CALCULATING TOTAL CURRENT

The total current in a parallel circuit is the sum of the currents in all the branches

I.t = I1 + I2 + In + ....

Therefore to calculate the current in a kit, the meter probes must be close the the positive polatity of the voltage source, then the circuit must be "broken" and the meter is placed in series with the circuit, with the voltage source connected.

CURRENT CONSUMPTION

Current consumption is the current used to operate a device.  Take for example a LED which uses 20mA.  If you have a 5volt battery, and the voltage drop over a LED is 2volts and you wanted to connect 2 LEDS to the circuit, the total current consumed would be 20mA, and the total voltage is 4 volts.  

If you wanted to connect 4 LEDs, then a parallel circuit would be needed, each have a 20mA path.  Then the total current drawn would be 40mA, with the voltage the same across the two branches, in this case 5 Volts in each branch, hence each LED can have the 2 volts to operate.

BATTERY

The voltage of an AA battery is the same as an AAA battery, C cell or D cell. AA batteries, however, provide power for a longer period than AAA batteries, because their larger size allows them to store a greater mass of anode material which is consumed as it does electrical work

LEDs

The brightness of LEDs is measured in millicandela (mcd), or thousandths of a candela. Indicator LEDs are typically in the 50 mcd range; "ultra-bright" LEDs can reach 15,000 mcd, or higher (the 617 nm Luxeon Star (part number LXHL-NH94) can reach 825,000 mcd).

By way of comparison, a typical 100 watt incandescent bulb puts out around 1700 lumen - if that light is radiated equally in all directions, it will have a brightness of around 135,000 mcd. Focused into a 20° beam, it will have a brightness of around 18,000,000 mcd.

TRANSFORMERS

Transformers are usually AC activated devices.  The change in signal across the Inductors causes a Counter EMF which is voltage is the opposite polarity.

Transformer transfer voltage from one circuit to another.

The greater the amount of secondary windings in the coil, then the greater the amount of resistance and hence lower the current.

A 20:1 is a step down transformer
A 1:10 is a step up transfomer (volts)

So the 20:1 transformer would produce more current and less voltage compared to its primary windings (or circuit), as there is less amount of windings, hence less resistance and more current can flow. 
Note however, that the power is the same before the transfer of voltage (via th transformer)
This is the inverse for the 1:20 transformer.

There are different types of Transformers:  

Audio Transformers are used because of their effeciency.
Impedance Matching Transformers are used for maximum power output to the load (e.g speaker)
Power Transformers are used to supply appropriate voltages to components e.g. the PSU in a PC.

If the winding of a transformer has a large diameter wire, then its windings carry high current and low voltage.  The opposite is true - where a small diameter wire carries high voltage but low current.

AF/RF NOTES

The resistive coupling provides a minimal result. The waveform on the collector cannot be greater than the supply and some of the energy will be lost in the load-resistor. The rest of the energy will be delivered into the antenna. The antenna is seen as a very low impedance - resistance (about 50 ohms) and it is very difficult for the transistor to deliver its energy into a low impedance such as this. The actual transfer will be small and this reflects the low output of the design. 
(http://www.emitatoaresystem.go.ro/Bug1/FM-BugsIntro-P2.htm)

This is because there is an impedance mismatch and when there is a impedance mismatch, the power transfer is at the minimum.

A low impedance in the circuit to be fed the signal will result in low power being transferred from the transistor.

TRANSISTORS

COMMON-EMITTER (Input and Output signal is common to the Emitter)

Current through the transistor is controlled by the base-to-emitter bias. If both the base and emitter become more positive by the same amount at the same time (signal through emitter and base), current will not increase. It is the difference between the base and emitter voltages that controls the current flow through the transistor.

To eliminate this negative feedback caused by the emitter resistor, some way must be found to
remove the signal from the emitter. If the signal could be coupled to ground (decoupled) the emitter of the transistor would be unaffected. That is exactly what is done. A DECOUPLING CAPACITOR (C3 in view B) is placed between the emitter of Q1 and ground (across the emitter resistor). This capacitor should have a high capacitance so that it will pass the signal to ground easily. The decoupling capacitor (C3) should have the same qualities as the coupling capacitors (C1 and C2) of the circuit. Decoupling capacitors are also called bypass capacitors.

COMMON EMITTER AMPLIFIER

In a Common Emitter Amplifier, the output signal is 180Degrees out of phase to the input signal - why ?
As the Base Emitter of the signal goes positive, the forward bias current increases which causes an amplified signal to appear at the collector.  Now the Load which is based on the collector (e.g a resistor) - its voltage increases, causing a DECREASE at the collector point - this is the negative alteration of the signal at the collector point, since the voltage drop across the Load has increased.

