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Spy Circuits

lt's a bit like hundreds of people trying to push a person on a swing - most of them will get in the way of each a signal at 150MHz will try to push the swing when it is coming towards the pusher and the energy will be app time.

Ali the other signals will be pushing at the wrong time too and the only signal that pushes at exactly the correc one marked on the scalę. Its energy will not be lost in the tuned Circuit but appear on the output. This signal is stage via a 47p capacitor for amplification.

The RF stage is able to amplify signals in the 100MHz rangę as we have used a high frequency transistor ant appears at the collector.

Two further stages of amplification are needed to increase the signal so that it is large enough to be fed into £ biased in a standard self-bias configuration while G3 is biased in an unusual way. It is biased ON so that sma input do not appear at the collector. This means the noise generated by the first two stages is prevented from diodę pump. Only signals above a certain threshold on the base of Q3 appear on the collector. This signal is i diodę and fed into a 100n reservoir capacitor.

The other diodę (between the 1 n capacitor and negative raił) removes the negative portions of the waveform . the 1n capacitor so that it can supply positive pulses for the charging process.

The first transistor in the staircase (Q4) starts to turn on when ,6v is present on the reservoir capacitor. As the to ,65v the LED connected to the collector of 04 gets brighter and brighter. Due to the slight voitage drop acrc bias resistor, the voltage on the reservoir capacitor needs to be slightly higher than ,65v and once the first tra staircase is turned on fully, the next transistor (Q5) will begin to turn on as the voltage on the reservoir capaci slightly above 1.3v (,65v + ,65v).

This process continues with the middle LED getting brighter and brighter until it is fully turned on.

As the voltage on the reservoir capacitor increases, the top LED will come ON and illuminate fully.

The 3 LEDs will give plenty of rangę as you can read values such as a LED fully turned on or partialiy turned It is important to know that the lower transistor (G4), turns on FIRST and as the voltage on the reservoir capa then Q6 turns on. Without this, you will not be able to understand how the Circuit works.

2011-12-08

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