This material is intended for students who want to learn about the Emitter Follower Circuit and its working.

1. Emitter Follower Circuit
-Piyush Charan
Assistant Professor,
Deptt. Of ECE,
Integral University, Lucknow
Unit 2: Continued......
9/26/2019 1
Course on "Electronics Devices and Circuits" by
Deptt of ECE, Integral University, Lucknow.
Facilitator: Piyush Charan

2. Emitter Follower Circuit
•The Emitter follower circuit
has a unity Gain and
approximately follows the
input Current.
•Unlike other amplifier circuits
the output where the output
is taken across the collector
terminal, the output in a
emitter follower circuit is
taken across the Emitter
terminal.
9/26/2019 2
Course on "Electronics Devices and Circuits" by
Deptt of ECE, Integral University, Lucknow.
Facilitator: Piyush Charan

3. Basics of Emitter Follower Ckt.
• Emitter follower circuit has a prominent place in feedback
amplifiers. Emitter follower is a case of negative current
feedback circuit. This is mostly used as a last stage
amplifier in signal generator circuits.
• The important features of Emitter Follower are −
– It has high input impedance
– It has low output impedance
– It is ideal circuit for impedance matching
• All these ideal features allow many applications for the
emitter follower circuit. This is a current amplifier circuit
that has no voltage gain.
9/26/2019 3
Course on "Electronics Devices and Circuits" by
Deptt of ECE, Integral University, Lucknow.
Facilitator: Piyush Charan

4. Construction
• The constructional details of an emitter follower circuit are
nearly similar to a normal amplifier. The main difference is
that the load RL is absent at the collector terminal, but
present at the emitter terminal of the circuit. Thus the
output is taken from the emitter terminal instead of
collector terminal.
• The biasing is provided either by base resistor method or
by potential divider method. The following figure shows
the circuit diagram of an Emitter Follower.
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Course on "Electronics Devices and Circuits" by
Deptt of ECE, Integral University, Lucknow.
Facilitator: Piyush Charan

5. Operation of Emitter Follower
• The input signal voltage applied between base and
emitter, develops an output voltage Vo across RE, which
is in the emitter section. Therefore,
• The whole of this output current is applied to the input
through feedback. Hence,
9/26/2019
Course on "Electronics Devices and Circuits" by
Deptt of ECE, Integral University, Lucknow.
Facilitator: Piyush Charan
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6. Operation of EF Ckt. Contd..
• As the output voltage developed across RL is proportional
to the emitter current, this emitter follower circuit is a
current feedback circuit. Hence,
• It is also noted that the input signal voltage to the
transistor (= Vi) is equal to the difference of Vs and Vo i.e.,
• Hence the feedback is negative.
9/26/2019
Course on "Electronics Devices and Circuits" by
Deptt of ECE, Integral University, Lucknow.
Facilitator: Piyush Charan
6

7. Characteristics
• The major characteristics of the emitter
follower are as follows −
– No voltage gain. In fact, the voltage gain is
nearly 1.
– Relatively high current gain and power gain.
– High input impedance and low output
impedance.
– Input and output ac voltages are in phase.
9/26/2019
Course on "Electronics Devices and Circuits" by
Deptt of ECE, Integral University, Lucknow.
Facilitator: Piyush Charan
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8. Voltage Gain of Emitter Follower
• As the Emitter Follower circuit is a prominent one, let us
try to get the equation for the voltage gain of an emitter
follower circuit. Our Emitter Follower circuit looks as
follows −
9/26/2019
Course on "Electronics Devices and Circuits" by
Deptt of ECE, Integral University, Lucknow.
Facilitator: Piyush Charan
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9. Voltage Gain of Emitter Follower
• If an AC equivalent circuit of the above circuit is drawn, it
would look like the below one, as the emitter by pass
capacitor is absent.
9/26/2019
Course on "Electronics Devices and Circuits" by
Deptt of ECE, Integral University, Lucknow.
Facilitator: Piyush Charan
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10. Voltage Gain of Emitter Follower
• The AC resistance rE of the emitter circuit is given by
• Where,
9/26/2019
Course on "Electronics Devices and Circuits" by
Deptt of ECE, Integral University, Lucknow.
Facilitator: Piyush Charan
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11. Voltage Gain of Emitter Follower
• In order to find
the voltage gain of
the amplifier, the
above figure can
be replaced by the
following figure.
9/26/2019
Course on "Electronics Devices and Circuits" by
Deptt of ECE, Integral University, Lucknow.
Facilitator: Piyush Charan
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12. Voltage Gain of Emitter Follower
• Note that input voltage is applied across the ac resistance
of the emitter circuit i.e., (r’E + RE). Assuming the emitter
diode to be ideal, the output voltage Vout will be
• Input voltage Vin will be
• Therefore, the Voltage Gain of emitter follower is
9/26/2019
Course on "Electronics Devices and Circuits" by
Deptt of ECE, Integral University, Lucknow.
Facilitator: Piyush Charan
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13. Voltage Gain of Emitter Follower
• Or
• In most practical applications,
• So, AV ≈ 1. In practice, the voltage gain of an emitter
follower is between 0.8 and 0.999.
9/26/2019
Course on "Electronics Devices and Circuits" by
Deptt of ECE, Integral University, Lucknow.
Facilitator: Piyush Charan
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