

A dirt cheap “spectrum analyzer” with an RTL-SDR dongle - sizzle
http://hansvi.be/wordpress/?p=91

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tzs
Can someone briefly explain for those of us who are electronically challenged
what the purpose of diodes in series is (D1 and D2)?

Googling on "diodes in series", I found some discussion about how you can do
this if you need a reverse breakdown voltage that is higher than that of one
diode, but aren't D1 and D2 always forward biased in this circuit?

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cnvogel
D1 and D2 are normal diodes in forward direction[1], in series they drop
2⋅0,7V = 1.4V. R2 just pulls them down. So the cathode of D2 will sit 1.4V
below supply voltage.

Q1 in this schematic is acting as a current source: If conducting a reasonable
amount of current, a transistor needs a base-emitter voltage of Ube = 0.6V.
This is a PNP transistor[2], so base needs to be 0.6V below emitter, hence the
voltage over R1 will be U(d1d2) - Ube = U(R1) = 0.8V. The current then is of
course set (to first order, ignoring the base-current contributing) to be
U(R1)/R1.

Why such a "complicated" circuit and not only use R1 to bias D3? Because a
current source has a much higher (theoretically: infinite) impedance: Z=dU/dI,
but dI=0 (stays constant) whatever the change in voltage (dU > 0) might be.

\-------------------

1) forward direction: A diode that is operated, like a LED supposed to light,
will have a certain voltage drop. For a standard silicon diode, this is 0.7V,
a red LED would have 1.6V or so.

2) PNP transistor: A transistor has three terminals, base is the cross-bar,
emitter is the diagonal line with the arrow, collector is the other one. a NPN
transistor runs on a base current (technical direction + to -) into the base
terminal and conducts current from emitter to collector, a PNP transistor runs
the reverse.

Edit: emitter/collector mixup

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millebe
I think, to put it more simply, those two diodes exist to cause a 1.4 volt
drop to the transistor.

The transistor is a PNP transistor, so the base (middle terminal) must be
about .6V lower than the emitter (top terminal) for the transistor to turn on.

I'm guessing one diode wouldn't turn on the transistor reliably, so the author
used two in series. This also probably allows for a more convenient value of
R1.

~~~
scoot
So why not just use an appropriate resistor?

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nitrogen
The voltage drop of a resistor is proportional to current.

~~~
iiiears
So in many appliances the LED serves two functions, Light and part of the
active circuit to lower voltage? - This is why HN has for me replaced slashdot
and digg. Thank You.

