Transistors Flashcards
The purpose of a transistor is to ______
The purpose of a transistor is to control signals. Transistors control a large signal with a small signal.
The word TRANSISTOR is derived from the words ______. These two words best describe transistor operation.
The word TRANSISTOR is derived from the words TRANSfer and resISTOR. These two words best describe transistor operation.
Name the definition.
A semiconductor (four electrons in the outermost orbit) with impurities added. The impurities have five electrons in the outermost orbit. This produces free electrons that move throughout this material.
N-TYPE MATERIAL
Name the definition
Asemiconductor (four electrons in the outermost orbit) with impurities added. The impurities have three electrons in the outermost orbit. This produces holes that move throughout the this material.
P-TYPE MATERIAL
The purpose of an transistor amplifier is to _________.
increase signal strength.
The transistor must be biased with _______ before use.
The transistor must be biased with a DC voltage before use.
Placing a positive voltage on the base and a negative voltage on the emitter forward biases the emitter to base (EB) PN junction. This junction is also known as the _____
LOW RESISTANCE JUNCTION
Transistors transfer a low resistance to a high resistance. Recall from the diode lessons that a forward biased PN junction offers very little opposition to current flow, or in other words, has a low resistance.
Transistor bias is accomplished how?
By forward biasing the emitter to base PN junction (EB) and reverse biasing the collector to base PN junction (CB).
Current flows through the forward biased BE junction. This current is called ____.
Ib.
Placing a large positive voltage on the collector reverse biases the collector to base PN junction. This junction is also known as_____.
HIGH RESISTANCE JUNCTION
Transistors transfer a low resistance to a high resistance. Recall from the diode lessons that a reverse biased PN junction offers very high opposition to current flow, or in other words, has a high resistance.
Transistors are solid state devices constructed from _______ materials just like diodes. Unlike diodes, transistors have three elements with two junctions.
Transistors are solid state devices constructed from N-type and P-type materials just like diodes. Unlike diodes, transistors have three elements with two PN junctions.
The _____ is heavily doped so it produces (emits) a large number of current carriers inside the semiconductor material.
EMITTER
The _____ is very thin and lightly doped. Its construction allows most of the current carriers to pass from the emitter to the collector.
BASE
The _____is moderately doped. It collects the current carriers from the base. The collector is the largest of the 3 areas because it has to dissipate most of the heat generated.
COLLECTOR
How does the current flow from the base to the collector in the PN juction?
Current does not flow from the base to the collector; the PN junction is reverse biased.
What is the name of the current that flows from the emitter to the collector?
lc
Why does current flow from the emitter to the collector ?.
Because the base is thin and lightly doped.
The base cannot use all the current carriers from the heavily doped emitter. The current carriers (electrons in NPN transistors) are attracted to the positive voltage on the collector.
Total current flowing through the transistor circuit is found by what equation?
Total current flowing through the transistor circuit (Ie) is found by: Ie = Ib + Ic
What is the difference between PN junctions (EB) for NPN and PNP transistors
NPN - positive voltage on the base and a negative voltage on the emitter
PNP - negative voltage on the base and a positive voltage on the emitter.
What type of current carriers are in P-type material?
Holes or electrons
Holes
P-type material has a missing electron in each impure atom. This creates holes, which move. Holes are the current carriers in P-type material.
Name the parts of this transistor schematic symbol.
What does the direction of the arrow indicate?
The direction of the arrow on the emitter indicates an NPN or a PNP transistor.
Identify the leads of this transistor
The base lead is in the middle.
The emitter is on the left or next to a tab when the semicircle is the top half of the transistor viewed from the bottom.
The collector is on the right when the semicircle is the top half of the transistor viewed from the bottom.
What are two types of signals applied to transistors?
Input - The input is the signal that is changed to produce an output. Input signals are either AC or DC.
Bias - The bias is the signal that allows the transistor to operate. Bias signals are DC voltages.The transistor must be biased with a DC voltage before use.
Transistor bias is accomplished by forward biasing the ____ to ____ and reverse biasing the___ to _____.
Transistor bias is accomplished by forward biasing the emitter to base PN junction (EB) and reverse biasing the collector to base PN junction (CB).
In an operating NPN transistor, which PN junction is forward biased and which PN junction is reverse biased?
Forward EB, Reverse CB
In an operating PNP transistor, which PN junction is forward biased and which PN junction is reverse
Forward EB, Reverse EC
Forward EB, Reverse CB
Forward CB, Reverse EB
Forward EB, Reverse CB
In an NPN transistor, the base DC bias voltage must be _______.
negative compared to the emitter
positive compared to the emitter
positive compared to the collector
positive compared to the emitter
In a PNP transistor, the base DC bias voltage must be _______.
negative compared to the emitter
positive compared to the emitter
negative compared to the collector
negative compared to the emitter
The emitter to base PN junction is similar to a diode. Approximately____ volts is needed to forward bias the junction (0.3 volts for a germanium transistor).
