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The search for the ideal transistor

30 Mar 2015  | Sergio Franco

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The advantages of CFAs compared to ordinary op amps (also called voltage-feedback amplifiers or VFAs) are fast dynamics. Equation (6) indicates that the loop gain of this circuit is


T = zc/R2           (8)


so the closed-loop bandwidth is given by the frequency at which |zc| = R2, also called the crossover frequency fx (see Ref. [4]). So long as R2 << Rc, this frequency is fx = 1/(2πR2Cc). With R2 on the order of 103 Ω and Cc on the order of 10–12 F, fx will be on the order of 108Hz. Note also that fx is set by R2 regardless of R1. Compared to VFAs, where fx is inversely proportional to the noise gain 1 + R2/R1, the fx of a CFA appears to be independent of the noise gain (for a more detailed analysis of higher-order effects, see Ref. [3]). Another dynamic advantage of CFAs is their relative immunity from slew-rate limitations because Cc is driven directly by the input buffer, which can supply virtually any current to rapidly charge/discharge Cc.


Back to the voltage-feedback amplifier (VFA)
Let us admit, we are so used to VFAs, that the presence of a buffer directly across the input terminals tends to make us feel uneasy (which is exactly how I felt the first time I run into a CFA). Yet, the fast dynamics of CFAs are quite tempting... Wouldn't be possible to modify a CFA so as to obtain a VFA that retains at least some of the dynamic advantages of the original CFA? This issue too has been addressed quite sometime ago by adding a third voltage buffer (see Q15-Q16-Q17-Q18 of figure 7) to turn node vN into a high resistance input, along with a resistance R between the outputs of the first buffer and this new buffer to generate the control current previously denoted as iN.


Figure 7: CFA-derived VFA.


To analyse the circuit, consider the current through R, assumed to flow from left to right, which is (Vp – Vn)/R. The current mirrors convey this current to the gain node C, where it produces the voltage zc(Vp – Vn)/R. This voltage is then buffered to the output node to give Vo, so the open-loop voltage gain is



where Eq. (4) has been used. Again, a well-designed circuit has R << Rc, in turn implying a large dc value for a. Owing to its inherently fast current-mode operation, this op amp type is especially suited to high-speed applications. A popular example is the LT1363—70MHz, 1000V/µs op amp.


Closure
Our quest for the ideal transistor has brought us to rediscover a series of circuits that have already been around for quite some time. Does this imply that when you sit down to try inventing something new, you better keep in mind the notorious dictum: "Everything that can be invented has [already] been invented"? Or that...? Well, I won't try answering now, because I'm going to lock my office door, pull out from a secret drawer my own tee shirt and cutoffs, change my clothes, and go out to soak up some sun myself.


References
[1] http://online.sfsu.edu/sfranco/BookAnalog/AnalogJacket.pdf

[2] http://www.ti.com/lit/an/sboa117a/sboa117a.pdf

[3] http://online.sfsu.edu/sfranco/BookOpamp/OpampsJacket.pdf

[4] http://www.edn.com/electronics-blogs/analog-bytes/4426049/Inverting-vs-Noninverting-


About the author
Sergio Franco is an author and (now emeritus) university professor who was drawn to analogue electronics by unusual circumstances. After graduating in Physics, he secured a Fulbright Scholarship to pursue graduate work in Computer Science with the venerable ILLIAC III Computer Project at the University of Illinois. After his doctorate, he left academia to work in industry. Then, in 1980, he returned to academia (summers off!), where he contributed to the formation of many hundreds of analogue engineers, now happily (and gainfully!) employed in Silicon Valley. Along with book writing, this has been his most satisfying career highlight. He enjoys writing, especially when it comes to using intuition to demystify unduly contrived concepts.


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