Common base

Common 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 common base amplifier.

BJT amplifier as the common base has low input impedance but produces high output impedance. It has various applications, such as a current buffer and microphones.

First, let's discuss the amplifiers and the requirement of using the base region as the common base.

Amplifiers

The 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 current amplifier (amplifies the input current), voltage amplifiers (amplifies the input voltage), transresistance amplifier (amplifies the input current to the output voltage), and the transconductance amplifier (amplifies the input voltage to the output current).

BJT Amplifiers

BJT 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 Transistor

Here, we will consider a common base amplifier of an NPN transistor. It consists of a P layer (doped with trivalent impurity) sandwiched between two N layers (doped with Pentavalent impurities).

Circuit connection

The 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:

Collector: Lightly doped, largest width

Base: Medium doped, smallest width

Emitter: Highly doped, medium width

Working

The input of the common base is the E region. The electrons from the voltage source V_BE flows towards the E region. It is an N-type and thus contains electrons as majority carriers. The electron from the E regions starts moving towards the B due to repulsion between the same charge carriers at the junction. The B region is small to hold such large concentration of carriers and passes it further to the collector. The collector does not have its own current due to the reverse-biased connection.

Equations

The ratio of the collector current and the emitter current is given by:

I_C/I_E = a

Where,

a is the common-base current gain (a < 1).

The ratio of the collector current and the base current is given by:

I_C/I_B = B

Where,

B is the common-emitter current gain (B > 1).

The equation of the common base transistor is given by:

I_E = I_B + I_C

Parameters

The 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 gain, current gain, input, and output impedance will be calculated based on the above circuit diagrams.

Voltage Gain

V_O = -I_O

V_O = I_CR_C

I_C/I_E = a

I_C = aI_E

Putting the value of collector current in output voltage, we get:

V_O = aI_ER_C

The emitter current is given by:

I_E = V_I/r_e

So, the output voltage is:

V_O = a(V_I/r_c)R_C

Gain = Output voltage/Input voltage

A_V = V_O/V_I = aR_C/r_C

Current gain

The current gain is given by:

Output current/Input current

A_I = I_O/I_I

The collector region is the output region. Thus, the output current depends on the collector current.

I_O = -I_C

We know, I_C = aI_E

So, I_O = - aI_E

The emitter region is the input region. Thus, the input current depends on the emitter current.

I_I = I_E

So, gain is:

A_I = I_O/I_I

A_I = - aI_E / I_E

A_I = - a

Input Impedance

There are two resistance connected across the input, R_E and r_e.

The input impedance Z_I is given by:

Z_I = R_E||r_e

It is the parallel connection of the two resistances R_E and r_e.

Output Impedance

The 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_C.

Thus, the output impedance is given by:

Z_O = R_C

We will also discuss examples based on the above parameters later.

Low-frequency parameters

The low-frequency common base amplifier is shown below:

The low-frequency parameters of the common base amplifier are listed as follows:

Open-circuit voltage gain

The 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_mro + 1)R_C/ (ro + R_C)

If ro in greater than Rc, the voltage gain will be:

Av = g_mroR_C/ro

Av = g_mR_C

Short-circuit current gain

The 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_o)/ (rπ + (B + 1)r_o)

If B in greater than 1, the current gain will be:

Ai = (rπ + Br_o)/ (rπ + (B)r_o)

Ai = 1

Input resistance

We know, I = V/R

Or

R = V/I

The input resistance can be represented as:

Ri = Vi/Ii

Ri =((r_o + R_C ||R_L)R_E) / (r_o + R_E + R_C ||R_L/(B + 1))

Output resistance

The output resistance can be represented as:

Ro = Vo/-Io

Ro = R_C || ([1 + g_m (rπ || R_S)] r_o + rπ || R_S)

If, r_o is less than r_E, the output resistance will be:

Ro = R_C || r_o

If, r_o is greater than r_E, the output resistance will be:

Ro = R_C || ([1 + g_m (rπ || R_S)]

Common base as a current follower

The current follower is also known as the current buffer, which follows the input current. If the output current of the transistor follows the input current, it is known as a current follower. If produced by the load, it is not affected by any current or voltages. The circuit of the current follower is shown below:

The Common base Bipolar Junction transistor is a bilateral transistor. The AC source is connected at the input, and the load resistance is connected across the output. The output resistance Ro connects the input and the output. If the collector resistance is present, the output resistance of the transistor will still be large. It follows the current division at the output and allows the majority current to pass through the load transistor. It is one of the essential conditions for the transistor to work as a current follower.

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 amplifier

The 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.

Applications

The 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.

Examples

Let's discuss an example based on the common base BJT amplifiers.

Example: The configuration of the common base transistor is given below:

Find the voltage gain, input impedance, and output impedance of the circuit. Assume that the transistor is ideal.

Solution:

Voltage Gain

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.

Input Impedance

The input impedance can be calculated using the formula:

I_E = V_I/r_e

Ie = 0.5 mA =0.5 x 10^-3A

Vt or threshold voltage = 26Mv = 26 x 10^-3V

So,

Re = V/Ie

Re = 26 x 10^-3V/0.5 x 10^-3A

Re = 52 Ohms

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.

Output Impedance

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

Zo = 990 Ohms