Question on the source follower - Page 1
Forum section: electronics / general

author: anjali
author_rank: Newbie
timestamp: June 21, 2024, 02:46:44 pm
content: Hey all,So I have a question on the source follower circuit (Might find silly)I was reading through the analog electronics of Behazad Razavi. So for this source follower, the Vout is taken from the source. I get that. But my question is,is this Vout is actually the Vsb?Because, while finding out the gain by derivating the large signal model, dVsb/ dVin is taken as dVout/dVin.Also, dont really know how to post questions here..Really appreciate the help.Thanks

author: TimFox
author_rank: Super Contributor
timestamp: June 21, 2024, 03:46:52 pm
content: Is your "Vsb" the voltage between source and positive supply rail (assuming N-channel FET)?Assuming that the voltage between supply rail and the supply common (or negative rail) is constant, then the derivative of the voltage on the source terminal compared with the derivative of the voltage on the gate terminal does give you the large-signal voltage gain.Being large-signal, that gain will depend somewhat on the magnitude of the voltage swings on gate and source.

author: pcprogrammer
author_rank: Super Contributor
timestamp: June 21, 2024, 03:50:30 pm
content: Welcome to the forum.Well you asked a question, so that ticks one box.About your other issue, might be helpful to add a quote from the article or book you are reading.According toWikipediathe voltage gain of the circuit is almost 1, just like the emitter follower.

author: TimFox
author_rank: Super Contributor
timestamp: June 21, 2024, 03:54:19 pm
content: The voltage gain of a source follower will be less than that of a comparable emitter follower, since the FET transconductance dIs/dVgsis lower than a BJT at the same current.Therefore, the difference voltageVgsthat subtracts from the output voltage will swing more thanVbeof the BJT for the same load current (from external load and the resistor from source to negative power rail or ground).

author: RJSV
author_rank: Super Contributor
timestamp: June 21, 2024, 06:34:40 pm
content: Sometimes, when I'm having some questions, usually non-circuit related, but still technically 'electronics' or straight engineering, that's when I decide, to place my question in the section labeled as 'Dodgy Subjects'.But your question seems fine, in the section; 'General Subjects', fine.That kind of question is often with some very good, and advanced explanations (as you might have noticed).I sometimes gravitate to the 'Beginner's' if my question or subject is basic, first year college stuff, but still, there is usually more complexity than I can absorb and understand.Really, really nagging technical issues are, sometimes covered by the personal, one to one messaging, but that feature seems used rarely, at least in this EEVBLOG forum.Welcome!- - Rick B.

author: TimFox
author_rank: Super Contributor
timestamp: June 21, 2024, 07:14:41 pm
content: A simple continuation of my statement above is that a follower (cathode, emitter, or source) can be modeled as a unity-gain non-inverting amplifier followed by an output resistor equal to 1/gm.The BJT emitter follower will pull base current from the source, but the other two (to first order) do not load the circuit before the grid or gate.