br />

− VBE)<br />

=<br />

VBE<br />

− ( 0.5 + α)<br />

− Vsig<br />

(3.1)<br />

where Vsig<br />

is the logic voltage swing and α is a constant between −1<br />

and 1. As shown in Fig. 3.4, α is 0<br />

when the output stays and increases when the output is rising. The maximum α is dependent on the<br />

output rise time; the slower, the larger. SPICE simulation predicts the maximum α is between 0.2 and<br />

0.4 when ICS<br />

= 70 µ A and CL<br />

ranged from 0.1 to 1.25 pF and temperature ranges from 0°<br />

C to 80°<br />

C. When<br />

Vsig<br />

is 0.6 V, VCE.Q2<br />

may become as low as 0.36 V for an instance in switching, but never stays in the<br />

saturation region.<br />

Only inverting structures are possible in the LS-APD-ECL circuit. If the input is fed to the base of Q2<br />

to construct noninverting structures, is given by<br />

V<br />

CE.Q2<br />

VCE.Q2<br />

= ( VOL<br />

− αVsig)<br />

− ( VOH<br />

− VBE)<br />

=<br />

VBE<br />

− ( 1 + α)<br />

− Vsig<br />

(3.2)<br />

In order to keep Q2 out of the saturation region, Vsig<br />

should be lower than 0.45 V, which is impractical.<br />

Because transistor QD self-terminates at the point where the output reaches VBE<br />

above VREG,<br />

VOL<br />

becomes<br />

a direct function of VREG.<br />

On the other hand, as VREG goes lower, both QU and QD turn on more deeply,<br />

resulting in larger steady-state current. Simulated VOH, VOL, ISS(H), and ISS(L) dependence on VREG are shown<br />

in Fig. 3.6. In order to keep enough noise margins between VOL and the circuit threshold, VBB, VREG should be lower than about −2.2 V. At the same time, in order to restrict ISS(L) to an acceptably low level,<br />

VREG should be higher than about −2.4 V. Accordingly, VREG needs to be controlled very tightly around<br />

−2.3 V within the small error indicated in the figure.<br />

A VREG voltage regulator circuit for the LS-APD-ECL circuit is presented in Fig. 3.7. An automated<br />

bias control (ABC) circuit [7] is employed to automatically adjust ISS(L) constantly even under process<br />

deviation, power supply voltage change, and temperature change. A replica circuit of the LS-APD-ECL<br />

circuit generates a reference VREG level, VR, when ISS(L) is one-eighth of the current ICS. A monitored VREG FIGURE 3.6 Output voltage and crossover current vs. V REG in LS-APD-ECL circuit.

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