THE COMBINATION
15
COMMON-EMITTER AMPLIFIER
AND EMITTER-FOLLOWER
PURPOSE AND BACKGROUND
The purpose of this experiment is to demonstrate the operation of
a combination common-emitter amplifier and emitter-follower cir-
cuit. This type of circuit, sometimes referred to as a phase-splitter
or a paraphase amplifier, produces two identical output signals to
identical loads, except that they are 180° out-of-phase with each
other. The output signal from the collector is simply a common-
emitter amplifier whose voltage gain is 1 in addition to being 180°
out-of-phase with the input signal. The output signal is from the
emitter-follower and is in-phase with the input signal.
Text References: 6-2, Transistor AC Equivalent Circuits; 6-3,
Common-Emitter Amplifiers.
REQUIRED PARTS AND EQUIPMENT
Resistors (1/4 W): [0 2N3904 npn silicon transistor
O Four 1 kQ [ 0-15 V dc power supply
O Two 10 kQ [ Signal generator
Capacitors (25 V): [0 VOM or DMM (preferred)
O Two 2.2 uF [J Dual trace oscilloscope
O 100 uF O Breadboarding socket
137
USEFUL FORMULAS
Voltage gain from base to collector
1) A, = Uout1
Uin
(2) A, Rc | Rp
(Rg | Re2) +re
Voltage gain from base to emitter
@) A, = P2
Vin
Re | Rz
@) A, = _RBs||Rip
Y Re||Rio) +re
Transistor ac emitter resistance (at normal room temperature)
5) ro= 25 mV
Ig
Quiescent dc base voltage
Ry
(6)
6) Vg Vg = (R1+Rz) Ve cc
Quiescent dc emitter voltage
() Vg = VB — Vb
Quiescent dc emitter current
®) Ig = Ye (I¢ = Ig for large B)
Re
Quiescent dc collector voltage
9) Ve = Vec — IcRe
PROCEDURE
1. Wire the circuit shown in Figure 15-1, omitting the signal gen-
erator and the power supply.
2. After you have checked all connections, apply the 15-V supply
voltage to the breadboard. With a VOM or DMM, individually
measure the transistor dc base, collector, and emitter voltages
with respect to ground, recording your results in Table 15-1.
Based on the resistor values of Figure 151, determine the ex-
pected values of these three voltages (Equations 6, 7, and 9),
assuming a base-emitter voltage drop of 0.7 V, and compare
them with the measured values in Table 15-1.
3. Connect Channel 1 of your oscilloscope to point I (vi,) and Chan-
138 nel 2 to point OA (voy1). Then connect the signal generator to
the circuit as shown in Figure 15-1, and adjust the sine wave
output level of the generator at 0.5 V peak-to-peak at a frequency
of 5 kHz.
+15V
FIGURE 15-1 Schematic diagram of circuit.
You should observe that the output signal level (vout1) is the
same as the input level (viy). In addition, there is a 180° phase
shift between the output and input signals.
4. Using the measured value for the dc emitter voltage obtained
in Step 2, calculate the dc emitter current (Equation 8) and the
resultant ac emitter resistance, r. (Equation 5). Record these
values in Table 15-2.
Measure the ac peak-to-peak voltage across the 1-k() load re-
sistor (Rz;) with the oscilloscope. Calculate the expected volt-
age gain from base to collector using Equation 2 in the “Useful
Formulas” section of this experiment, and record this value in
Table 15-3. Now measure the actual voltage gain by dividing
the peak-to-peak output voltage voun by the peak-to-peak input
voltage vi, (Equation 1), recording your result in Table 15-3.
Measure the ac peak-to-peak voltage across the 1-k() load re-
sistor (Rz2) with the oscilloscope. Calculate the expected volt-
age gain from base to emitter using Equation 4 in the “Useful
Formulas” section of this experiment, and record this value in
Table 15-3. Now measure the actual voltage gain by dividing
the peak-to-peak output voltage vouz by the peak-to-peak input
voltage vin (Equation 3), recording your result in Table 15-3. 139
7. Connect Channel 1 of your oscilloscope to point OA (veu1) and
Channel 2 to point OB (vgyu2). You should observe that both
output signal levels are the same as the input level, except for
a 180° phase shift between them.
WHAT YOU HAVE DONE
This experiment demonstrated the operation and characteristics of
a combination common-emitter amplifier and emitter-follower cir-
cuit, which is sometimes referred to either as a phase splitter or a
paraphase amplifier. This type of circuit produces two identical out-
put signals to identical loads, except that they are 180° out-of-phase
with each other.
140
Name Date
THE COMBINATION COMMON-EMITTER AMPLIFIER
AND EMITTER-FOLLOWER
OBJECTIVES/PURPOSE:
SCHEMATIC DIAGRAM:
©1992 Macmillan Publishing Company. Al rights reserved 141
Name Date
DATA FOR EXPERIMENT 15
TABLE 15-1
Parameter | Measured Value | Expected Value | % Error
Vi
Ve
Ve
Parameter Value
Ig (calculated)
re (calculated)
TABLE 156-3
Phase Measured Expected
Output Vin Vout Shift Gain Gain % Error
0A
OB
142 ©1992 Macmillan Publishing Company. Al rights reserved.
Name Date
RESULTS AND CONCLUSIONS:
REVIEW QUESTIONS FOR EXPERIMENT 15
1. The voltage gain at either output for the phase-splitter circuit
of Figure 15-1 is
(a) significantly less than 1 (b) essentially equal to 1
(c) significantly greater than 1
The two output signals are out-of-phase with each other by
(@ 0° (b) 45° (c) 90° (d) 180°
If Ry in the circuit of Figure 15-1 is omitted, vouu
(a) increases significantly (b) decreases significantly
(c) remains essentially the same
If Ry, in the circuit of Figure 15-1 is omitted, voutz
(a) increases significantly (b) decreases significantly
(c) remains essentially the same
5. If R, in the circuit of Figure 15-1 is omitted, vout1
(a) increases significantly (b) decreases significantly
(c) remains essentially the same
©1992 Macmillan Publishing Company. All rights reserved 143
