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Common Base Differential Amplifier
On both sides of the amplifier circuit, we see two NPN transistors with the bases biased
at just below 2V (assuming a supply of 12V. At 6V supply, the bias is just above 1V)
Each transistor has approximately 11 mA current passing at quiescent state. This, in
addition to the low impedance inputs, provides some headroom for dealing with large
signals if they are present. Bear in mind that ideally, we want to be able to amplify
signals of 1uV or less on our wire loop. However, every signal from ELF to upper HF
may arrive at the loop, and so it is that we need to allow for this probability together with
the possibility of large variance in signal strength. In other words we need the
amplifier to have a relatively large dynamic range.
The Output
The output is taken from the transformer which is used to convert the balanced
output to an unbalanced output suitable for feeding along 50Ω coaxial cable. Here
we must note that there is a DC path to ground through the output winding of the
transformer. No capacitor coupling is used as it is assumed that the input of our
receiver will have a capacitor.
We may choose to add a capacitor if DC coupling is not desired. If this is done,
we should use a value of 100nF, which is placed between the output post of the
RCA socket and the pad.
The other side of the transformer is coupled to both the collector load resistors of the
differential amplifier via 100nF capacitors and thus removes the DC component which is
present and stops the two sides of the differential pair being shorted together!
ASSEMBLY INSTRUCTIONS
RFLEGO Active Antenna Receiving Loop
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G.D.COWNE - MØOOZ
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