When the Base Emitter signal goes into the negative (assuming an AC sinusoidal signal), the forward bias is reduced causing a reduction in the current across the collector, which in turn is felt across the Load resistor, which causes a reduction in voltage across the Load, which in turn causes an INCREASE in voltage across point C (collector) of the Transistor.

Power (WATTS)

The Power output of a transistor is calculated from the Source Power (Vcc) over the Load of the transistor, (RL) usually in the Collector.  The law is

P = V * I

Which means, the greater the Source power, then the greater the power output.
The power dissipated in the load comes from the powersupply (Vcc)

Resistors and Capacitors in Parallel on BASE of Transistor

Adding a capacitor in parallel with the base resistor will permit transistorQ2 to switch at a higher speed. The capacitor passes a current spike that clears the current carriers out of the junction area and permits the transistor to cut off quickly.When the transistor is switched on via a base voltage or signal, the capacitor passes the needed voltage directly to the base and drives the transistor quickly into saturation.

SATURATION

When a transistor is in the SATURATED state, the COMMON-EMITTER Junction is acting like a short (or a piece of wire), therefore, the VOLTAGE DROP is very very small at the COLLECTOR, and hence the voltage drop across the transistor is close to ZERO.

RESISTORS

Resistors are used in circuits to impede to flow of electrons.  However, they do more than this basic function.  They also act as a VOLTAGE creation device as well as a CURRENT creation device.  Given a voltage through a circuit does not create a current unless there is some sort of resistance.  Once a current flows through a resistor, a VOLTAGE DROP is created over this device.

In Amplifier circuits, such as FM bugs, the lower the current flow due to a higher load resistance value (output stage), the greater the VOLTAGE drop across this resistor and hence the more POWER that is consumed.

Resistors also limit the amount of the signal that can go through the circuit, such as when used with a feedback circuit, so that the majority of the output signal goes to the next stage.

Therefore, at the output stage, greater the LOAD resistance (e.g.resistor), the greater the VOLTAGE consumed and hence LOWER the POWER transferred.

When the impedance is matched between the output circuit and the input, we have maximum 
POWER transfer take place.

If maximum current is required from the output of circuit1 into circuit2, then circuit2 should have a lower impedance (resistance) as its input into circuit2 as we want maximum current to flow through.

TANK CIRCUIT

A TANK CIRCUIT consists of a capacitor and an inductor in PARALLEL
The inductors voltage increases as the capacitor slowly discharges.  Once the capacitor is totally drained, the electromagnetic field surrounding the Inductor begins to collapse.  This voltage drop will feed into the capacitor and the capacitor begins to store this charge in its electrostatic field.

The capacitor will now sense that the inductor has no energy and will begin to discharge its current through the inductor.  This cycle continues.

In a PARALLEL Resonant circuit, and given a small amount of resistance in any realistic circuit, as the capacitor discharges, the current in the Inductor increases. Therefore the net result in such a circuit is that zero current is drawn from the source (be it a battery or a transistor). As one component draws current, the other returns it to the source. Now in a parallel circuit, voltage is the same across all branches.

So if the reactance is the same across a capacitor and the same across the inductor, then the current is the same across both components. So the net affect is 0 amps

Now R = V/I so we have a VERY HIGH IMPEDANCE in a parallel circuit. This is in contrast to a series LC circuit, where the impedance is low and hence high current.

RESONANCE

Resonance is a characteristic of AC Circuits consisting of an Inductor and a Capacitor.

When the values of inductance, capacitance and the applied frequency are such that the 
inductive reactance and the capacitive reactance cancel each other out, we have a state called RESONANCE and the circuit is said to be TUNED or resonant.  

Reactance is the opposition to the change of current flow in an AC circuit.

The value of an inductor (in Henries) and a capacitor (in Farads) produce a resonance at only ONE frequency.

IN A SERIES L and C circuit

At resonance, the difference between the inductive reactance and the capacitive reactance is 0.
This means that the resistance or impedance (if a resistor is included in the circuit) is 0. This is because the capacitive inductance is inverse to the inductive inductance at the resonant frequency.
Therefore, at resonance, the reactances of the two (L and C) cancel each other out and hence there is no (or very little) impedance (or resistance), hence the current is VERY HIGH

Now since impedence/reactance is very low in an resonant circuit, we can use OHMS law to calcualte Current

I = V / R
I = 10 / 1 = 10amps for example -

Main thing to understand is that in RESONANT series circuits (tuned circuits), IMPEDANCE is very low.

Now, because impedance is very low, current is high and hence the voltage drop across the devices is high since voltage and current are directly proportional.

