Electronic Devices and Circuits 3
Lecture 4
The linear amplifier
Voltage Amplification
DC analysis
AC analysis
Voltage gain
Current gain
Power gain
The LinearAmplifier
A linear amplifier provides amplification of a signal without any distortion so that
the output signal is an exact amplified replica of the input signal. A voltage-
divider biased transistor with a sinusoidal ac source capacitively coupled to the
base through C1 and a load capacitively coupled to the collector through C2.
The coupling capacitors block dc and thus prevent the internal source resistance.
ICQ, in phase with the base current. VCEQ, 180o out of phase with the base voltage.
The Linear Amplifier
The ac load line differs from the dc load line because the effective ac collector
resistance is RL in parallel with RC and is less than the dc collector resistance RC
alone.
Comparison of the AC Beta (βac) to the DC Beta (βDC)
For a typical transistor, a graph of IC versus IB is nonlinear. If you pick a Q-
point on the curve and cause the base current to vary an amount then the
collector current will vary an amount as shown in part (b). At different
points on the nonlinear curve, the ratio will be different, and it may
also differ from the ratio at the Q-point. Since the
values of these two quantities can differ slightly.
h Parameters
A manufacturer’s datasheet typically specifies h (hybrid) parameters (hi, hr,
hf, and ho) because they are relatively easy to measure.
The four basic ac h parameters and their descriptions are given in Table.
Relationships of h Parameters and r Parameters
Because datasheets often provide only common-emitter h parameters, the
following formulas show how to convert them to r parameters.
DC analysis
To analyze the amplifier in Figure
AC analysis
To analyze the ac signal operation of an amplifier, an ac equivalent circuit
is developed as follows:
1. The capacitors C1, C2, and C3 are replaced by effective shorts because
their values are selected so that XC is negligible at the signal frequency and
can be considered to be 0 Ώ.
2. The dc source is replaced by ground.
This is why a dc source is called an ac ground.
AC analysis
Signal (AC) Voltage at the Base An ac voltage source, Vs, is shown
connected to the input in Figure (b). If the internal resistance of the ac
source is 0 Ώ ,then all of the source voltage appears at the base terminal.
If, however, the ac source has a nonzero internal resistance, then three
factors must be taken into account in determining the actual signal voltage
at the base.
The total input resistance is expressed by the following formula:
AC analysis
Input Resistance at the Base To develop an expression for the ac input
resistance looking in at the base, use the simplified r-parameter model of
the transistor.
Output Resistance The output resistance of the common-emitter amplifier
is the resistance looking in at the collector and is approximately equal to
the collector resistor.
AC analysis
Voltage Gain
The ac voltage gain expression for the common-emitter amplifier is developed using
the model circuit in Figure.
Attenuation is the reduction in signal voltage as it passes through a circuit and
corresponds to a gain of less than 1.
Voltage Gain
Effect of the Emitter Bypass Capacitor on Voltage Gain
The emitter bypass capacitor, which is C2 in Figure. provides an effective
short to the ac signal around the emitter resistor, thus keeping the emitter at ac
ground.
With the bypass capacitor, the gain of a given amplifier is maximum and
equal to
The value of the bypass capacitor must be large enough so that its reactance
over the frequency range of the amplifier is very small (ideally 0 Ώ )
compared to RE.
Voltage Gain
Voltage Gain Without the Bypass Capacitor
Without the bypass capacitor, the emitter is no longer at ac ground. Instead,
RE is seen by the ac signal between the emitter and ground and effectively
adds to re’ in the voltage gain formula.
The effect of RE is to decrease the ac voltage gain.
Voltage Gain
Effect of a Load on the Voltage Gain
A load is the amount of current drawn from the output of an amplifier or other
circuit through a load resistance. When a resistor, RL, is connected to the
output through the coupling capacitor C3.
The effect of RE is to decrease the ac voltage gain.
Stability of the Voltage Gain
Stability is a measure of how well an amplifier maintains its design values over
changes in temperature or for a transistor with a different β Although bypassing
RE does produce the maximum voltage gain, there is a stability problem because
the ac voltage gain is dependent on depends on IE and on
temperature.
Voltage Gain
Voltage Gain
Current Gain
