Lecture Notes for Analog Electronics - The Electronic Universe ...

Typically, we can consider v or i to be sinusoidal functions, e.g. v(t) =vocos(ωt + φ), and
their amplitudes vo and io (sometimes also written as v or i when their is no confusion) are
small compared with V0 or I0, respectively.
5.3 Emitter Follower
The basic emitter follower configuration is shown below in Figure 19. An input is fed to
the base. The collector is held (by a voltage source) to a constant DC voltage, VCC. The
emitter connects to a resistor to ground and an output. As we shall see, the most useful
characteristic of this circuit is a large input impedance and a small output impedance.
Vin
Vcc
R
Vout
Figure 19: Basic emitter follower.
For an operating transistor we have Vout = VE = VB − 0.6. Hence, vout = vE = vB. From
this, we can determine the voltage gain G, equivalent to the transfer function, for the emitter
follower:
G ≡ vout/vin = vE/vB = 1 (22)
From Eqn. 20, IE =(β+1)IB⇒iE =(β+1)iB. Therefore, we see that the follower exhibits
“current gain” of output to input equal to β + 1. Assuming the output connection draws
negligible current, we have by Ohm’s Law iE = vE/R. Using this in the previous expression
and solving for iB gives iB = iE/(β +1)=(vB/R)/(β + 1). Now we can define the input
impedance of the follower:
Zin = vin/iin = vB/iB = R(β + 1) (23)
By applying the Thevenin definition for equivalent impedance, we can also determine the
output impedance of the follower:
Zout = vin/iE = =
(β +1)iB
Zsource
(24)
β +1
where Zsource is the source (i.e. output) impedance of the circuit which gave rise to vin.
Hence, the emitter follower effectively increases input impedance (compared to R) bya
factor β +1≈100 and reduces output impedance, relative to that of the source impedance
of the previous circuit element, by a factor β +1≈100. We will return to this point next
time.
23
vin