[sdiy] ad844 current feedback amplifier??

[Richie Burnett]

Current Feedback (CFB) op-amps are a special type of operational amplifier 
designed for wideband applications.  They are typically used in things like 
RF amplifiers and line-drivers, video distribution amplifiers, and broadband 
transducer drivers for things like ultrasound or laser modulation etc.  They 
also often form the final output stage of function generators that go up 
into the tens of MHz range.

The main merit of the current feedback topology over the normal voltage 
feedback scheme is that you arrive at a design where the gain can be 
controlled independently of the bandwidth.  The feedback resistor typically 
sets the -3dB bandwidth point for the amplifier stage, and the resistor from 
the inverting input to ground sets the gain.  This can be contrasted with 
the normal gain/bandwidth tradeoff you get with a standard voltage feedback 
op-amp.  (It's hard to get high-gain and a very wide bandwidth with 
conventional VFB op-amps.)

The CFB op-amp will operate correctly in some circuits designed for standard 
VFB op-amps, but others require the topology to be changed...  For instance 
the standard VFB op-amp integrator circuit with a capacitor between the 
output and the inverting input doesn't work with a CFB op-amp, and the 
De-boo integrator is the preferred CFB integrator realisation.  Placing a 
capacitor across the feedback resistor to limit the bandwidth is also not 
recommended, and can actually result in instability.

In short the current-feedback op-amp has little to offer the audio market, 
but if you want an op-amp to drive a few hundred milliwatts into a 50 ohm 
load at 100 MHz it is likely to be one of a small number of possible 
solutions.  (I recently used the THS3001 CFB op-amp from TI to amplify a 
20MHz sinewave from ±300mV up to ±12v and drive a capacitive load with low 
distortion for a medical diagnostic application.)

I hope this helps clarify their purpose,

-Richie,