High Performance Audio Stages Using Transimpedance Amplifiers

Transimpedance or current feedback amplifiers are still relatively new to audio applications, since they are pri­ marily designed for video circuits. While the AD846 (an early high-per­ formance example) was successfully used in TAA,1 transimpedance amps may not be completely familiar de­ vices (The AD846 data sheet contains a detailed discussion of transimped­ ance amplifier theory). Interestingly, they possess several performance at­ tributes quite useful in audio circuits. Among their virtues are very high slew rate and bandwidth, high output cur­ rent, and the ability to drive low im­ pedance loads with excellent linear­ ity. Because of these factors, more de­ signers and audiophiles will use them. Viewed from the exterior of an ap­ plication, a circuit using a transim­ pedance amp can be deceiving. It can look just like one using a more con­ ventional op amp. Both transimped­ ance amps and conventional (voltage input) op amps use a common sym­ bol, and are often called op amps. Nevertheless, their behavior can often be quite different. The simplified gain expression for a non-inverting voltage-gain stage based on a transimpedance amp is similar to one using a conventional voltage input op amp. This gain "G" is expressed as: