Would a collector resistor in an emitter follower improve linearity? - Page 1
Forum section: electronics / beginners

author: Deactivated-1
author_rank: Frequent Contributor
timestamp: August 18, 2023, 08:10:43 pm
content: I was looking at some old german hifi schematics and many times the collector has a resistor on emitter followers, for example there's an emitter follower with a 4.7k emitter resistor and 1k collector resistor, no signal gets taken from the collector. the B+ is 25V. 11V at the cathode so there would be only ~2V of drop across the resistor, so it wouldnt be to reduce power dissipation across the NPN transistor.It also doesnt seem likely that they're doing this to limit current since it's driving a cathodyne ... or emitterodyne then?phase inverter.I was thinking, maybe it's to reduce distortion. Since it would shift the circuit towards the linear region of the transistor the more current gets pulled trough the transistor, so a slightly more linear emitter follower.What do you think?

author: Kim Christensen
author_rank: Super Contributor
timestamp: August 18, 2023, 10:32:00 pm
content: Can you post a schematic of the circuit in question? Hard to tell without one.

author: TimFox
author_rank: Super Contributor
timestamp: August 18, 2023, 10:42:19 pm
content: One reason for a (small) resistor in series with the collector of an emitter follower is to avoid oscillation, but usually it is to prevent overcurrent with a shorted load.There are interesting effects on the input impedance of an emitter follower if the load (on the emitter) is reactive, and you can get negative conductance at the base.This old article discusses the input impedance, but does not suggest a collector resistor as a cure:https://www.hifisystemcomponents.com/downloads/articles/Prevent-Emitter-Follower-Oscillation.pdf

author: David Hess
author_rank: Super Contributor
timestamp: August 18, 2023, 10:47:05 pm
content: A collector resistor may be added to an emitter follower to suppress oscillation; the idea is to have a lossy element on at least one lead if not two.I have also seen it done to prevent destructive current during overload. Sometimes the resistor is really a fuse.

author: David Hess
author_rank: Super Contributor
timestamp: August 19, 2023, 12:25:26 am
content: I think the collector resistor is there to prevent excessive current when power is still applied. If the tone controls are all set one way, then the reactive elements are effectively shorts from the emitter of T202 to ground unless the capacitances are charged. R206 limits the current through T202, but that still leaves the base-emitter current which is limited by R204.

author: Kim Christensen
author_rank: Super Contributor
timestamp: August 19, 2023, 02:39:16 am
content: R204 as probably added to limit the peak current into the base of T202 for the positive half of the audio waveform if the circuit was being severely overdriven. Without it, the limiting factor would be the risetime of the signal, the ESR of C202, the beta of T201, etc.

author: magic
author_rank: Super Contributor
timestamp: August 19, 2023, 08:11:24 am
content: Quote from: ELS122 on August 18, 2023, 08:10:43 pmFirst of all, this transistor is in the "linear" region (though I think it's usually called the active region with BJTs) and the resistor pushes it in the direction of saturation.Note that this naming is the opposite of FETs.Alex Nikitin, a member of this forum, published a short article where he claims that FET distortion can be reduced in similar way.http://www.ant-audio.co.uk/Theory/Parametric_Linearisation.pdfQuoteThis is because transconductance would naturally increase with drain current, but it also falls down at extremely low drain voltage.For voltage followers, transconductance is the inverse of output impedance, of course.This could possibly work on BJT too, but:1. as with FETs, proper cancellation would likely only occur at very low collector voltage, maybe less than 1V2. as with FETs, getting this to track over temperature would be nightmare3. it's unknown how wide the useful range of collector current would beAnd further,4. in your circuit, T201 works leisurely at almost constant bias, it's T202 which would need corrections first5. maybe it is famous and has "superb sound quality", but real sign of working compensation would be low measured 2nd harmonic distortion

author: magic
author_rank: Super Contributor
timestamp: August 19, 2023, 07:31:02 pm
content: No, quite the opposite.Increasing Ic increases gm.Decreasing Vce decreases gm.A collector resistor causes Vce to decrease when Ic increases.Theoretically the two effects could cancel, but in normal circuits Ic dominates over Vce.I'm quite sure that this is not the purpose of this resistor.But you asked if it's possible so the answer is "yes, maybe".