Practical applications of BJTs in common base configuration - Page 1
Forum section: electronics / beginners

author: Picuino
author_rank: Super Contributor
timestamp: December 18, 2024, 07:24:30 pm
content: Well, I have read that a common-base bipolar transistor is often used to amplify antenna signals (because of its high speed) and I also remember a practical application amplifying the sound signal received by a low impedance speaker (8 Ohm).Do you know more practical applications of this configuration?

author: jwet
author_rank: Frequent Contributor
timestamp: December 18, 2024, 07:34:55 pm
content: Low impedance input with decent voltage gain but I gain of 1. Kind of the opposite of an emitter follower that has large current gain but V gain of 1. RF circuits a lot since 50 ohms is all over.Its also what the upper transistor in a cascode is. In this case, it lets you make a common emitter like amplifier without taking a big penalty for "miller c". You move the load for your CE load circuit above a common base. Since its base is grounded, you only pay for Ccb once instead of mulitplying it by the gain of the stage as in a common emitter. Can extends gain/BW a lot (50x).The other place that it comes up is in level shifters. You can pull on the emitter and get the same current out of the collector at some high voltage provided by the Vce of common base stage- a little like a cascode but for a different purpose.

author: Zero999
author_rank: Super Contributor
timestamp: December 18, 2024, 08:59:36 pm
content: Bidirectional digital level shifterLevel shift bi-direc BJT 5V.asc

author: Picuino
author_rank: Super Contributor
timestamp: December 22, 2024, 09:16:11 am
content: Thank you both.The level shifter I didn't even know it could exist.I have been looking for information on the internet, but it is really hard to find schematics on common base. There are a few general circuits, almost all the same, and few practical applications.

author: Kleinstein
author_rank: Super Contributor
timestamp: December 22, 2024, 09:35:55 am
content: The common base is often used in combination with a 2nd transistor. E.g. the difference amplifier / long tailed pair can be seen as emitter follower + common base. Another one the cascode circuit, with a common emitter or common source transistor at the base side. This is used for better revese isolation, reduction of the effect of the base-collector capacitance, less effect from the output side and also for high output voltages (e.g. common in CRT intensity control in old TVs to get some 100 V output signal).

author: Calvin
author_rank: Regular Contributor
timestamp: December 22, 2024, 09:36:12 am
content: Hi,You can build I/V converters for current output DACs from common-Base topologies ... great sounding ones btw.regardsCalvin

author: Picuino
author_rank: Super Contributor
timestamp: December 22, 2024, 11:58:48 am
content: Summarizing.Common base combined with other transistors:* Cascodehttps://en.wikipedia.org/wiki/CascodeCommon emitter stage feeding into a common base stage.* Differential amplifier (https://en.wikipedia.org/wiki/Differential_amplifier#Long-tailed_pair)Common collector stage feeding into a common base stage.Common base as translator:* I/V converter.* Level shifter.Common base as amplifier (low impedance input, large voltage amplification, fast operation):* Antenna 50 Ohm amplifier.* Speaker (acting like microphone) amplifier.* High output voltage CRT intensity control.Regards.

author: magic
author_rank: Super Contributor
timestamp: December 22, 2024, 12:39:00 pm
content: Sometimes you have no other choice because the signal is too close to ground. This sort of "folded cascode" circuit is common in single supply opamps, the example schematic below is CMOS but similar bipolar circuits also exist.The input pins can be at or slightly below ground, the emitters/sources are a little above ground and the collectors/drains must be lower, so there isn't enough voltage there to drive the base/gate of the next stage - the next stage needs to be common base/gate.(I mean, OK, LM358 solves this differently by adding emitter followers in front of the differential pair and shifting its voltage up. But doing so adds offset voltage and noise.)