At 09:32 AM 12/2/97 -0500, you wrote:
>Paul B. wrote:
>
>>> halved because the input impedances of the amps are now in parallel.
>>> This is a faulty analysis, though, and would only be true if the input
>>> impedance of the amplifier was comparable to the source impedance of
>>> the transducer.  In fact, the amplifier impedance can be _much_ higher
>>> than the transducer impedance.
>>
>>  REALLY? Well, in that case, wouldn't it be far more sensible to match
>>the impedance better by using a step-UP transformer before resorting to
>>parallel amplifiers?
>>
>>  Hint:  That's how they do it in microphone pre-amplifiers.  They do
>>however not infrequently trade off poorer noise figure for lower cost
>>and/ or slightly flatter frequency response by omitting the transformer
>>and using a second pre-amp module to provide a balanced input.  This is
>>however, just that, a trade-off.
>>
>
>I defy you to come up with a transformer (or any magnetics, for that
>matter) that has anything approaching within several orders of magnitude a
>2000 ohm to 10**12 ohm impedance difference.
>
>Besides, I'd say the impedance mismatch is the reason why the amplifier is
>useful--it doesn't load the transducer significantly. The object is not to
>maximize power transfer from the transducer to the amp, but to amplify the
>signal.
>
>--BN

While it is true that an op amp is primarily a voltage amplifier and it is
not meant to be impedance matched, one could get getter gain and SN ratio
by using a step up transformer between a signal source and an op amp. Lets
say that the source has a 10K impedance and the op amp has a 10**12 imput
Z. Using a 10K:100K impedance match xfrmr will not change the impedance
match situation dramatically, but it seems to me that it  will give 10
times more gain and improve the signal to noise ratio almost as much (since
the op amp's internal noise is presumably much larger than that generated
in the signal source or transformer).

Sean

+--------------------------------+
| Sean Breheny                   |
| Amateur Radio Callsign: KA3YXM |
| Electrical Engineering Student |
+--------------------------------+
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