Any transistor amplifier, uses a transistor to amplify the signals which is connected in one of the three configurations. For an amplifier it is a better state to have a high input impedance, in order to avoid loading effect in Multi-stage circuits and lower output impedance, in order to deliver maximum output to the load. The voltage gain and power gain should also be high to produce a better output.

Let us now study different configurations to understand which configuration suits better for a transistor to work as an amplifier.

CB Amplifier

The amplifier circuit that is formed using a CB configured transistor combination is called as CB amplifier.

Construction

The common base amplifier circuit using NPN transistor is as shown below, the input signal being applied at emitter base junction and the output signal being taken from collector base junction.

The emitter base junction is forward biased by V_EE and collector base junction is reverse biased by V_CC. The operating point is adjusted with the help of resistors Re and R_c. Thus the values of I_c, I_b and I_cb are decided by V_CC, V_EE, R_e and R_c.

Operation

When no input is applied, the quiescent conditions are formed and no output is present. As V_be is at negative with respect to ground, the forward bias is decreased, for the positive half of the input signal. As a result of this, the base current I_B also gets decreased.

The below figure shows the CB amplifier with self-bias circuit.

As we know that,

$$I_C \cong I_E \cong \beta I_B$$

Both the collector current and emitter current get decreased.

The voltage drop across R_C is

$$V_C = I_C R_C$$

This V_C also gets decreased.

As I_CR_C decreases, V_CB increases. It is because,

$$V_{CB} = V_{CC} - I_C R_C$$

Thus, a positive half cycle output is produced.

In CB configuration, a positive input produces a positive output and hence input and output are in phase. So, there is no phase reversal between input and output in a CB amplifier.

If CB configuration is considered for amplification, it has low input impedance and high output impedance. The voltage gain is also low compared to CE configuration. Hence CB configured amplifiers are used at high frequency applications.

CE Amplifier

The amplifier circuit that is formed using a CE configured transistor combination is called as CE amplifier.

Construction

The common emitter amplifier circuit using NPN transistor is as shown below, the input signal being applied at emitter base junction and the output signal being taken from collector base junction.

The emitter base junction is forward biased by V_EE and collector base junction is reverse biased by V_CC. The operating point is adjusted with the help of resistors R_e and R_c. Thus the values of I_c, I_b and I_cb are decided by V_CC, V_EE, R_e and R_c.

Operation

When no input is applied, the quiescent conditions are formed and no output is present. When positive half of the signal is being applied, the voltage between base and emitter V_be is increased because it is already positive with respect to ground.

As forward bias increases, the base current too increases accordingly. Since I_C = βI_B, the collector current increases as well.

The following circuit diagram shows a CE amplifier with self-bias circuit.

The collector current when flows through R_C, the voltage drop increases.

$$V_C = I_C R_C$$

As a consequence of this, the voltage between collector and emitter decreases. Because,

$$V_{CB} = V_{CC} - I_C R_C$$

Thus, the amplified voltage appears across R_C.

Therefore, in a CE amplifier, as the positive going signal appears as a negative going signal, it is understood that there is a phase shift of 180^o between input and output.

CE amplifier has a high input impedance and lower output impedance than CB amplifier. The voltage gain and power gain are also high in CE amplifier and hence this is mostly used in Audio amplifiers.

CC Amplifier

The amplifier circuit that is formed using a CC configured transistor combination is called as CC amplifier.

Construction

The common collector amplifier circuit using NPN transistor is as shown below, the input signal being applied at base collector junction and the output signal being taken from emitter collector junction.

The emitter base junction is forward biased by V_EE and collector base junction is reverse biased by V_CC. The Q-values of I_b and I_e are adjusted by R_b and R_e.

Operation

When no input is applied, the quiescent conditions are formed and no output is present. When positive half of the signal is being applied, the forward bias is increased because V_be is positive with respect to collector or ground. With this, the base current I_B and the collector current I_C are increased.

The following circuit diagram shows a CC amplifier with self-bias circuit.

Consequently, the voltage drop across R_e i.e. the output voltage is increased. As a result, positive half cycle is obtained. As the input and output are in phase, there is no phase reversal.

If CC configuration is considered for amplification, though CC amplifier has better input impedance and lower output impedance than CE amplifier, the voltage gain of CC is very less which limits its applications to impedance matching only.

Comparison between CB CE CC Amplifiers

Let us compare the characteristic details of CB, CE, and CC amplifiers.

Characteristic	CE	CB	CC
Input resistance	Low (1K to 2K)	Very low (30-150 &ohm;)	High (20-500 K&ohm;)
Output resistance	Large (≈ 50 K)	High (≈ 500 K)	Low (50-1000 K&ohm;)
Current gain	B high	α < 1	High (1 + β)
Voltage gain	High (≈ 1500)	High (≈ 1500)	Less than one
Power gain	High (≈ 10,000)	High (≈ 7500)	Low (250-500)
Phase between input and output	reversed	same	same

Due to the compatibility and characteristic features, the common-emitter configuration is mostly used in amplifier circuits.

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