Feature: Electronic design
the standing current of the common-base transistors of the Figure 10 folded cascode TIS, must each be biased to provide at least as much quiescent current as the TIS’s tail current established by transistors Q23 and Q25. Tis is necessary to prevent one of the common-base transistors Q16 and Q17 from being prematurely starved of current when the TIS’s tail current swings entirely from one transistor of its differential pair to the other. For the same reason, the standing current provided by sources Q34 and Q36 in the TIS of Figure 13 must at least each be equal to the quiescent current provided by each of the TIS’s tail sources Q38 and Q39.
Figure 7: The bases of the complementary output transistors are connected to the low-impedance outputs of complementary cross-coupled emitter-followers in a futile attempt to differentially drive the output devices
Compromise Te circuits of Figures 4, 7, 8 and 9 are effectively single voltage gain stage designs which do not maximise forward-path gain or improve forward-path linearity enough to justify their use. Te best compromise is obtained by adopting a two-stage design, consisting of a transadmittance stage driving a transimpedance stage, with each stage designed to generate as much forward-path gain as possible. A modest amount of local (minor loop) negative feedback is then applied to each stage to enhance forward-path linearity and promote stability of the major negative feedback loop, without excessively reducing the forward-path gain necessary for adequate major loop gain. As noted previously, the forward-path
Figure 8: A cosmetic variation on the scheme of Figure 7
transimpedance gain of the second stage of a two-stage amplifier is approximately equal to the product of its current gain and the effective impedance at its output. Terefore, it may at first appear that increasing the second stage’s forward-path transimpedance gain is a simple matter of either increasing its current gain by making it a compound configuration of an emitter-follower driving a common emitter stage, or increasing the effective impedance at its output by making it a cascode. In practice, however, the extent to which the output impedance of the TIS can be maximised by using a cascode is severely limited by the input impedance of the buffer to which the TIS is connected, and only approaches a maximum when the output of the TIS is connected to the input of a Class-A MOSFET source follower. A far more significant increase in the
50 December 2021/January 2022
www.electronicsworld.co.uk
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