Transistor as an Amplifier

Transistor As An Amplifier
A transistor can be used for amplifying a weak signal.
When a transistor is to be operated as amplifier, three different basic circuit connections are possible.

These are (i) common base, (ii) common emitter and (iii) common collector circuits.
Whichever circuit configuration, the emitter-base junction is always forward biased while the collector-base junction is always reverse biased.

Common base amplifier using a p-n-p transistor

In common base amplifier, the input signal is applied across the emitter and the base, while the amplified output signal is taken across the collector and the base. This circuit provides a very low input resistance, a very high output resistance and a current gain of just less than 1. Still it provides a good voltage and power amplification. There is no phase difference between input and output signals.

The common base amplifier circuit using a p-n-p transistor is shown in figure. The emitter base input circuit is forward biased by a low voltage battery VEB. The collector base output circuit is reversed biased by means of a high voltage battery VCC. Since, the input circuit is forward biased, resistance of input circuit is small. Similarly, output circuit is reverse biased, hence resistance of output circuit is high.
The weak input AC voltage signal is superimposed on VEB and the amplified output signal is obtained across collector-base circuit. In the figure we can see that,

VCB = VCC – ic RL

The input AC voltage signal changes net value of VEB. Due to fluctuations in VEB, the emitter current ie also fluctuates which in turn fluctuates ic. In accordance with the above equation there are fluctuations in VCB, when the input signals is applied and an amplified output is obtained.

Input characteristics graph of common base transistor

Output characteristics graph of common base transistor

Current gain, Voltage gain and Power gain

Current gain : Also called AC current gain (αac), is defined as the ratio of the change in the collector current to the change in the emitter current at constant collector-base voltage.

Thus, αac or simply $\large \alpha = \frac{\Delta i_c}{\Delta i_e} $ (VCB = constant)

As stated earlier also, α is slightly less than 1.

Voltage gain : It is defined as the ratio of change in the output voltage to the change in the input voltage. It is denoted by AV. Thus,

$\large A_V = \frac{\Delta i_c \times R_o}{\Delta i_e \times R_i} $

but $\large \alpha = \frac{\Delta i_c}{\Delta i_e} $ = the current gain.

$\large A_V = \alpha \frac{ R_o}{ R_i} $

Since, Ro >> Ri, AV is quite high, although α is slightly less than 1.

Power gain : It is defined as the change in the output power to the change in the input power. Since, P = Vi

Therefore, power gain = current gain × voltage gain

or,  $\large Power \; gain = \alpha^2 . \frac{ R_o}{R_i} $

Important Points in Common Base Amplifier

1. The output voltage signal is in phase with the input voltage signal.
2. The common base amplifier is used to amplify high (radio) – frequency signals and to match a very low source impedance (~20 Ω) to a high load impedance (~100 k Ω).

Common emitter amplifier using a p-n-p transistor
Figure shows a p-n-p transistor as an amplifier in common emitter mode. The emitter is common to both input and output circuits. The input (base-emitter) circuit is forward biased by a low voltage battery VBE. The output (collector-emitter) circuit is reverse biased by means of a high voltage battery VCC.
Since, the base-emitter circuit is forward biased, input resistance is low. Similarly, collector-emitter circuit is reverse biased, therefore output resistance is high. The weak input AC signal is superimposed on VBE and the amplified output signal is obtained across the collector-emitter circuit.
In the figure we can see that,

VCE = VCC – ic RL

When the input AC voltage signal is applied across the base-emitter circuit, it fluctuates VBE and hence the emitter current ie. This in turn changes the collector current ic consequently VCE varies in accordance with the above equation. This variation in VCE appears as an amplified output.

Input characteristics graph of common emitter transistor

Output characteristics graph of common emitter transistor

Current gain, Voltage gain and Power gain

Current gain : Also called AC current gain (αac), is defined as the ratio of the change in the collector current to the change in the emitter current at constant collector-base voltage.

Thus, βac or simply $\large \beta = \frac{\Delta i_c}{\Delta i_b} $ (VCE = constant)

Voltage gain : It is defined as the ratio of change in the output voltage to the change in the input voltage. It is denoted by AV. Thus,

$\large A_V = \frac{\Delta i_c \times R_o}{\Delta i_b \times R_i} $

but $\large \beta = \frac{\Delta i_c}{\Delta i_b} $ = the current gain.

$\large A_V = \beta \frac{ R_o}{ R_i} $

Power gain : It is defined as the change in the output power to the change in the input power. Since, P = Vi

Therefore, power gain = current gain × voltage gain

or,  $\large Power \; gain = \beta^2 . \frac{ R_o}{R_i} $

Important Points in Common Emitter Amplifier
(i) The value of current gain β is from 15 to 50 which is much greater than α .
(ii) The voltage gain in common-emitter amplifier is larger compared to that in common base amplifier.
(iii) The power gain in common-emitter amplifier is extremely large compared to that in common base amplifier.
(iv) The output voltage signal is 180° out of phase with the input voltage signal in the common emitter amplifier.

Transconductance (gm):
There is one more term called transconductance (gm) in common emitter mode. It is defined as the ratio of the change in the collector current to the change in the base to emitter voltage at constant collector to emitter voltage. Thus,

$\large g_m = \frac{\Delta i_c}{\Delta V_{BE}}$ ; (VCE = constant)

The unit of gm is Ω-1 or siemen (S).

By simple calculation we can prove that,

$\large g_m = \frac{\beta}{R_{in}}$

Advantages of a transistor over a triode valve
A transistor is similar to a triode valve in the sense that both have three elements. While the elements of a triode are, cathode, plate and grid the three elements of a transistor are emitter, collector and base. Emitter of a transistor can be compared with the cathode of the triode, the collector with the plate and the base with the grid.

Transistor has following advantages over a triode valve
(i) A transistor is small and cheap as compared to a triod valve. They can bear mechanical shocks.
(ii) A transistor has much longer life as compared to a triode valve.
(iii) Loss of power in a transistor is less as it operates at a much lower voltage.
(iv) In a transistor no heating current is required. So, unlike a triode valve, a transistor starts functioning immediately as soon as the switch is opened. In case of valves, they come in operation after some time of opening the switch (till cathode gets heated).

Drawbacks of a transistor over a triode valve:
Transistor have following drawbacks as compared to valves
(i) Since, the transistors are made of semiconductors they are temperature sensitive. We cannot work on transistors at high temperatures.
(ii) In transistors noise level is high. Keeping all the factors into consideration, transistors have replaced the valve from most of the modern electronic devices.

3. In transistors, the base region is narrow and lightly doped, otherwise the electrons or holes coming from the input side (say emitter in CE-configuration) will not be able to reach the collector.

Also Read :

→ Energy Levels & Energy Bands
→ Intrinsic semiconductor & Extrinsic Semiconductor
→ P-N Junction & P-N Junction Diode
→ Junction Diode as Rectifier
→ Zener diode , Photo diode & LED

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