0.7 volts
What causes current to flow?
Once the EB junction is forward biased and the CB junction is reverse biased, current flows.
Current carriers from the emitter are attracted where? Why?
Most of the current carriers from the emitter are attracted to the collector because the collector is large and moderately doped compared to the base which is thin and lightly doped. This is why the collector current is large, the base current is small and most of the current carriers pass on to the collector. .
Because there are a limited number of current carriers in the base, what can be done to only slightly increases base current?
Increase the base bias
Because there are a limited number of current carriers in the base, increasing base bias only slightly increases base current. What else would result by increasing the base voltage?
Since the emitter is heavily doped, an increase in base voltage causes more current carriers to enter the base. This causes collector current to increase significantly.
What happens if you increase the base bias too much?
Increasing the base bias too much causes the collector current to remain constant because all the available current carriers in the emitter are used up.
The point when Ic no longer increases although base bias voltage increases is called saturation. The transistor acts as a short between the collector and emitter.
What happens if you decrease the base bias too much?
Decreasing the base bias below the forward bias level of the emitter base junction stops current flow altogether.
Since the base is more negative than the emitter, Ib stops. Ic stops because the current carriers cannot enter the base.
The point where Ib and Ic stop is called ____. The transistor acts as an open between the collector and emitter.
The point where Ib and Ic stop is called cutoff. The transistor acts as an open between the collector and emitter.
Name this type of basic bias circuit?
Fixed biased transistor
RL is a load resistor and Rb is the base bias resistor. Note that Rb connects directly to Vcc. The value of the resistors are chosen so that there is a higher positive voltage on the collector than the base. Thus, the voltage drops from +Vcc forward bias EB and reverse bias CB.
The disadvantage of fixed biasing is that small changes in base current, due to temperature changes, affect the voltage on the collector. The changes are small, but small voltage changes on the base cause large changes in collector current.
For a PNP transistor, +Vcc is changed to -Vcc.
Name this type of basic bias circuit?
Self biased transistor
The temperature disadvantage from a fixed is overcome by using self bias.
RL is a load resistor and Rb is the base bias resistor. Note that Rb is connected DIRECTLY to the collector. If collector current increases due to temperature, the voltage drop across RL increases, decreasing the voltage across Rb. This reduces base bias, which returns Ic to normal.
The disadvantage of self bias is that control of the base bias over the collector current, β (beta), is reduced.
For a PNP transistor, +Vcc is changed to -Vcc.
Name this type of basic bias circuit?
Combination Bias
The disadvantage of fixed and self bias is overcome by a combination of fixed and self bias.
There are many types of combination bias. The voltage divider is the most widely used.
R1 and R2 form a voltage divider that provides a fixed bias to the base. Note that the base resistor connects directly to +Vcc.
R3 self biases the emitter. The emitter voltage is determined by how much current is flowing through R3, which is controlled by the base voltage.
C1 is used to keep any AC input signal off the emitter. This stabilizes the DC bias voltages.
For a PNP transistor, +Vcc is changed to -Vcc.
_____ amplifiers are biased so the input signal occurs within the limits of cutoff and saturation. Collector current (Ic) flows for the complete 360° cycle of the input signal. The bias point is normally mid-way between cutoff and saturation. The output signal is a replica of the input signal, good fidelity with low distortion.
Class A
______amplifiers are biased so collector current is zero for half of the input signal (one alternation). The DC bias point is at cutoff. Collector current (Ic) flows during only half (180°) of the input signal.
Class B
_____ amplifiers are biased so that collector current is zero for a portion of one alternation. The bias point is above cutoff, so collector current flows for more than 180° but less than 360°.
Class AB
_______amplifiers are biased so that collector current is zero for most of the input cycle. The bias point is below cutoff, so collector current flows for less than 180°.
Class C
Which transistor amplifier circuit configurations is this?
The common emitter configuration (CE)
the most frequently used amplifier. It provides good voltage, current, and power amplification.
The input signal is applied to the base, the output is taken from the collector, and the emitter is common.
The common emitter is the only configuration which provides a phase reversal.
Which transistor amplifier circuit configurations is this?
The common collector configuration (CC)
The input signal is applied to the base, the output is taken from the emitter, and the collector is common.
The output signal is in phase.
Which transistor amplifier circuit configurations is this?
The common base configuration (CB)
The input signal is applied to the emitter, the output is taken from the collector, and the base is common.
The output signal is in phase with the input.
