Common Base Amplifier

Both the input and output circuits share the base terminal. The output signal is obtained between the collector and base, while the input signal is applied between the emitter and base.

• Because of their high voltage gain and low input impedance, common base amplifiers are helpful in high-frequency circuits.
• A CB amplifier's output signal is in phase with the input signal, in contrast to certain other amplifier topologies.

Circuit Diagram

In a common base amplifier:

• The base terminal of a common base amplifier is linked to ground.
• The emitter receives the input signal.
• The collector provides the output.
• To keep transistors operating properly, coupling capacitors and biasing resistors are utilized.
• Small changes in the input signal result in exaggerated variations in the output signal because the transistor operates in the active area.

Working Principle

The following describes how a standard base amplifier works:

• The emitter terminal receives a little AC input signal.
• Depending on the input signal, the emitter current varies.
• A significant amount of the emitter current travels to the collector because the transistor is operating in the active region.
• The collector load resistor receives the fluctuating collector current.
• Amplification results from a higher output voltage.
• The input signal directly regulates the emitter current, which in turn regulates the collector current since the base terminal stays at a constant potential.

Input Impedance

The ratio of input voltage to input current is known as the input impedance of a common base amplifier.

Z_{in} = \frac{V_{in}}{I_{in}}

Because the input is applied to the forward-biased emitter junction, a CB amplifier typically has a low input impedance. The amplifier may react fast to high-frequency signals when the input impedance is low.

Output Impedance

The ratio of output voltage to output current when the input source is turned off is known as the output impedance.

Z_{out} = \frac{V_{out}}{I_{out}}

Because the output is drawn from the collector terminal, a common base amplifier has a high output impedance. The amplifier can provide a large voltage gain thanks to its high output impedance.

Voltage Gain

The ratio of output voltage to input voltage is known as voltage gain.

A_v = \frac{V_{out}}{V_{in}}

The voltage gain of a CB amplifier can be estimated as follows:

A_v \approx g_m (R_C \parallel R_L)

where:

• g_m = Transconductance of the transistor
• R_C = Collector resistance
• R_L = Load resistance

High voltage gain is usually produced by this setup.

Characteristics

A CB amplifier's primary features are as follows:

• Minimal input resistance
• High resistance to output
• Elevated voltage gain
• Current gain is little less than 1.
• There is no phase reversal between the input and output

Advantages

• High Voltage Gain: Because the collector resistance transforms tiny changes in current into bigger voltage variations, the CB amplifier may produce a high voltage amplification.
• Good High-Frequency Performance: Because the Miller effect is absent, the arrangement performs well at high frequencies, increasing bandwidth.
• Improved Bandwidth: The common base configuration is appropriate for RF circuits because it provides a larger bandwidth than other configurations.
• Stable Operation: In some high-frequency applications, the circuit tends to be more stable because the base terminal is fixed.

Disadvantages

• Low Input Impedance: The signal source may be loaded when the input is attached to the emitter terminal, which produces extremely low input resistance.
• Current Gain Less Than Unity: Because the collector current is marginally lower than the emitter current, the current gain is marginally less than one.
• Limited Use: CB amplifiers are less frequently utilized in general-purpose amplification than conventional emitter amplifiers.

Applications

• Radio Frequency (RF) Amplifiers: Used in RF circuits because of their good high-frequency response.
• Impedance Matching Circuits: Match low input impedance sources with high-impedance stages.
• Stages of voltage amplification: Used in circuits that need a high voltage gain.
• Communication Systems: Assists communication receivers in boosting weak signals.