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At a glance of CB configuration

In our previous blog, we learned about BJT and its mode of operations, so in today’s blog, we will learn about the CB configuration of BJT, Input characteristics, Input Resistance, Output characteristics, Output Resistance of common base configuration.

Introduction

The Bipolar Junction transistor has three terminals

Emitter (E)
Base (B)
Collector(C)

But to connect the circuit connections, four terminals are required two terminals for input and another two terminals for output. We use one terminal as common for both input and output to use three-terminal devices as a four-terminal device.

However, we construct the circuits using these and the combinations of these connections are known as transistor configurations.

There are three different configurations of transistors

(CB)Common base configuration
(CC) Common collector configuration
(CE) Common emitter configuration.

The behavior of transistor configurations according to gain.

(CB) Common Base Configuration: voltage gain only
(CC) Common Collector Configuration: only current gain
(CE) Common Emitter Configuration: both current gain and voltage gain

CB Configuration (common base)

As the name suggests in the common base configuration, the base is used as a common terminal both for input and output.

Here the weak signal input is applied between the base and emitter terminals and the corresponding amplified output signal is taken between the base and collector terminals. however as the base is shared, it is grounded base terminal grounded.

The input voltage and current parameters are VEB and IE, and the output voltage and parameters are VCB and IC. The current flowing through the emitter, which is the input current is always higher than the base current; therefore the output collector current is less than the input emitter current.

For common base configuration, the current gain is equal or less than unity. The amplifier circuit configuration of the common base configuration is known as a non-inverting amplifier circuit. however, due to the high voltage gain values, the construction of a common base configuration is complicated.

A series resistor RE is connected between supply and emitter in the circuit to limit the emitter current IE.

The input characteristics of this configuration are looks similar to the characteristics of a photodiode. The output characteristics common base represents a forward-biased diode. The common base configuration of the transistor has high output impedance and low input impedance. Also, the resistance gain is elevated, i.e., the ratio of output resistance to input resistance is high.

The voltage gain for common base configuration of circuit is given below.

AV = Vout / Vin = (IC×RL) / (IE ×Rin)

The current gain can be given as,

α = Output current/Input current

α = IC/IE

The common base configuration circuit is frequently used in single-stage amplifier circuits; however, in microphone preamplifiers or radiofrequency amplifiers due to their high-frequency response.

Input characteristics of CB Configuration

Input characteristics of a common base configuration are obtained between the input current and the input voltage when the output voltage is constant.

To obtain the input characteristics we need to keep the output voltage VCB constant and vary the input voltage VEB for different points and record the input current IE value.

For different output voltage levels repeat the same process. By using obtained values plot the graph between IE and VEB parameters. Input characteristics curve is the curve plotted between emitter current IE and the emitter-base voltage VEB at constant collector-base voltage VCB. Below fig. shows input characteristics for the common base configuration. The equation to calculate the input resistance Ri value is given below.

To analyze the input characteristics curve following points are essential.

1. The curve is a diode characteristic in the forward region; for a specific value of VCB (collector-base voltage), the PN emitter junction is forward-biased.

2. Flow of base current and emitter current is directly proportional to each other when the base current increases, the emitter current increases slightly.

Input Resistance CB Configuration

The input resistance is the ratio of change in emitter-base voltage to the resulting change in emitter current when the collector-base voltage is constant.

However, The input resistance can be expressed by using the formula

Ri = ∆V_EB/ ∆I_E

Collector–base current increases with an increase in the value of collector-base voltage VCB. The value of input resistance is very low, and their value varies from a few ohms to 10 ohms.

Output characteristics CB Configuration

For common base configuration, output characteristics are obtained between the output current and output voltage while keeping current input.

Therefore in the output characteristics, the emitter current is constant, and vary the VCB value for different points and record the IC values at each point. Repeat the same process at change in IE (emitter current) values.

The below figure shows the output characteristics of the common base configuration. The equation to calculate the output resistance value is given below.

To obtain the output characteristics the curve is plotted between the collector current and collector-base voltage VCB while the emitter current IE is constant.

To analyze the output characteristics curve following points are essential.

1. Collector current IC is almost equal to the emitter current IE. When the active region at the collector-base junction is reverse biased, the transistor continuously operates in the active region.

2. The circuit used to obtain output characteristics always has very high output resistance Ro. The output curve of the active regions is almost flat. therefore the significant charges in VCB produce only a small change in IC.

3. Collector-base junction is forward bias when VCB is positive and the collector current decrease suddenly. Therefore this condition is known as saturation state in which the collector current does not depend on the emitter current.

4. When the current flowing through the emitter is zero, the collector current will not become zero. however, there is always a tiny amount of current present there which flows through the circuit are known as the reverse leakage current, i.e., ICBO. however this reverse leakage current is temperature-dependent, and its value ranges from 2 to 5 μA for the germanium transistor. And 0.1 to 1.0 μA for silicon transistor

Output Resistance

The output resistance is the ratio of change in collector-base voltage to the change in collector current when the emitter current IE is constant.

The output resistance is represented as,

R_o= ∆V_CB/ ∆ I_c

For the output characteristics of the common base configuration, the change in collector current is very negligible with the shift in collector-base voltage VCB.

The value of output resistance is very high in the order of several kilometers.

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