Sean Breheny wrote: > It seems that there should be some impedance matching device which > would work all the way down to DC (even if it has to be active and > have a net power loss). For example, a common-base transistor amp > naturally has a low input impedance and a high output impedance. > However, if I just implement it in the normal way, it has overall > power gain AND adds noise. Is there some way to reduce the > input-referred noise (at the expense of power gain)? In other words, > can one make an active version of a transformer which approaches the > low noise quality of the transformer but works down to DC (or almost > DC)? Current amps, like the common-base amp, are the best you are going to be able to do unless you use a transformer. And they are not noiseless, although they will be quieter than other topologies. They have their quirks, since they are, after all, current amps. And since you are way down at 50 ohms all the other resistances are so significant it will make it difficult to model. Beautiful transformers are readily available with 5Hz to 20KHz response. They just tend to be rather large and expensive. I'm most familiar with 600 ohm input impedance for microphone use but I bet 50 ohms is also an off-the-shelf impedance. Cheerful regards, Bob -- http://www.piclist.com PIC/SX FAQ & list archive View/change your membership options at http://mailman.mit.edu/mailman/listinfo/piclist