Although transistors can be used for many other things, amplification is the main application for them. The PN junctions of the transistor must be appropriately biased by external voltages for the transistor to function as an amplifier. The transistor operates in three distinct modes: Active, Saturation, and Cutoff, depending on the external bias voltage polarities that are applied. An amplifier is only functional in the active mode of the transistor. Transistors can be employed as amplifiers in addition to switches.
A three-layer semiconductor device with amplifier capabilities is called a transistor. The transistor can regulate a greater signal flow between the collector and emitter by applying a small signal to the base. This makes transistors valuable in electronic circuits as amplifiers for music, radio signals, and more. The development of modern technology has been significantly influenced by the transistor’s capacity to amplify signals. In essence, transistors are amplifiers used for amplification. “Amplification” is the process of increasing a weak signal’s amplitude without altering its frequency or form. ‘Transistor biasing’ is the process of accomplishing this by having the transistor’s input circuit remain forward-biased and its output circuit remains reverse-biased throughout the whole signal. The weak signal is received by the transistor’s base, and an amplified output is produced by the collector circuit. The signal must only increase in strength and not change in shape for amplification to occur.
Why is the Transistor works as An Amplifier ?
A transistor strengthens a weak signal by acting as an amplifier. The emitter base junction is kept in a forward biased state by the DC bias voltage provided to it. The signal’s polarity has no bearing on the preservation of this forward bias. An amplifier coupled to a transistor is depicted in the figure below.
Transistor as an Amplifier
VEB = Emitter Base Voltage
VCB = Collector Base Voltage
VI = Input Power Supply
IB= Base Current
IE=Emitter Current
RL=Load Resistance
In the case of common emitter configuration, we have to calculate different parameters of the Amplifier configuration. We have to calculates Voltage, Input Resistant, Output Resistant, Effective Collector load, Current Gain, Voltage Gain, Power gain, Trans Conductance.
Input Resistance Ri
It is defined as the ratio of the negligible change in Base-Emitter voltage to the negligible in the Base Current, at constant Collector-Emitter voltage.
Input Resistance (Ri)= VBE / IB
Where,
Ri = input resistance
VBE= base-emitter voltage
IB = base current.
Output Resistance RO
It is defined as the ratio of small change in Collector-Emitter voltage to the small change in collector current at constant base currentB.
Output resistance(RO) = ΔVCE / ΔIC
Where,
RO = output resistance
VCE = Collector Emitter Voltage
IC = Collector current
IB = Base Current
Effective Collector Load
The load is linked to a transistor’s collector. For a single-stage amplifier, the output voltage is obtained from the transistor’s collector; for a multi-stage amplifier, it is obtained from a transistor circuit’s cascaded stages.
It is, by definition, the entire load as perceived by the collector current of the A.C. The effective collector load for single-stage amplifiers is RC plus Ro combined in parallel.
Effective Collector Load, RAC = RC // RO
RAC = (RC×RO) / (RC+RO)
Hence for the Single Stage Amplification Effective load is equivalent to the collector load.
Current Gain
Current gain is the gain in terms of current that is noticed when changes in input and output currents occur. It is, by definition, the relationship between the change in base current (ΔIB) and the change in collector current (ΔIC).
β = Current gain,
β = ΔIC / ΔIB
ΔIC = Small Change in collector current.
ΔIB = Small change in base current.
Voltage Gain
Voltage gain is the amount of voltage gained when variations in input and output currents are noted. It is, by definition, the ratio of the input voltage change (ΔVBE) to the output voltage change (ΔVCE).
Voltage gain, AV = ΔVCE / ΔVBE
Power Gain
The gain observe in power output siganl when input signal is passed through the Amplifer is called as Power gain.
AP = Power gain
AP = Current gain × Voltage gain
AP = β × AV
Transistor as an Amplifier Working
When a transistor’s base receives a little input signal, very little current passes between the emitter and base of the transistor. The input signal is then amplified when this low current causes a significantly higher current to flow between the emitter and collector.
The amount of amplification that a transistor can provide is determined by the bias current applied to its base. The current that flows across a transistor’s base-emitter junction in the absence of an input signal is known as bias current. The greatest output signal that can be produced and the transistor’s operating point are determined by the amount of bias current. The transistor can be used as a voltage amplifier or a current amplifier, depending on the type of circuit layout used. The base receives the input signal and the collector receives the output signal in a conventional emitter configuration. In this design, the transistor acts as a voltage amplifier, increasing the output signal value by amplifying the input signal voltage.
