Npn transistor as amplifier
The arrangement of the three terminals affects the current and the amplification of the transistor. The behavior of Bipolar junction transistors is also very different for each circuit configuration. The three different circuit configurations produce different circuit characteristics with regards to input impedance, output impedance and gain. These characteristics affect whether the transistor exhibits voltage gain, current gain or power gain. One of the primary operations of a bipolar junction transistor is to amplify the signal of the current.
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Small-Signal Bipolar Transistor
A signal is just a general term used to refer to any particular current, voltage, or power in a circuit. An amplifier is the device that provides amplification the increase in current, voltage, or power of a signal without appreciably altering the original signal. Transistors are frequently used as amplifiers. Now take a look at the NPN version of the basic transistor amplifier in figure above and let's see just how it works.
By inserting one or more resistors in a circuit, different methods of biasing may be achieved and the emitter-base battery eliminated. In addition to eliminating the battery, some of these biasing methods compensate for slight variations in transistor characteristics and changes in transistor conduction resulting from temperature irregularities.
Notice in figure above that the emitter-base battery has been eliminated and the bias resistor Rb has been inserted between the collector and the base. Resistor Rb provides the necessary forward bias for the emitter-base junction. Current flows in the emitter-base bias circuit from ground to the emitter, out the base lead, and through Rb to Vcc. Since the current in the base circuit is very small a few tens microamperes and the forward resistance of the transistor is low, only a few tenths of a volt of positive bias will be felt on the base of the transistor.
However, this is enough voltage on the base, along with ground on the emitter and the large positive voltage on the collector, to properly bias the transistor.
With Q1 properly biased, direct current flows continuously, with or without an input signal, throughout the entire circuit. The direct current flowing through the circuit develops more than just base bias; it also develops the collector voltage Vc as it flows through Q1 and Rl. Notice the collector voltage on the output graph. Since it is present in the circuit without an input signal, then the output signal starts at the Vc level and either increases or decreases.
These dc voltages and currents that exist in the circuit before the application of a signal are known as QUIESCENT voltages and currents the quiescent state of the circuit. Resistor Rl, the collector load resistor, is placed in the circuit to keep the full effect of the collector supply voltage off the collector. This permits the collector voltage Vc to change with an input signal, which in turn allows the transistor to amplify voltage.
Without Rl in the circuit, the voltage on the collector would always be equal to Vcc. The coupling capacitor Cc is another new addition to the transistor circuit. It is used to pass the ac input signal and block the dc voltage from the preceding circuit.
This prevents dc in the circuitry on the left of the coupling capacitor from affecting the bias on Q1. The coupling capacitor also blocks the bias of Q1 from reaching the input signal source. The input to the amplifier is a sine wave that varies tens of millivolts above and below zero.
It is introduced into the circuit by the coupling capacitor and is applied between the base and emitter. As the input signal goes positive, the voltage across the emitter-base junction becomes more positive.
This in effect increases forward bias, which causes base current to increase at the same rate as that of the input sine wave. Emitter and collector currents also increase but much more than the base current. With an increase in collector current, more voltage is developed across Rl. Since the voltage across Rl and the voltage across Q1 collector to emitter must add up to Vcc, an increase in voltage across Rl results in an equal decrease in voltage across Q1.
Therefore, the output voltage from the amplifier, taken at the collector of Q1 with respect to the emitter, is a negative alternation of voltage that is larger than the input, but has the same sine wave characteristics. During the negative alternation of the input, the input signal opposes the forward bias.
This action decreases base current, which results in a decrease in both emitter and collector currents. The decrease in current through Rl decreases its voltage drop and causes the voltage across the transistor to rise along with the output voltage.
Therefore, the output for the negative alternation of the input is a positive alternation of voltage that is larger than the input but has the same sine wave characteristics. By examining both input and output signals for one complete alternation of the input, we can see that the output of the amplifier is an exact reproduction of the input except for the reversal in polarity and the increased amplitude tens of millivolts as compared to a few volts.
The PNP version of this amplifier is shown above. With a negative Vcc, the PNP base voltage is slightly negative with respect to ground, which provides the necessary forward bias condition between the emitter and base. When the PNP input signal goes positive, it opposes the forward bias of the transistor.
This action cancels some of the negative voltage across the emitter-base junction, which reduces the current through the transistor. Therefore, the voltage across the load resistor decreases, and the voltage across the transistor increases.
