From Electronics I Laboratory Manual

A common-emitter amplifier1 in general will include three coupling capacitors that play an important role in the low-frequency response of the system.  These capacitors are the input and output coupling capacitors2 and the emitter biapass capacitor3.

Figure 19.1 shows a typical common-emitter amplifier with a source, Vs (including its source resistance, Rs) and a load, RL.  The low-frequency response of this amplifier is determined in part by these three capacitors (Ci, Co and CE)The result is essentially a combination of three high-pass filter networks that allow signals having frequencies greater than the cutoff frequency of the dominant network to pass through while attenuating all others.  This frequency is the so-called 3-dB point.  At this frequency, as you should know, the voltage is only 70.7% of the mid-frequency gain, or in decibels, it is the frequency at which the gain is 3-dB below the mid-frequency gain.

Each one of the three capacitors makes a contribution to the overall frequency response of the amplifier.  Each one behaves like the capacitor in a high–pass filter.  Therefore, each one will contribute with a cutoff frequency of its own. 

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Power Amplifiers
Power amplifiers deliver a specific amount of AC power to a load. They are used in audio frequency and radio frequency applications.
Gate Turn-off (GTO) Thyristors
Gate turn-off thyristors (GTOs) are four-layer PNPN devices that act as switches, rectifiers, and voltage regulators. Like other thyristors, GTOs can be turned on by the application of a positive gate signal (g > 0); however, unlike other more conventional devices that can be turned off only at a zero crossing of current, GTOs can be turned off at any time by the application of a gate signal equal to zero.
Unijunction Transistors (UJT)
Unijunction transistors (UJT) are three-terminal devices that exhibit a negative resistance characteristic.
Diacs are bi-directional diodes that switch AC voltages and trigger silicon controlled rectifiers (SCRs) and triacs. Except for a small leakage current, diacs do not conduct until the breakover voltage is reached.

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