Digital Circuit Design Course
LESSON 5:
Emitter Follower Design
Editor 's Note : The material below is a serialization of the Kurz Kasch correspondence course for electronics, designed
spec ifically for the coin -operated amusemen~ industry. Th is course is copyrighted and. owned_ by Kur~ K_asch of f?ayto n,
Oh io and its reprinting is being sponsored jotr_Jtly by Kurz Kasch and Play Meter magaztne. Th1s matenal 1s authonzed fo r
publication exclusively by Play Meter magaztne.
The operation and design of
emitter followers is the topic of
this lesson . The circuit is
analyzed· both mathematically
and graphically.
Definition: An emitter follower. or
common collector circuit, is a
circuit configuration in which the
output is from the emitter to
ground. In the case of the
inverter, or common emitter
circuit of Lesson Four, the output
was taken from the collector to
ground . The emitter follower is
used primarily as a current ampli-
fier. It is used in digital circuits to
drive high current devices, such
as lamps and relays , and also to
drive low impedance trans-
mission lines.
Emitter Follower Operation:
Figure 5-1 is a schematic diagram
of an emitter follower. Notice that
the collector is connected
directly to Vee and the resistor
connecting the emitter to ground .
The input to an emitter follower is
from base to ground, and the out-
put is from emitter to ground. The
circuit operates as follows:
During time, t less than 0, the
input is at a low level (OV). the
base is then zero biased, and the
only current flowing is the
leakage current. The leakage
current is too small to cause a
voltage drop across RE worth
accounting for; therefore, all of
the voltage is across the transis-
tor, collector to emitter. This
operating condition is graphically
shown at point A on the emitter-
follower load load line of fig. 5-2.
At point A, VeE is approximately
equal to Vee and le is ap-
proximately zero. Because all of
the voltage is across the transis-
tor, the voltage across RE must
then be zero .
12
The base-emi tter diode will begin to
forward bias at the knee in the curve
of fig . 5-2, and this point is termed
VB E(cu tin). As a general rule-for
silicon transistors VBE(cutin ) = 0.5 V
and for germanium transistors :
= 0 .1
VBE (cut in)
V.
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IIIIHII
II\ ~ J
I • II
Our pol
~v"
ov 1· 0 1 -
F ig . 5· 1. An em iUtr fo llower chc~o~l t . The Input and output
w avef orms are In phase .
.
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At t = 0 the voltage [Vin] rises from
OV to a level high enough to
forward-bias the base emitter
diode, a current begins to flow in
the emitter circuit [IE]. IE causes
a voltage drop across RE which
subtracts from VeE. The voltage
dropped across RE will continue
to rise with the input voltage and
continue to subtract from
VeF. Let us assume that Vin has
risen to Vin3 . This operating
condition is represented at point
B. Projecting the voltage inter-
cept, we find that VeE has de-
creased and that VE has in-
creased proportionately . The
current le can be found by pro-
jecting the current intercept.
Also , notice that le has increased .
When the input rises to the steady
voltage V, the emitter voltage will
also reach a steady state voltage.
This operating cond ition is
shown at point C. Notice that VE
has risen to almost Vee and that
VeE is almost zero. The steady
state voltage across the em itter
resistor [VE] will be equal to the
input voltage minus the voltage
base to emitter, or
VE
\ f' .. 0
\ Ct-: • 0
F ig . 5·2. The family of Input -out put t tates fOf an emi tter ·
foll ower . The output voltage W i ng the Int ercept of the load
lint wllh a partlc u ler V 111 cwrve .
VE
O urpul
Fie. 54. Aneqw lvel"t c lreu lt of tM Input circu it of en tm l lttr·
follower . The output volt ... le Ye t: Ieee t"-n the Input volta .. .
= Vin -VBE
An equivalent circuit of the
base-emitter relationship is
shown in fig . 5-3. The diode
represents the base-emitter
diode. This illustrates clearly why
the o·utput voltage is VBE less than
the input voltage.
Also, note from fig. 5-1 that the
input and output voltages are in
phase.
Collector Dissipation:
Because the transistor is
basically a resistive device, as
current flows there will be dissi-
pation and , therefore, heat. The
effects of heat regarding VF and lA
fora diode and regarding leo fora
PLAY METER, May 1, 1981