Nicholas Wetherell
Aug 24, 2022
TA: Azzam Qureshi
Instructor: Jorge Hurtarte
ECE 171L
Experiment 5: Differential Amplifiers
Introduction and Background:
In this lab we will investigate the characteristics of a BJT differential amplifier biased
with a Widlar current mirror. The use of a differential amplifier is to reject signals that are
common between the two inputs applied at the base of both transistors, but amplify the
differential signals. The amplified output signal vo will be measured between the collector
voltages VC3 and VC4.. (Figure 1) A differential amplifier functions most ideally when it is
perfectly symmetrical, therefore in this experiment we use a CA3046 IC to provide a
differential pair of transistors for the amplifier and two additional transistors for the
Widlar current mirror.
Experimental Section:
The circuit in figure 1 was built on a breadboard. The resistor RC3 was measured to
be 9.90 kΩ, RC4 was measured to be 9.89 kΩ, and the resistors in the current mirror were
measured to be 3.83 kΩ at the reference input and 99.2 Ω at the output emitter. The DC
voltages were supplied VCC = 12 V and VEE = -10 V. They were decoupled with a 0.1 uF
capacitor connecting the supply rails to ground.
A special op amp circuit was also constructor in order to provide a small-signal
differential input to the bases of the transistors in the current mirror (Vb1 and Vb2). The
circuit was constructed as shown in figure 2.
Results and Discussion:
Part 1:
Currents were measured by taking voltage measurements and applying Ohm’s law.
The reference current IREF was measured to be 4.995 mA. The output current Io was
measured to be 0.670 mA. The collector current of transistor Q3 IC3 was measured to be
0.335 mA. The collector current of transistor Q4 IC4 was measured to be 0.335 mA.
Part 2:
The quiescent collector voltages VC3, and VC4 were measured to be 6.64 V and 6.65 V
respectively. The DC output offset voltage (Vo) was measured to be 0.01 V.
Part 3:
Two AC signal generators were used to add an AC signal between the base and
ground of each of the transistors in the differential amplifier. The amplitude of the AC
signals was varied and the voltages VC3, VC4, and Vo were measured and became the basis of
figure 3. The average between VC3 and VC4 was taken at each point to give VCM, the common
mode input voltage. The common mode gain of the amplifier Acm, was determined by taking
the ratio of Vo over Vcm. The curve in figure 3 represents the common mode gain (Acm) as a
function of the common mode input voltage (Vcm).
∂𝑉𝐶3
∂𝑉𝐶𝑀
∂𝑉𝐶4
and ∂𝑉 were determined by plotting VC3 vs VCM and applying a linear regression as
𝐶𝑀
∂𝑉𝐶3
∂𝑉𝐶4
shown in figure 4. ∂𝑉 = 1.00008 and ∂𝑉 = 0.99983. They each practically responded 1
𝐶𝑀
𝐶𝑀
to 1 with the common mode voltage.
Part 4.
The voltage responses of VC2, VBE3, VBE4, VC3, and VC4 with varying DC voltage applied
to VCM were measured and the results are shown in figure 5.
It can be observed that VBE3, VBE4, and VC2 all increased linearly from approximately 0
with increasing VCM . VC3 and VC4 remained constant at approximately 6.66 Volts until VCM
reached 7 volts. Then VC3 and VC4 intersected VC2 and began to increase linearly as well.
Part 5.
The small signal input generator circuit was used to apply a differential input vd to
the node that connects the bases of Q1 and Q2. vd was varied and the differential mode
voltage gain was measured. (Figure 6)
From the linear portion of this figure the gain Add can be determined to be
approximately 370 via linear regression.
The amplifier’s transfer characteristics were then plotted as a function of branch
current (IC3) over total current (I). (Figure 7) It can be observed that the branch current
(IC3) over the total current (I) increases linearly with input signal voltage at a rate of
0.144/V.
𝐶𝑀𝑅𝑅 = |
𝐴𝑑𝑑/2
𝐴𝑐𝑐
| = (370/2)/. 01 = 18500
eq 1.
The common mode rejection ratio of the amplifier could be determined from the
common mode gain and differential mode gain according to equation 1.
Conclusion:
In this experiment a differential amplifier was successfully designed and its common
mode and differential mode characteristics were experimentally determined. The
differential mode gain, common mode gain, and CMRR were all successfully determined
experimentally.