TC 9-62
2-36. By inserting one or more resistors in a circuit, different methods of biasing may be
achieved and the emitter-base battery eliminated. In addition to eliminating the battery,
some of these biasing methods compensate for slight variations in transistor characteristics
and changes in transistor conduction resulting from temperature irregularities. Notice in
Figure 2-14 that the emitter-base battery has been eliminated and the bias resistor (RB) has
been inserted between the collector and the base. Resistor RB provides the necessary
forward bias for the emitter-base junction. Current flows in the emitter-base bias circuit
from ground to the emitter, out the base lead, and through RB to VCC. Since the current in
the base circuit is very small (a few hundred microamperes) and the forward resistance of
the transistor is low, only a few tenths of a volt of positive bias will be felt on the base of
the transistor. However, this is enough voltage on the base, along with ground on the
emitter and the large positive voltage on the collector, to properly bias the transistor.
2-37. With Q1 properly biased, DC flows continuously, with or without an input signal,
throughout the entire circuit. The DC flowing through the circuit develops more than just
base bias; it also develops the collector voltage (VC) as it flows through Q1 and RL. Notice
the collector voltage on the output graph. Since it is present in the circuit without an input
signal, then the output signal starts at the VC level and either increases or decreases. These
DC voltages and currents that exist in the circuit prior to the application of a signal are
known as QUIESCENT voltages and currents (the quiescent state of the circuit).
2-38. Resistor RL, the collector load resistor, is placed in the circuit to keep the full
affect of the collector supply voltage off the collector. This permits the collector voltage
(VC) to change with an input signal, which in turn allows the transistor to amplify voltage.
Without RL in the circuit, the voltage on the collector would always be equal to VCC.
2-39. The coupling capacitor (CC) is another new addition to the transistor circuit. It is
used to pass the AC input signal and block the DC voltage from the preceding circuit. This
prevents DC in the circuitry on the left of the coupling capacitor from affecting the bias on
Q1. The coupling capacitor also blocks the bias of Q1 from reaching the input signal
source.
2-40. The input to the amplifier is a sine wave that varies a few millivolts above and
below zero. It is introduced into the circuit by the coupling capacitor and is applied
between the base and emitter. As the input signal goes positive, the voltage across the
emitter-base junction becomes more positive. This in effect increases forward bias that
causes base current to increase at the same rate as that of the input sine wave. Emitter and
collector currents also increase, but much more than the base current. With an increase in
collector current, more voltage is developed across RL. Since the voltage across RL and the
voltage across Q1 (collector to emitter) must add up to VCC, an increase in voltage across
RL results in an equal decrease in voltage across Q1. Therefore, the output voltage from the
amplifier, taken at the collector of Q1 with respect to the emitter, is a negative alternation
of voltage that is larger than the input, but has the same sine wave characteristics.
2-41. During the negative alternation of the input, the input signal opposes the forward
bias. This action decreases base current, which results in a decrease in both emitter and
the voltage across the transistor to rise along with the output voltage. Therefore, the output
for the negative alternation of the input is a positive alternation of voltage that is larger
than the input, but has the same sine wave characteristics. By examining both input and
output signals for one complete alternation of the input, we can see that the output of the
2-14
TC 9-62
23 June 2005
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