IN A CIRCUIT THAT DOES HAVE SOME RESISTANCE

Now, in a RESONANT series circuit, we know that the REACTANCE is zero.  Add a resistor then we have IMPEDANCE (impedance is reactance of the Inductor and Capacitor and the Resistor).  Now since Reactance is 0, then impedance must equal the resistance or Z = R

Increasing the capactance, and given a fixed frequency, will decrease the resonant frequency.  Why?  Because the smaller the area of the charge plates on the capacitor, the greater the opposition to the flow of the alternating source signal - likewise, the greater the area of the charge plates on the capacitor (larger the capactance value), the smaller the reactance and hence, the lower the resonant frequency. Because the charge plates on the larger capacitor is larger, the more of a charge can be taken causing the opposition to be less and hence the lower the resonant frequency.

Remember the formulae for Capactive Reactance =
Xc = 1 / 2piFC
where C is in Farads

IN A PARALLEL L C Circuit

At Resonance, in a Parallel LC Circuit (also know as a TANK Circuit) - current is flowing into one device whilst it is flowing out of another.  There is an inverse relationship between inductive reactance and capacitive reactance.
Now, in a parallel circuit, the voltage is the same across all branches, but the current is different. Now, since the impedance is the same in a tuned parallel circuit, using OHMS law we can deduce that the current is the same across the capacitor and the inductor, but they are inverse in relation.  Hence, the net affect is that there is 0 Amps in the parallel resonant circuit.  This implies, the IMPEDANCE IS VERY HIGH IN A PARALLEL RESONANT CIRCUIT.

For a tank circuit, the source needs to be fedback to keep the tank oscillating - this is acheived by feedback through an amplifier device.

REACTANCE

REACTANCE

The effect of inductive reactance is to cause the current to lag the voltage, while that of capacitive reactance is to cause the current to lead the voltage. Therefore, since inductive reactance and capacitive reactance are exactly opposite in their effects, what will be the result when the two are combined? It is not hard to see that the net effect is a tendency to cancel each other, with the combined effect then equal to the difference between their values. This resultant is called REACTANCE; it is represented by the symbol X; and expressed by the equation X = XL - XC or X = XC - X L. Thus, if a circuit contains 50 ohms of inductive reactance and 25 ohms of capacitive reactance in series, the net reactance, or X, is 50 ohms - 25 ohms, or 25 ohms of inductive reactance.

CAPACITORS

A Capacitor is a device that stores an electrical charge in an ELECTROSTATIC Field.
When two CHARGED particles, one positive the other negative, are placed in close proximity, a ELECTROSTATIC Field is developed. Because an electrostatic field is polarised positive to negative, the field is drawn from positive to negative.
If two charges are the same for example, then the charges will repel each other.

When a capacitor is placed in a circuit which is connected to a DC source, the negative plate gains an electron whilst the positive plate loses an ELECTRON. This creates a DIFFERENCE of POTENTIAL.


As the negative plate gains more electrons, it REPELS the negative voltage coming from the battery since like charges REPEL. Similarly, as the positive plate loses an ELECTRON and becomes more postive, it repels the positive voltage from the battery. A characteristic of a circuit or device to oppose the change of current or voltage in a circuit is called REACTANCE. This is a form of CAPACITIVE REACTANCE.

The EMF will tend to force the current from postive to negative, against the battery source, which is from negative to positive.

REMEMBER, as the voltage builds up across a capacitor due to the electrostatic field, the voltage drop will reduce across another device on the circuit, like a resistor or transistor.  In a parallel circuit, Voltage is the same everywhere, in a series circuit, CURRENT is the same everywhere.

If you think about it for a moment, you should be able to understand why they allow AC to pass through: AC keeps reversing its polarity. As long as the AC switches fast enough to prevent the cap from becoming fully charged in any direction, then the cap will partially charge in one direction, and as the AC polarity reverses, the cap will start to discharge, then charge in the opposite direction. Very low-capacitance caps may partially block the AC because they become fully loaded before the AC cycle is complete, however.

IN AC Circuits:
When the frequency increases, the capacitor will act like a short, allowing the signal through, therefore, higher the frequency, the more of the signal is passthough, (given the capacitor size is the same)
Larger the cap, more of the signal will pass through, since the formula for Capacitive reactance is
Xf = 1 / 2pi*f*C
ONE THING THAT I FOUND

When a capacitor is connected in SERIES in a circuit that is used with AC, such as a FM bug, the capacitor is used for coupling.

When a capacitor is connected in PARALLEL, meaning the positive end is connected to the positive rail and the negative end (or the other end of the capacitor) is connected to the negative rail, then it is used to hold a charge in its electrostatic field.

The size of the capacitance of a Capacitor matters in AC Circuits.  RF can use lower capacitors becuase the charging and discharging of the capacitor plates happen so quickly, that the capacitor does not block any signals - if a small farad capacitor was used in AF, then the plates would charge to the potential and block most of the Audio signal, hence we need a larger farad capacitor.