## Common baseCommon base BJT (Bipolar Junction Transistor) amplifiers are BJT with the common or grounded region as the base. Based on this, BJT is also classified as a common emitter and common collector amplifier. The emitter region is the input in the common base amplifier, and the collector region is the output. The base is connected to the ground as the common terminal to both the input and the output and thus named BJT amplifier as the common base has low input impedance but produces high output impedance. It has various applications, such as a First, let's discuss the amplifiers and the requirement of using the base region as the common base. ## AmplifiersThe amplifier is an electronic device that amplifies the input signal and produces the desired output. Amplifying refers to the increase in the power or the voltage of the input signal. Amplifiers take energy from the external connected power supply, control the output, and produce the output waveform with large amplitude. There are various types of amplifiers such as ## BJT AmplifiersBJT works as an excellent amplifier generally in the active forward mode. It provides the high output voltage, power gains, and current. A small input voltage to the emitter region of the BJT provides a large output voltage. Thus, Bipolar Junction Transistors provide good amplification. ## Common Base TransistorHere, we will consider a common base amplifier of an NPN transistor. It consists of a ## Circuit connectionThe circuit of the common base BJT is shown below: The three regions of the BJT are emitter (E), base (B), and collector (C). The two voltage sources are connected between the three regions. The voltage source connected between B and C is VCB and the voltage source connected between B and E is VBE. The current from the E and C regions are represented by IE and IC. Since, the base region is grounded (0 Volts), it has no current. ## Note: Common base amplifier is a type of BJT only with the common base between the input and the output.The doping concentrations of these three regions are:
## WorkingThe input of the common base is the E region. The electrons from the voltage source V ## EquationsThe ratio of the collector current and the emitter current is given by:
Where, a is the common-base current gain (a < 1). The ratio of the collector current and the base current is given by:
Where, B is the common-emitter current gain (B > 1). The equation of the common base transistor is given by: I ## ParametersThe equivalent circuit of the common base amplifier is shown below: The current gain of the common base amplifier is less than 1. The AC equivalent circuit is shown below: The above circuit diagram is drawn by keeping all the DC (Direct Current) sources as a short circuit. The parameters like ## Voltage GainV V I I Putting the value of collector current in output voltage, we get: V The emitter current is given by: I So, the output voltage is: V Gain = Output voltage/Input voltage A ## Current gainThe current gain is given by: Output current/Input current A The collector region is the output region. Thus, the output current depends on the collector current. I We know, I So, I The emitter region is the input region. Thus, the input current depends on the emitter current. I So, gain is: A A
## Input ImpedanceThere are two resistance connected across the input, R The input impedance Z
It is the parallel connection of the two resistances R ## Output ImpedanceThe output resistance is the resistance connected across the output of the above equivalent circuit of the common emitter. There is only resistance connected across the output, i.e., R Thus, the output impedance is given by:
We will also discuss examples based on the above parameters later. ## Low-frequency parametersThe low-frequency common base amplifier is shown below: The low-frequency parameters of the common base amplifier are listed as follows: ## Open-circuit voltage gainThe voltage gain is a unitless quantity and is defined as the ratio of output voltage to input voltage. Voltage gain = Output voltage/Input voltage It is given by: Av = (g If ro in greater than Rc, the voltage gain will be: Av = g
## Short-circuit current gainThe current gain is defined as the ratio of output current to input current. Voltage gain = Output current /Input current It is given by: Ai = (rπ + Br If B in greater than 1, the current gain will be: Ai = (rπ + Br
## Input resistanceWe know, I = V/R Or R = V/I The input resistance can be represented as: Ri = Vi/Ii Ri =((r ## Output resistanceThe output resistance can be represented as: Ro = Vo/-Io Ro = R If, r Ro = R If, r Ro = R ## Common base as a current followerThe current follower is also known as the The Common base Bipolar Junction transistor is a bilateral transistor. The AC source is connected at the input, and the The current gain may reach unity as long as AC source resistance at the output is large as compared to the emitter resistance. The current gain of the BJT current follower is unity. ## Common base as a voltage amplifierThe common base amplifier also works as a voltage amplifier. According to the common base as a current follower, the high output resistance allows the majority current to pass through the load transistor. It is an ideal condition for the common base to work as a current follower. But in the case of voltage, the high output resistance is not the desirable condition for the voltage division at the output. The voltage gain can be calculated for a small load and large impedance values. IN the case of large impedance, the voltage gain is determined based on the load resistance and input resistance (RL/Rs) ratio. As discussed above, if the AC source replaces the Thevenin voltage source, the common base transistor as a voltage amplifier starts behaving like the current follower. ## ApplicationsThe applications of the common base are as follows: - It is commonly used for amplifiers than requires low input impedance, such as microphones.
- It is used in very high and ultra high frequency amplifiers because it performs better at high frequencies. It is due to the input-output impedance and the high voltage amplifications.
- It is used for impedance matching. If the circuit has high input resistance, the common base provides it with the low output resistance. It is known as
**impedance matching**.
## ExamplesLet's discuss an example based on the common base BJT amplifiers.
Voltage Gain is given by: Output voltage/Input voltage Since the given transistor is ideal, we have assumed the input and output current to be equal. So, the voltage gain is: Av = (1 k Ohms) || (100 K Ohms) / Re Av = 990 Ohms/ 52 Ohms Av = 19.03 or 19 Gain is a unitless quantity and thus has no units.
The input impedance can be calculated using the formula: I Ie = 0.5 mA =0.5 x 10 Vt or threshold voltage = 26Mv = 26 x 10 So, Re = V/Ie Re = 26 x 10 Re = It is the resistance across the input, i.e. emitter region. The input impedance of the transistor is the resistance of the emitter region = Re = 52 Ohms.
The output impedance is the equivalent resistance value of the resistors connected across the output of the transistor. There are two resistors connected across the output. Thus, the output impedance is: Zo = (1 k Ohms) || (100 K Ohms) Zo = 1 x 100/ (1 + 100) K Ohms Zo = 100/101 k Ohms Zo = 0.990 k Ohms Zo = 0.990 x 1000 Ohms
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