A transistor amplifier must have a DC biasing circuit for several reasons. Especially, we will need two separate voltage supply to provide the desired class bias voltage to the emitter-collector and emitter-base. In fact it is actually only done in certain applications, but found that biasing voltage can be obtained separately from a single supply. Second, the transistor is very sensitive to temperature and creates a condition called thermal runaway. Thermal runaway will quickly destroy the bipolar transistor, because the collector current out of control quickly and will improve to the level of damage and the temperature will rise if there is no stabilizing the temperature at the amplifier to eliminate this effect.
A common class for refractive surgery is a Class A, AB, B, and C. All of these classes use the same arrangement of the components to her on the operation of the DC bias transistor Q-point or different.
Location of the variation bias point Q for different amplifier classes
Class A bias on the amplified signal current flows in the form of a full circle, 3600 so that the output signal never reaches saturation or cutoff, so stay on the operation of the linear parameter. The output represents the strengthening of the similarity of the input signal accurately. Because of low efficiency, this class is typically used only for small signal (small-signal) which is not power applications, especially as a low distortion linear amplifier in the RF and IF. Reduction in efficiency occurs because of the DC power needed over time with or without the RF input signal to generate a constant current always flows through through this amplifier.
Class AB bias is obtained by lowering a little Q-point on the amplifier. Efficiency is slightly higher than Class A due to the static output current (Ic) flowing through the amplifier will be smaller and its shape is not a full circle, usually 3000 for power amplifier applications. But the single-ended power amplifier class AB will produce more distortion than Class A because of the clipping on the output. Class AB is also a common bias for push-pull audio power amplifier and linear RF power amplifier is a push-pull
Class B has a very high level of efficiency. Currently there is no input signal, the power dissipation that occurs close to zero. This is because almost no collector current flows, because the bias is reduced to cope with the slightest connection (junction) 0.6 V base-emitter junction. The output signal is close to 1800 in which this condition occurs because the RF signal is a half circle forward bias on the basis of the semi-circle while the other is the reverse bias on the emitter-base, causing a reduction in output signal.
Class C amplifiers are even more efficient than Class B because it only consumes less leakage current when no RF input signal. When the input signal is given, the class C amplifier will be strengthened less than a half circle and will only supply a pulse at its output terminal. Konduksinya angle of 1200 or less because of the connection (junction) emitter-base reversed a bit of bias
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