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Texas Instruments IncorporatedAmplifiers: Op Amps

Expanding the usability of current-feedback amplifiers

By Randy Stephens (Email: r-stephens@)

Systems Specialist, Member Group Technical Staff

Introduction

Although current-feedback (CFB) amplifiers have beenaround as long as the widely utilized voltage-feedback (VFB)amplifiers, their acceptance has been sporadic. One of thereasons for this is quite simple—they have a different

name and therefore must be difficult and very hard to use.This is simply not true. There are numerous papers1, 2, 3comparing the differences between the two amplifiertypes that show they are more similar to each other thandifferent. In fact, for numerous circuits, a CFB amplifiermay actually yield better results due to its inherent slew-rate advantage, lack of a gain-bandwidth product, and reasonably low noise for the performance.

Almost every paper written about CFB amplifiers cautionsreaders that placing a capacitor directly in the feedback path,without any resistance in series, will cause the CFB ampli-fier to oscillate. This is true, as the compensation of theamplifier is tied directly to the feedback impedance. Since acapacitor has low impedance at high frequencies, this essen-tiallyplaces a short in the feedback path that inadvertentlydefeats amplifier compensation, resulting in instability.Because of this limitation, there are a handful of commoncircuits that are not recommended for use with a CFBamplifier. These include integrators, some types of filters,and special feedback-compensation techniques. But whatif there was a way to make these circuits work? And whatif the solution was as simple as adding a single component?This would make it feasible to implement a CFB amplifierfor just about every application for which a VFB amplifiercould be used, with the benefits of the CFB amplifier.

Compensation

This article does not explain the compensation theory ofVFB and CFB amplifiers, as there are many papers writtenon this topic. The only thing that is important is thatthere must be resistance, or impedance, in the feedbackpath at the open-loop intersection point to make the CFBamplifier stable.

Figure 1 shows a traditional VFB amplifier, a THS4012,configured in a noninverting gain of +5 with a simple low-pass gain filter set at approximately 1 MHz by the straight-forward 1/(2πRFCF) formula.

If a CFB amplifier like the THS3112 is simply droppedinto this circuit, it willoscillate and the circuit willbecome useless. A method of compensating the CFB

amplifier in this circuit is to insert a resistance, or imped-ance (Z), in the feedback path as shown in Figure 2.It can easily be seen that regardless of the impedance of the feedback path represented by RFand CF, theimpedance Z is in the amplifier’s feedback loop dictatingthe compensation of the amplifier. The interesting thingabout this configuration is that the feedback resistance(RF), which normally dictates the compensation of the

amplifier, can now be essentially any resistance desired.The reader should keep in mind that this is still a high-speed amplifier with speeds over 100 MHz; so the feedbackresistance should always be kept less than a few kilohmsto minimize the effects of parasitic capacitances on theoverall circuit. Conversely, minimizing the resistance toomuch will place too much of a load on the amplifier, typically degrading performance.

One of the drawbacks of adding the impedance Z in thismanner is that the summing node at the inverting terminalis now separated from the virtual summing node. This can