amplifier is an exact reproduction of the input except for the reversal in polarity and the
increased amplitude (a few millivolts as compared to a few volts).
2-42. Figure 2-14 shows the PNP version of the basic transistor amplifier. The primary
difference between the NPN and PNP amplifier is the polarity of the source voltage. With a
negative VCC, the PNP base voltage is slightly negative with respect to ground, which
provides the necessary forward bias condition between the emitter and base.
2-43. When the PNP input signal goes positive, it opposes the forward bias of the
transistor. This action cancels some of the negative voltage across the emitter-base junction
that reduces the current through the transistor. Therefore, the voltage across the load
resistor decreases and the voltage across the transistor increases. Since VCC is negative, the
voltage on the collector (VC) goes in a negative direction (as shown on the output graph)
toward -VCC (for example from -5 volts to -7 volts). Therefore, the output is a negative
alternation of voltage that varies at the same rate as the sine wave input but is opposite in
polarity and has a much larger amplitude.
2-44. During the negative alternation of the input signal, the transistor current increases
because the input voltage aids the forward bias. Therefore, the voltage across RL increases,
and consequently, the voltage across the transistor decreases or goes in a positive direction
(for example from -5 volts to -3 volts). This action results in a positive output voltage,
which has the same characteristics as the input except that it has been amplified and the
polarity is reversed.
2-45. The input signals in the preceding circuits were amplified because the small
change in base current caused a large change in collector current. By placing resistor RL in
series with the collector, voltage amplification was achieved.
TYPES OF BIAS
2-46. One of the basic problems with transistor amplifiers is establishing and
maintaining the proper values of quiescent current and voltage in the circuit. This is
accomplished by selecting the proper circuit-biasing conditions and ensuring these
conditions are maintained despite variations in ambient (surrounding) temperature, which
cause changes in amplification and even distortion (an unwanted change in a signal).
Therefore, a need arises for a method to properly bias the transistor amplifier and at the
same time stabilize its DC operating point (the no signal values of collector voltage and
collector current). As mentioned earlier, various biasing methods can be used to
accomplish both of these functions. Although there are many biasing methods, only the
three basic types (base-current bias [fixed bias], self-bias, and combination bias) will be
Base-Current Bias (Fixed Bias)
2-47. The first biasing method, called BASE-CURRENT BIAS or sometimes FIXED
BIAS, was used in Figure 2-14. This method consisted basically of a resistor (RB)
connected between the collector supply voltage and the base. Unfortunately, this simple
arrangement is quite thermally unstable. If the temperature of the transistor rises for any
reason (due to a rise in ambient temperature or due to current flow through it), collector
current will increase. This increase in current also causes the DC operating point,
sometimes called the quiescent or static point, to move away from its desired position
(level). This reaction to temperature is undesirable because it affects amplifier gain (the
number of times of amplification) and could result in distortion.
23 June 2005