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Spy Circuits

slightly morę and the base is fed with current, the transistor will conduct and current will flow in the collector-e This is the case with the transistor in the audio stage. The emitter is held rigid and the base is fed with currenl above 0.65v.

The other way to tum on an NPN transistor is to hołd the base firm and lower the emitter. Once the emitter is by 0.65v, the transistor turns on and if the emitter is lowered slightly morę, the transistor turns on morę.

This may be difficult to visualise, but this is occurring in the oscillator stage.

Let's advance a few cycles and see what is happening.

The transistor turns on and the collector pushes the top of the 10p capacitor towards the emitter.

The energy in the capacitor gets converted to a smali voltage and larger current. We said before, that a capac The smali voltage pushes the emitter lower and this turns ON the transistor. The result is a little bit of energy capacitor in the tuned Circuit.

At this instant the coil does not get energy from the transistor because a coil resists any quick flow of current like a very large resistor.

The energy in the 10p is now spent and the transistor turns off slightly. The energy from the capacitor in the ti to the coil and when the capacitor can no-longer keep the flux in the coil increasing, there comes a point whet collapse. This flux produces a voltage that is larger than before and is opposite to the previous voltage.

This is one of the amazing things of a coil and capacitor in parallel. The voltage at the lower end of the coil-ca is higher than the supply and this raises the top piąte of the 10p capacitor. This rises the voitage on the emitte transistor off com pletely.

This allows the tank Circuit to produce its amazing HIGH VOLTAGE without the transistor loading the Circuit. But he coil/capacitor is a very delicate arrangement and it is producing the high voltage at very Iow current as original Iow voltage smali current into high voltage very Iow current.

If we put a load on the Circuit at point "A" we will reduce the voltage and may freeze the Circuit.

The following 9 circuits show different ways to "tap-off" the waveform and amplify it in a stage called a BUFFE The BUFFER converts a HIGH IMPEDANCE signal into a LOW IMPEDANCE signal as the antenna is comm 50 OHM LOAD

The simplest buffer is shown in the following Circuit. It is a com mon emitter stage with a resistive load.

1. STABILITY

\/ Antenna I 165cm

BC547 is not suitable as a buffer

A BC 547 transistor is not very good at amplifying at 100MHz. The Circuit above does not give a greater rangę transistor version, but the stability is much greater. The antenna can be touched without the bug drifting off fre

2. INCREASED RANGĘ

To increase the rangę, the output must be increased. This can be done by using an RF transistor and adding effectively converts morę of the current taken by the Circuit into RF output. The output is classified as an untui

2011-12-08

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