Hi all,

I have a very low-level (microvolt) audio source with a 50 ohm
impedance (it is a diode mixer in a direct conversion receiver). I
also have an audio amplifier which has a gain of about 10000 in
voltage, 5 uV input noise, flat audio response down to about 10Hz, and
a differential audio input with a 10K impedance.

I want to feed the signal from the mixer into the audio amp. I realize
that I should provide a flat 50 ohm termination for the mixer and that
I should also filter out the RF products coming out of the mixer.

However, it would be nice to transform that 50 ohm impedance to
something closer to 10K because it would be free (noise free) gain.

I could use a transformer, but I'd like to avoid it for several
reasons all relating to the fact that it doesn't have good low
frequency response. This not only spoils the nice low-end ability of
the output amp, but also could be a problem because down the line a
bit I am planning on allowing this circuit to do synchronous detection
(the LO is produced by a DDS which is digitally tunable on the fly,
and is controlled by a PIC. If I feed some of the audio into an ADC
channel on the PIC, I can shift the frequency and even phase of the
DDS to match the carrier of an AM station, providing much better
demodulation quality. However, this needs (almost) DC response from
the audio up to the ADC).

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)?

Thanks,

Sean
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