2-18
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.
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 collector currents. The decrease in
current through R
L
decreases its voltage drop and causes 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 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).
The PNP version of this amplifier is shown in the upper part of the figure. 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.
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, which 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). Thus, the output is a negative
alternation of voltage that varies at the same rate as the sine wave input, but it is opposite in polarity and has
a much larger amplitude.
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.
In summary, the input signals in the preceding circuits were amplified because the small change in base
current caused a large change in collector current. And, by placing resistor RL in series with the collector,
voltage amplification was achieved.
Q14. What is the name of the device that provides an increase in current, voltage, or power of a signal
without appreciably altering the original signal?
Q15. Besides eliminating the emitter-base battery, what other advantages can different biasing methods
offer?
Q16. In the basic transistor amplifier discussed earlier, what is the relationship between the polarity of the
input and output signals?
Q17. What is the primary difference between the NPN and PNP amplifiers?
TYPES OF BIAS
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)
