Electronics

Physics

Some formulas that are fundamental:

• Ohm's Law: V=IR

From Ohm's law, power generation is P=VI (watts)

Fundamentals

V_cc is the power supply voltage. It is the voltage supplied at the collector.

Diodes

   Anode (+)         Cathode (-)
   ----------[   |]----------

   Electronic symbol:
             | \  | 
   ----------|   >|---------
             | /  |

• Forward bias = on, lets current pass through
• Reverse bias = off, restricts current completely

The I-V curve shows relationship between voltage and current. The ideal diode will allow all current through at any forward voltage and no current at any reverse voltage. A real diode will have some limitations:

• Forward voltage (Vf) - voltage at which the diode turns on and allows forward current to flow. This is the voltage that is required for the diode to be in forward bias
• Breakdown voltage (Vbr) - the negative voltage which the diode fails to stop current from flowing backwards. You can expect the diode to withstand this voltage in the reverse bias before reverse current flows freely. A diode in breakdown is not necessarily broken. What breaks it is the amount of current that goes through it in this state because the diode does not limit the current.
• Forward voltage drop - the voltage that is 'lost' by going across the diode

Diodes will have a maximum forward current with the limitation typically because of heat dissipation. This means there will be two maximums: One that is constant (maximum forward continuous current, IF) and one that is peak.

Determining the forward voltage with a multimeter can be done by using the test option across the diode. Test by connecting the black probe to cathode (the side of the diode with the black stripe) and the red probe to anode.

Example Components:

• 1N4148 Signal Diode
• 1N4001 Rectifier Diode (for rectifying power)

Schottky diode

Similar to a normal diode but:

• Has a lower forward voltage drop
• Faster switch action

In general, a shottky diode has better characteristics than a typical diode but is more expensive.

Zener Diode

• Has a specific zener breakdown voltage which allows current to flow in reverse bias
• Zener breakdown voltage is typically labeled with the device. Eg. 5V1 = 5.1 volts

Can be used to create a constant voltage reference by exploiting the breakdown voltage. Any excess voltage will be 'spilled' over via the zener diode.

The resistor is required in order to limit current that can pass through the diode. Because of this design, attempting to use a zener diode as a voltage regulator for a load is inefficient since a lot of power is wasted with the resistor. For high load or a large voltage difference, the resistor will generate a lot of heat (P=IV).

Transient Voltage Suppression (TVS) Diode

A TVS diode is similar to two zener diodes that are connected together in opposite direction. Connecting this to a circuit will prevent voltage from spiking past a certain voltage. It does not conduct in either direction until the breakdown voltage is reached.

Transistors

There are two types: Bipolar Junction Transistor (BJT) and Field-Effect Transistor (FET)

There are two types of BJT transistors: NPN and PNP.

In a NPN transistor, current flows in to the base terminal and the larger regulated current flows from the collector to the emitter. The emitter emits negative charge to the collector.

In a PNP transistor, current flows out of the base terminal, and the larger regulated current flows from the emitter to the collector. The emitter emits positive charge (or holes) to the collector.

In a PNP transistor, because current flows from the emitter to the collector, the voltage at the emitter terminal must be higher than the collector terminal. This is the opposite of a NPN transistor where the voltage at the collector is higher than the emitter.

PNP transistors are typically drawn upside down because nodes near the top of the page are at a higher voltage.

A NPN transistor has the arrow pointing out. Remember by mnemonic Not Pointing iN.

NPN transistors will have a complementary counterpart with similar characteristics.

NPN	PNP	V_CE	I_C	P_D
BC547	BC557	45V	100mA	500mW
BC337	BC327	45V	800mA	625mW
TIP 29	TIP 30	40V	1A	2W
TIP3055	TIP2955	60V	15A	90W
2n3904	2n3906	40V	200mA	625mW

Transistors come in a few different packages:

• TO-90 - signal transistors; dissipates a few hundred milliwatts of heat
• TO-220 - power transistors (looks like fets); dissipates few watts or more with a heatsink

Modes:

Cutoff

Fully off; blocks all current.

Cutoff happens when:

• NPN transistor: Voltage between base and emitter is less than some voltage threshold (V_BE < V_TH), where V_TH is some positive value such as 0.7V.
• PNP transistor: Voltage between base and emitter is greater than some voltage threshold (V_BE > V_TH), where V_TH is some negative value such as -0.7V.

NPN: V_E > V_B < V_C PNP: V_E < V_B > V_C

There is a specific forward voltage required before semiconductors conduct (like a diode); until that happens, the current flowing through the transistor is 0.

When the forward voltage is met, the base and the emitter acts as a short circuit and no current is limited. This means it is important to limit the amount of current that can flow from the base to the emitter (with a resistor).

Active Mode

Partially on. Also known as the linear region because there is a relationship between the current flowing through the transistor and the current flowing through the base.

I_C = h_FE * I_B, where h_FE is the current gain, also sometimes written as a uppercase beta.

h_FE may change from operating conditions.

NPN: V_E < V_B < V_C PNP: V_E > V_B > V_C

Example: A typical transistor has a h_FE of 100. If a transistor has 1mA flowing through the base (I_B), then 100mA can flow through the collector (I_C). The current flowing through the emitter will be equal to the sum of the base and collector (I_E = I_C + I_B)

Saturation Mode

Fully on; Acts like a short circuit between the collector and emitter.

Changing the current flowing through the base will not affect the current through the collector.

NPN: V_E < V_B > V_C PNP: V_E > V_B < V_C

Ideally, the voltage drop between collector and emitter (V_CE) is 0. In practice, it is typically something small at around 0.2V. This voltage drop is also known as the saturated emitter-collector voltage or V_CE(Sat).

On a PNP transistor, the voltage drop is negative since the collector has a higher voltage than the emitter and base.

It is important to limit the amount of current that can flow from the collector to emitter because the transistor acts as a short circuit.

Important characteristics

• V_CEO is the maximum voltage between the collector and emitter.
• V_CBO is the maximum voltage between the collector and base.
• V_EBO is the maximum voltage between the emitter to the base base
• I_C is the continuous collector current. This is the amount of power that can be dissipated by the transistor while operating in saturation mode (ie. smallest voltage drop V_CE(Sat) * I_C). If operating in any other mode, V_CE could be higher and the power generated could be higher!
• I_CP is the pulse collector current
• h_FE is the current gain and may be different at different voltages and collector currents.

Power dissipation is the total energy that can be dissipated by the component. For every degrees above 25 degrees Celsius, subtract 5 milliwatts from the maximum rating. Silicon will degrade past 150 degrees celsius.