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Spy Circuits hUp:/Avwv.talkingdectromcs.convprojects/Spy%2C£ircuits/SpyCircui.

This is the case with the transistor in the audio stage. The emitter is held rigid and the base is fed with current, once the base is above 0.65v.

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

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

Lefs advance a few cycles and see what is happening.

The transistor turns on and the collector pushed 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 capacitor can do this.

The smali voltage pushes the emitted lower and this turns on the transistor. Morę current flows though the 470R and this has the effect of turning off the transistor. The two actions fight against each other and the capacitor wins. The end result is the transistor is tumed on and a littie bit of energy is pumped into the 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 into it and it acts like a very large resistor.

The energy in the 10p is now spent and the transistor turns off slightly. The energy from the capacitor passes to the coil and when the capacitor can no-longer keep the flux in the coil increasing, there comes a point where the flux starts to 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-capacitor combination is higher than the supply and this raises the top piąte of the 10p capacitor. This rises the voltage on the emitter and turns the transistor off completely.

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 it is converting the 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-off¹ the waveform and amplify it in a stage called a BUFFER.

The BUFFER converts a HIGH IMPEDANCE signal into a LOW IMPEDANCE signal as the antenna is commonly referred to as a 50 OHM LOAD.

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

1. STABILITY

2 z9

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