Introduction :
We have seen that V-I characteristics of an active device such as BJT are non-linear. The analysis of a non-linear device is complex. Thus to simplify the analysis of the BJT, its operation is restricted to the linear V-I characteristics around the Q-point i.e. in the active region. This approximation is possible only with small input signals. The term small signal amplifier refers to the use of signal that takes up a relatively small percentage of an amplifier's operational range. With small input signals the transistor can be replaced with small signal linear model. This model is also called small signal equivalent circuit.
We know that the reactance of the capacitance is inversely proportional to the frequency, Zc = 1/2pifC. Thus for low frequencies the reactances of junction capacitances of the transistor are very high. Since these junction reactances appear in parallel with junctions, their effect is ignored at low frequencies and transistor analysis is further simplifies.
Small Signal Low Frequency Transistor Amplifier Circuits:
An amplifier is used to increase the signal level; i.e. the amplifier is used to get a larger signal output from a small signal input. We will assume a sinusoidal signal at the input of the amplifier. At the output, signal must remain sinusoidal in waveform, with frequency same as that of the input.
To make the transistor work as an amplifier, it is to be biased to operate in the active region, i.e. base-emitter junction is to be forward biased, while base-collector junction to be reversed biased.
Let us consider the common emitter amplifier circuit using self bias or voltage divider bias as shown below
In the absence of input signal, only dc voltage are present in the circuit. This is known as zero-signal or no-signal condition or quiescent condition for the amplifier. The dc collector-emitter voltage, Vce, the dc collector current Ic and dc base current Ib is the quiescent operating point for the amplifier. On this dc quiescent operating point, we superimpose ac signal by application of ac sinusoidal voltage at the input. Due to this base current varies sinusoidally.
Since the transistor is biased to operate in the active region, the output is linearly proportional to the input. The output current i.e. the collector current is β times larger than the input base current in common emitter configuration. Hence the collector current will also vary sinusoidally about its quiescent value, Icq. The output voltage will also vary sinusoidal.
The variations in the collector current and the voltage between collector and emitter due to change in the base current are shown graphically with the help of load line in below fig.
We have seen that V-I characteristics of an active device such as BJT are non-linear. The analysis of a non-linear device is complex. Thus to simplify the analysis of the BJT, its operation is restricted to the linear V-I characteristics around the Q-point i.e. in the active region. This approximation is possible only with small input signals. The term small signal amplifier refers to the use of signal that takes up a relatively small percentage of an amplifier's operational range. With small input signals the transistor can be replaced with small signal linear model. This model is also called small signal equivalent circuit.
We know that the reactance of the capacitance is inversely proportional to the frequency, Zc = 1/2pifC. Thus for low frequencies the reactances of junction capacitances of the transistor are very high. Since these junction reactances appear in parallel with junctions, their effect is ignored at low frequencies and transistor analysis is further simplifies.
Small Signal Low Frequency Transistor Amplifier Circuits:
An amplifier is used to increase the signal level; i.e. the amplifier is used to get a larger signal output from a small signal input. We will assume a sinusoidal signal at the input of the amplifier. At the output, signal must remain sinusoidal in waveform, with frequency same as that of the input.
To make the transistor work as an amplifier, it is to be biased to operate in the active region, i.e. base-emitter junction is to be forward biased, while base-collector junction to be reversed biased.
Let us consider the common emitter amplifier circuit using self bias or voltage divider bias as shown below
Since the transistor is biased to operate in the active region, the output is linearly proportional to the input. The output current i.e. the collector current is β times larger than the input base current in common emitter configuration. Hence the collector current will also vary sinusoidally about its quiescent value, Icq. The output voltage will also vary sinusoidal.
The variations in the collector current and the voltage between collector and emitter due to change in the base current are shown graphically with the help of load line in below fig.
Graphical representation of base current, collector current, and
collector-emitter voltage awings
The collector current varies above and below its Q point value in-ohase with the base current, and the collector-to-emitter voltage varies above and below its Q point value 180 degrees out-of-phase with the base voltage.
When one cycle of input is completed, one cycle of output will also be completed. This means the frequency of output sinusoidal is the same as the frequency of input sinusoid. Thus in the amplification process, frequency of the output signal does not change, only the magnitude of the output is larger than that of the input.
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