The Common-Collector Amplifier
The common-collector amplifier is usually referred
to as the emitter follower because there is no phase
inversion or voltage gain. The output is taken from
the emitter. The common-collector amplifier’s main
advantages are it’s high current gain and high input
resistance.
The Common-Collector Amplifier
The input resistance can be
determined by the simplified
formula below.
Rin(base) ≅ βac(r’e + Re)
Rin(tot) = Rin(base)//R1 // R2
Output Resistance is very
low:
Rout ≅ RE (Rs /βac)
The Common-Collector Amplifier
The Voltage Gain can be determined by the simplified
formula below.
Av = Re/(r’e + Re)
and is always less than 1 but
if r’ e << Re then Av ≅ 1 is a good approximation.
The current Gain:
Ai ≅ βac with
R1 // R2 >> Reβac
The Power Gain:
Ap = AvAi ≅ Ai
with
Av ≅ 1
Example
Example
Example 6.9: Calculate Input Resistance, Voltage Gain,
Current Gain and Power Gain for the given circuit.
DC analysis: VE = R2 /(R1+R2 ) - VBE = 4.3V
DC analysis: IE = VE /RE = 4.3mA
AC Analysis: r’e =25mV/IE = 5.8
Example
Example 6.9: Calculate Input Resistance, Voltage Gain,
Current Gain and Power Gain for the given circuit.
Re = RE // RL : 1k//1k = 500
Rin(base) ≅ βac(r’e + Re) but r’e is just 5.8
so Rin(base) ≅ βac(Re) = 175(500) = 87.5k
Rin(tot) = Rin(base)//R1 // R2 = 8.16k
Example
Example 6.9: Calculate Input Resistance, Voltage Gain,
Current Gain and Power Gain for the given circuit.
Av = Re/(r’e + Re) = 500/505.8 = 0.989
Ai = Ie/Iin = (Ve/Re)/(Vin/Rin(tot))=
= (VbAv/500)/(1/8.16k)
= 2mA/123A = 16.3
Conclusion
The output resistance is very low. This makes
it useful for driving low impedance loads.
The current gain(Ai) is approximately βac.
The voltage gain is approximately 1.
The power gain is approximately equal to
the current gain(Ai).
The Common-Collector Amplifier
The darlington pair is
used to boost the input
impedance to reduce
loading of high output
impedance circuits. The
collectors are joined
together and the emitter
of the input transistor is
connected to the base of
the output transistor.
The input impedance can
be determined the
formula below.
Rin = βac1βac2Re
Tutorial and Assignment
• Solve Example 6.9 and 6.10 and send to me on
Wednesday before class.
• Tutorial II : Qs 18 to 23 pages 319-320. To be
discussed on Thursday.
The Common-Collector Amplifier