JFET Audio preamp, diode set constant drain-source voltage, why not with BJT's? - Page 1
Forum section: electronics / beginners

author: Deactivated-1
author_rank: Frequent Contributor
timestamp: September 22, 2023, 09:20:40 am
content: Many JFET buffer stages I see have a diode or some other circuit that keeps the JFET source-drain voltage constant, I presume this is done for increased linearity right?But why isn't it done for BJT buffers? I've never seen an emitter follower set up like that.

author: Kleinstein
author_rank: Super Contributor
timestamp: September 22, 2023, 11:12:29 am
content: JFETs are not that ideal as BJTs. The drain voltage has more effect and often also the thermal offset drift is larger. A variable voltage causes a change in temperature and this way some extra error (though to a large part linear and mainly low frequency).As part of chips a constant collector - emitter voltage is still used with super beta transistors, as these usually have too low a maximum voltage. There are few discrete ones too and these would in many cases need a similar bootstrapping of the collector side too.The Circuit shown as the Radford preamp is the classic Sizlaky stage - not directly related.Anyway most modern circuits now use OP-amps and feedback - the open loop buffers are a bit outdated in the audio range.

author: CaptDon
author_rank: Super Contributor
timestamp: September 22, 2023, 01:29:49 pm
content: One thing for sure, that m.buffer circuit is a current limiter safety circuit, not a voltage stabilizer circuit.

author: Marco
author_rank: Super Contributor
timestamp: September 22, 2023, 01:48:44 pm
content: BJTs have a good transconductance, steady Vbe and small early effect.JFET have shit transconductance, hugely variable IDSS/Vgs and act as a voltage controlled resistor rather than a voltage controlled current source.BJTs need less help.

author: schmitt trigger
author_rank: Super Contributor
timestamp: September 22, 2023, 02:19:04 pm
content: For the two first circuits you posted, I believe that it’s purpose is to keep the JFET well below the pinchoff. In other words, in the ohmic region. Presumably for linearity, as you mentioned.

author: TimFox
author_rank: Super Contributor
timestamp: September 22, 2023, 02:29:29 pm
content: For a (PNP) BJT input stage on a low-noise RIAA preamp, I used a second PNP in cascode with the input device:the second base was driven through a capacitor from the emitter of the first device, to avoid modulation of the input current from common-mode voltage, due to the "Early effect" (variation of base current with CE voltage).At audio frequencies, that keeps the CE voltage of the first transistor reasonably constant.That input current is a real effect (not "equivalent"), and flows through the source impedance, which is not negligible for a MM phono cartridge.Since the voltage induced in the source impedance thereby is a real voltage at the input outside of the feedback loop, any non-linear distortion therefrom will not be reduced by the amplifier's negative feedback.

author: magic
author_rank: Super Contributor
timestamp: September 22, 2023, 06:14:33 pm
content: This can be used to increase AC input impedance of diamond buffers - base-collector capacitance of the first transistor pair is mostly bootstrapped away.Example below; red is the signal path and blue shows pre-driver collectors connected to the output node. In this case it appears to have been done not for reduced loading on the preceding stage (it is loaded much heavier by C2 anyway) but purely to get rid of Cbc nonlinearity and/or uncertainty and replace it with a more linear and precise type of capacitor.Off the top of my head I can't name any other instance of this trick, but AD797 shows that it can be done and it can work.

author: magic
author_rank: Super Contributor
timestamp: September 22, 2023, 06:40:26 pm
content: Quote from: schmitt trigger on September 22, 2023, 02:19:04 pmGenerally, there is nothing great about operation in the "linear" aka ohmic region of a FET. Or the analogous saturation region of a BJT. Output current varies not only with input voltage (as it's supposed to) but also with drain/collector voltage.That being said, there were attempts to exploit this effect to cancel transconductance modulation with drain current. But this requires careful design and probably won't happen by accident in the sort of circuit shown in the first post.Quote from: TimFox on September 22, 2023, 02:29:29 pmSimilar bootstrapped cascoding is common in precision opamps such as OP07, serving the same purpose of increasing input impedance i.e. reducing variation of input bias current with applied voltage.AD797 has it too - Q8,Q7,Q3,Q4.

author: TimFox
author_rank: Super Contributor
timestamp: September 22, 2023, 07:14:19 pm
content: Side comment about the "ohmic" region of JFET operation.(I agree that such low-voltage operation of JFETs is generally useless for amplification.)The low-voltage resistance (between D and S, controlled byVGS) can be useful as a voltage-controlled resistor in a gain control, so long as you keep the drain voltage low.On first look, the characteristic curves of a vacuum pentode and a JFET (family of curves forIvsVfor different input voltages) appear to be the same.However, at the far left of the graphs the pentode's curves (at each grid-cathode voltage) coalesce into one line, but in the same region for a JFET they remain separate (with different slopes) down to the origin.

author: magic
author_rank: Super Contributor
timestamp: October 08, 2023, 06:10:12 am
content: With bipolars it's sometimes called "folded darlington" and a complementary pair of those is called "diamond buffer".But you are also asking about the specific case of the 1st transistor having its collector/drain bootstrapped to the second transistor's output. I don't think there is any short name for this exactly. Maybe "folded darlington with bootstrapped driver collector", but I just made it up.

author: Circlotron
author_rank: Super Contributor
timestamp: October 08, 2023, 06:47:46 am
content: Quote from: ELS122 on September 22, 2023, 09:20:40 amIn the first circuit the BJTs are set up to supply constant current to the JFET, not constant voltage. They are effectively an infinite load resistance.

author: magic
author_rank: Super Contributor
timestamp: October 08, 2023, 07:18:02 am
content: The JFET is a source follower, operated at constant current (set by the bottom BJT)andconstant drain voltage (set by the middle BJT, which itself is loaded by the top BJT).

author: TimFox
author_rank: Super Contributor
timestamp: October 08, 2023, 02:43:30 pm
content: With followers (source, emitter, or cathode) the linearity is improved by using a constant-current source in the [source, etc.] lead, and cascoding another follower at the [drain, etc.] lead to keep the voltage across the follower itself constant.Ideally, this keeps the voltage across and current through the device constant (when feeding a high-impedance load), so the output follows the input almost exactly.