---
title: "Why CC amplifier is an emitter follower"
subject: "Electronics Devices and Circuit"
chapter: 2
key: "semester_3/electronics_devices_and_circuit:ce-emitter-follower-cc"
source: https://www.bctengineer.com/q/semester_3/electronics_devices_and_circuit/ce-emitter-follower-cc
site: BCT Engineering
---

# Why CC amplifier is an emitter follower

Electronics Devices and Circuit · Chapter 2: The Bipolar Junction Transistor

- Asked in: 2082 Shrawan, 2+3+3 marks; 2075 Chaitra, 1+6 marks (asked 2 times)
- Video: [ECE3400 Lecture 14: Common Collector Amplifiers (Emitter Followers) (Analog Electronics)](https://www.youtube.com/watch?v=WyYZhL7W8eo) — Lantertronics
- Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_3/electronics_devices_and_circuit/ce-emitter-follower-cc
- Whole chapter: https://www.bctengineer.com/md/semester_3/electronics_devices_and_circuit/chapter-2

## Question

Why common collector amplifier is known as emitter follower? Draw its ac equivalent circuit to find its input resistance and voltage gain.

## Short answer

It is called emitter follower because the emitter voltage follows the base voltage with a gain just below 1 and no phase inversion. AC analysis yields Rin = R1∥R2∥(β+1)(re+RE∥RL) and Av = (RE∥RL)/(re+RE∥RL) ≈ 1.

## Answer

#### Why it is called an emitter follower
In a common collector amplifier the input is applied to the base and the output is taken from the emitter. The emitter voltage is vE = vB - 0.7 V, so the output **follows** the input in phase with a voltage gain slightly below 1. The collector is connected to the supply (ac ground) and is common to input and output.

#### AC equivalent circuit (T-model)
```text
 vs--Rs--+----B                    
         |      \  ie=(β+1)ib
       R1||R2    re
         |        \
        GND        E---+---- vo
                       |
                    RE||RL
                       |
                      GND
```
Collector and VCC are at ac ground. RB = R1 || R2 is from the base to ground. R'E = RE || RL.

#### Input resistance
```text
vb = ie (re + R'E) = (β+1) ib (re + R'E)
Rib = vb/ib = (β+1)(re + R'E)
Rin = RB || Rib = R1 || R2 || (β+1)(re + RE||RL)
```
Example: β = 100, IE = 2 mA (re = 12.5 Ω), R'E = 2 kΩ: Rib = 101 × 2012.5 ≈ 203 kΩ.

#### Voltage gain
```text
vo = ie R'E ,   vi = vb = ie (re + R'E)

Av = vo/vi = R'E/(re + R'E)  ≈ 1   (0.99 in the example)
```
So Av is just below 1, positive, with no phase inversion. With source resistance: Avs = Av · Rin/(Rin + Rs).

It acts as a **buffer**: high Rin, low Rout, voltage gain ≈ 1 but a large current gain.

## Check yourself

The answer's key points as multiple-choice questions, with the correct option and why.

1. Why is the CC amplifier called an emitter follower?
   A. It inverts the signal
   B. The collector follows the emitter
   C. The emitter is grounded
   D. The emitter voltage follows the base voltage (vE = vB - 0.7 V) in phase
   Answer: D. The output at the emitter tracks the input at the base. (Key point: Name origin)

2. What is the base input resistance Rib?
   A. re + R'E
   B. (beta+1)(re + R'E)
   C. beta RC
   D. R1||R2
   Answer: B. vb = (beta+1) ib (re + R'E). (Key point: Input resistance)

3. What is Av?
   A. re/RE
   B. -gm RC
   C. R'E/(re + R'E), slightly below 1
   D. beta
   Answer: C. vo = ie R'E and vi = ie(re + R'E). (Key point: Gain)

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