Effect of an
oad on the
ain of a
Since the invention of CMOS circuits in 1963, active load amplifiers have
become more popular
in circuit design
than resistive load
amplifiers. One advantage of
active load circuits is that the power dissipated in
the circuit is reduced.
his experiment focuses on a second advantage which is characteristic
a specific type
of active load circuits: differential amplifiers
. The experiment shows that using an active
load differential amplifier over a resistive load amplifier results in a significant increase
In this experiment, we
build two topologies of differential amplifier
; one is the resistive load amplifier topology, and the other is the active load
Replacing the resistive load with an active load has the effect of
eliminating the collector resistance in the amplifier
replacing it with a
n open circuit
The gain of a typical differential amplifier
similar to the one used in this experiment
is proportional to the collector resistance
. The following formula is an expression of
the gain of the amplifier:
the collector resistance and
This means that in
an ideal case, the open loop gain of an active load amplifier is infinite
as opposed to the
gain of a resistive load amplifier which is finite.
In reality, BJT tra
nsistors have some fabrication constraints
that have the effect of
limiting the response of the amplifier
second order effects.
emitter and base
each device, and
have the effect
ing down or expanding the bandwidth of the amplifier.
also known as “Early” resistance
limits the mid
gain of the
. This resistance is the reason why there can be no infinite gain
in the active load
, as shown in the below formula:
The purpose is to show that replacing the active load with a
sistive load results
in a significant increase in the open loop gain of the
The apparatus used in this experiment consists
An oscilloscope, used to plot the output versus input response
the amplifier and calculate its gain. In this experiment, both analog and digital
oscilloscopes were used.
oscilloscopes are preferable because they
s on the plot, and users have the ability to export a picture of the
curves for further analysis.
A voltmeter, used for making sure that the operating point voltage
of the bipolar transistor is exactly 10 volts.
potentiometer, used in a voltage divider configuration to create a 10 volts
from the 15 volts supply.
NPN Bipolar transistors: two transistors are used in a “current sink”
configuration to establish a stable collector current;
transistors are the main
transistors of the differential amplifier circuit
Two PNP Bipolar transistors are
used to replace the resistive load and build an active
KΩ resistors are used as our resistive load resistors.
circuit elements such as coupling and de
bias resistors are also used to complete
the amplifier circuit.
The experiment consists
of building the amplifier circuit in Figure 1 and making
ements to calculate the open loop gain of the amplifier.
The open loop gain
or common mode
rejection ratio for a differential amplifier
good measure of the performance of the
amplifier as opposed to the closed loop gain,
because it is independen
t of the
attached to the amplifier. In this experiment, by
resistive “load” and
the amplifier’s internal loads
from external load
The closed loop gain of an amplifier is a
function of the exter
nal load attached to the
system; it is thus less suitable to use the closed
of an amplifier
Am灬ifier c潮fi杵ration use搠in
Calculating the open loop gain or common mode rejection ratio (
) of an amplifier
is a two
ess; the first step consists of calculating the differential gain of the
, which is
obtained when the first input
the amplifier is
connected to an AC voltage source, and the second input is connected to ground.
we calculate the common mode gain,
, which is obtained when the two
input terminals are connected to the same AC voltage source.
used to calculate the open loop gain of the amplifier:
he differential gain, and A
is the common mode gain.
resistive load amplifier circuit of Figure 1 is first built, with
terminal connected to an AC voltage source, and the
input terminal connected to
Then, the transfer curve on the oscilloscope is used to calculate the differential
gain of the amplifier. Second, both input terminals are connected to the AC source, and
the transfer curve is used to measure the common mode gain. Finally, these two
information are used to evaluate the open loop gain of the amplifier.
The same procedure is repeated for the active load differential amplifier, which is
built by replacing the resistors R2 and R3 from Figure 1 with two transistors configured
Figure 3 shows a typical differential
gain transfer curve
(second input terminal
connected to ground)
, obtained using a digital
oscilloscope. The differential gain can be
calculated using the following formula:
oe灬aci湧 the resistive l潡搠with
an active l潡d
Ty灩cal 摩ffere湴ial gain transfer
Similarly, Figure 4 shows a typical
mode gain transfer curve (both input terminals
connected to AC source signal).
mode gain is the slope of the linear curve in
Table 1 shows the results of our
measurements, as well as the calculated gain values for
each differential amplifier topology built.
Results of measurements
The purpose of the experiment was to
in a typical differential amplifier
circuit, replacing the resistive load with an active load results in a significant increase in
open loop gain. We can see in Table 1 that the active load has the effect of m
e gain by a factor of about 10. This proves that active loads provide much greater gain
than resistive loads, besides consuming less power.
This is clearly
one of the
active load circuit design has completely replaced the alternati
ve in commercial
Ty灩cal c潭m潮 m潤o gain