Since Vcc is negative, the voltage on the collector Vc goes in a negative direction as shown on the output graph toward -Vcc for example, from -5 volts to -7 volts. Thus, the output is a negative alternation of voltage that varies at the same rate as the sine wave input, but it is opposite in polarity and has a much larger amplitude. During the negative alternation of the input signal, the transistor current increases because the input voltage aids the forward bias.
Therefore, the voltage across Rl increases, and consequently, the voltage across the transistor decreases or goes in a positive direction for example: from -5 volts to -3 volts.
This action results in a positive output voltage, which has the same characteristics as the input except that it has been amplified and the polarity is reversed. In summary, the input signals in the preceding circuits were amplified because the small change in base current caused a large change in collector current.
And, by placing resistor Rl in series with the collector, voltage amplification was achieved. All Rights Reserved.
Differences between NPN & PNP Transistors and their Making
For a transistor to act as an amplifier, it should be properly biased. We will discuss the need for proper biasing in the next chapter. Here, let us focus how a transistor works as an amplifier. A transistor acts as an amplifier by raising the strength of a weak signal. The DC bias voltage applied to the emitter base junction, makes it remain in forward biased condition. This forward bias is maintained regardless of the polarity of the signal. The below figure shows how a transistor looks like when connected as an amplifier.
The Uses of NPN Transistors
Definition: The transistor in which one p-type material is placed between two n-type materials is known as NPN transistor. The NPN transistor amplifies the weak signal enter into the base and produces strong amplify signals at the collector end. In NPN transistor, the direction of movement of an electron is from the emitter to collector region due to which the current constitutes in the transistor. Such type of transistor is mostly used in the circuit because their majority charge carriers are electrons which have high mobility as compared to holes. The NPN transistor has two diodes connected back to back. The diode on the left side is called an emitter-base diode, and the diodes on the left side are called collector-base diode. These names are given as per the name of the terminals. The NPN transistor has three terminals, namely emitter, collector and base. The middle section of the NPN transistor is lightly doped, and it is the most important factor of the working of the transistor. The emitter is moderately doped, and the collector is heavily doped.
C1815 NPN Audio Frequency Amplifier Transistor 50V 150mA TO-92 Package - 5 Pieces Pack
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2sc1815 Equivalent
Transistor Circuit Types Include: Transistor circuit types Common emitter Emitter follower Common base Darlington pair Sziklai pair Current mirror Long tailed pair Constant current source Capacitance multiplier Two transistor amplifier High pass filter See also: Transistor circuit design This electronic circuit design shows a simple two transistor amplifier with feedback to offer a defined gain level that can be determined by the resistors in the circuit. The design includes PNP and NPN transistors and adopts the overall topology of the Sziklai pair, but with additional resistors included to define the gain. The two transistor amplifier offers a reasonably high impedance while providing a low output impedance. It is an ideal transistor amplifier circuit for applications where a higher level of gain is required than that which would be provided by a single transistor stage. The resistors R1 and R2 are chosen to set the base of TR1 to around the mid point.
Transistor ＜Understanding Transistors＞
Transistors are circuit elements designed to function either as amplifiers or as switches. At transistor has three parts: base, collector and emitter. The base is the controlling agent for a large supply of voltage, the collector is this large voltage supply and the emitter is the output for the transistor. A good analogy to use when explaining transistors as amplifiers is that of a tap. The gate is the faucet that controls the flow of water, the collector is the water supply and the emitter is the mouth of the tap from which the water comes out. Functioning as a switch allows the transistor to control current traveling through it and it can either allow current through it On or not Off. The name NPN transistors is based on the way they are created, that is, by placing two P-N junctions in parallel.
Start Learning English Hindi. This question was previously asked in. Forward bias the base-emitter junction and reverse bias the base-collector junction Forward bias the collector-base junction and reverse bias the emitter-base junciton Apply a positive voltage on the n-type material and a negative voltage on the p-type material Apply a large voltage on the base.
NPN transistors are a type of bipolar transistor with three layers that are used for signal amplification. It is a device that is controlled by the current. A negative-positive-negative transistor is denoted by the abbreviation NPN. A p-type semiconductor is fused between two n-type semiconductor materials in this configuration. It is divided into three sections: emitter, base, and collector. In an NPN transistor, the flow of electrons is what causes it to conduct.
Tardigrade Question Physics Consider an n-p-n transistor amplifier in common emitter configuration. The current gain of the transistor is If the collector current changes by 1 mA, what will be the change in emitter current? Assertion : By roughening the surface of a glass sheet its transparency can be reduced. Reason : Glass sheet with rough surface absorbs more light.
Get free ground shipping on all U. Shop now. Instead we are going to see how they are used practically, using a very simple yet accurate model.
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