Characteristics Curve
In active region transistor act as the amplifier , in the cutoff and saturation region it behave as the switch.
The slope of the curve gives the voltage gain.
Transistor as an Amplifier​ Circuit
In transistor as an amplifier circuit, the common emitter configuration usually used. The first half of BJT means Emitter-Base is forward biased and next halt of BJT means Collector emitter is reverse biased.
Circuit diagram
In the above figure the circuit diagram is given in which the CE configuration is shown,
the input signal in out of phase with output signal (180 degree phase difference).
Capacitors are used in the circuits to block the D.C (Direct Current ) from the circuit and only pass the A.C (Alternation Current).
IE =IB + IC
where,
IE = Emitter current.
Common Emitter Transistor Amplifier
Any small signal amplifier’s goal is to maximize input signal amplification while minimizing distortion to the output signal; in other words, the output signal should be an identical replica of the input signal, although magnified.
When utilized as an amplifier, the operational quiescent point must be properly chosen to achieve minimal distortion. This is actually the amplifier’s DC operating point, and with the right biasing configuration, you can determine its position at any point along the load line.
In order to produce a Class A type amplifier operation, the ideal location for this Q-point is as close to the load line’s center as is reasonably possible
Common Emitter Configuration
VOUT = Output Voltage
C1, C2 = Capacitor
Characteristics of CE Amplifier
In this setup, IB stands for input current, also known as base current, while IE stands for output current, also known as emitter current. Low output impedance and high input impedance characterize the common collector amplifier. Its current gain is large and its voltage gain is minimal. The typical collector amplifier has a medium power gain.
Characteristics of CE Amplifier
Advantages and Disadvantages of Transistor as an Amplifier
Given Below are Advantages and Disadvantages of Amplifier :
Advantage of Amplifier
- A low input impedance, inverting common emitter amplifier
- high output impedance
- high voltage gain
- high current gain.
Disadvantages of Amplifier
- It has a high output resistance.
- It responds poorly to high frequencies.
- It has high thermal instabilities.
- It’s voltage gain is very unstable.
Applications of Transistor as an Amplifier
- These types of amplifiers enable the amplification of radio signals.
- Amplifiers are used in wireless communication.
- Its used to Increase the strength of the signal.
Conclusion
We have different configuration of amplifiers but we are using common-emitter configuration in the case of the amplication. The output voltage of the common-emitter arrangement is inverted with regard to the input signal, which is a crucial characteristic to observe. In other words, when the input signal rises, the output voltage falls. In this case if we applied the input signal of 0 degree phase then output will be obesrved to be Phase of 180 degree. The common-emitter amplifier design is called an inverting amplifier.
Examples
Q1. The output resistance of a common base transistor amplifier is 100 kΩ, while the input resistance is 10 Ω. One kΩ is the collector load. Determine the voltage amplification if a 100 mV signal is applied between the emitter and the base. Let αac be almost equal to one.
Solution:
Ri = 10ohm
Ro = 100Kohm
Rl = 1kohm
Vin = 100mV
Input current (IE) = Signal / Ri => 100mV / 10 = 10mA.
αac is nearly one so IC = IE
Output voltage (VO) = ICRC = (10mA ) (1kohm) = 10V
Voltage Amplification = VOUT / Signal = 10 / 100mV = 100.
Q2 .If we are using a common base connection, IC = 0.95mA and IE = 1mA. Determine the IB value.
Solution :
IE = IB + IC
1 = IB + 0.95
IB = 0.05mA
FAQs on Transistor As An Amplifier
1. How may types of Configuration available in Transistor?
In transistor we have three types of configuration as list shown below:
- Common Emitter Configuration.
- Common Base Configuration.
- Common Collector Configuration.
2. Why we used common-emitter configuration for the Amplification?
Common emitter circuit configuration is used by most amplifiers because it provides both voltage and current gain, which leads to a higher power gain than common base emitter design. Its increased input resistance, which is of the order of load resistance, is another factor to take into account.
3. Mention the different types of region in which transistor works?
Different Types of Region are given Below:
- Saturation region
- Cutoff region
- Active region
4. In which region transistor act as amplifier
We have 3 different types of regions which is for the amplification behaviour but we use the Active region.
Share your thoughts in the comments
Please Login to comment...