---
title: "Ebers-Moll terminal currents; BJT configuration applications"
subject: "Electronics Devices and Circuit"
chapter: 2
key: "semester_3/electronics_devices_and_circuit:ebers-moll-terminal-currents-applications"
source: https://www.bctengineer.com/q/semester_3/electronics_devices_and_circuit/ebers-moll-terminal-currents-applications
site: BCT Engineering
---

# Ebers-Moll terminal currents; BJT configuration applications

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

- Asked in: 2074 Chaitra, 5+3 marks
- Video: [Ebers Moll Model of BJT (Basics, Circuit, Current Equations & Working) Explained](https://www.youtube.com/watch?v=ujDaUymXBg0) — Engineering Funda
- 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/ebers-moll-terminal-currents-applications
- Whole chapter: https://www.bctengineer.com/md/semester_3/electronics_devices_and_circuit/chapter-2

## Question

Find terminal currents of BJT using Ebers-Moll Model. Write applications of different BJT configurations.

## Short answer

IE = -IF + αR IR, IC = αF IF - IR, IB = (1-αF)IF + (1-αR)IR, where IF = IES[e^(VBE/VT)-1], IR = ICS[e^(VBC/VT)-1]. Common emitter provides high voltage gain and phase inversion; common base is used for high-frequency amplification; common collector serves as a buffer with high input and low output impedance.

## Answer

#### Terminal currents from the Ebers-Moll model
The model has two diodes: the emitter-base (DE) with current IF and the collector-base (DC) with IR, and two controlled sources αF IF and αR IR.

```text
  C o---+-----------+
        |           |
       ( ) αF·IF   DC | IR
        |           |
  B o---+-----------+
        |           |
       ( ) αR·IR   DE | IF
        |           |
  E o---+-----------+
```
```text
IF = IES [exp(VBE/VT) - 1]
IR = ICS [exp(VBC/VT) - 1]
```
Applying KCL at each terminal (currents into the transistor):
```text
IE = -IF + αR IR
IC =  αF IF - IR
IB = -(IE + IC) = (1 - αF) IF + (1 - αR) IR
```
with αF IES = αR ICS = IS. In the active region (VBC reverse, IR ≈ -ICS): IC ≈ αF IF = IS exp(VBE/VT), IE ≈ -IF, and IB ≈ IC/βF.

#### Applications of the BJT configurations
| Configuration | Properties | Applications |
|---|---|---|
| Common emitter | high Av and Ai, 180° phase, medium Rin and Rout | general voltage amplifiers, audio preamps, switching circuits |
| Common base | low Rin, high Rout, Av high, Ai ≈ 1, no phase shift | RF/high-frequency amplifiers, current buffers, cascode top stage |
| Common collector | Av ≈ 1, high Rin, low Rout, high Ai | buffer/impedance matching, output stage (power) driving low impedance loads, voltage regulators |

## Check yourself

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

1. What are IF and IR in Ebers-Moll?
   A. IF = IES[exp(VBE/VT)-1], IR = ICS[exp(VBC/VT)-1]
   B. IF = beta IB, IR = IC
   C. IF = VBE/RE
   D. IF = IR always
   Answer: A. Each junction diode carries a Shockley-type current. (Key point: Diode currents)

2. Which expression for IC is correct?
   A. IC = IF - alphaF IR
   B. IC = alphaR IR - IF
   C. IC = alphaF IF - IR
   D. IC = IF + IR
   Answer: C. The collector gets alphaF IF from the emitter diode minus the reverse diode current. (Key point: Collector current)

3. How is IB obtained?
   A. IB = IC/IE
   B. IB = -(IE + IC) = (1-alphaF)IF + (1-alphaR)IR
   C. IB = IE + IC
   D. IB = alphaF IF
   Answer: B. KCL at the transistor gives IB = -(IE+IC). (Key point: Base current)

4. Which application suits the common collector configuration?
   A. High voltage gain preamp
   B. Cascode top stage
   C. Buffer/impedance matching stage
   D. RF amplifier with low Rin
   Answer: C. CC has high Rin, low Rout and gain near 1, making it a buffer. (Key point: CC application)

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