is there a "base follower" transistor configuration? - Page 1
Forum section: electronics / beginners

author: jdutky
author_rank: Regular Contributor
timestamp: July 10, 2025, 07:05:58 am
content: Is there a configuration of a transistor amplifier where your input controls the collector-emitter current and you take your output from the base (so that rather than amplifying by beta you decrease the current by a factor of beta)?

author: Zero999
author_rank: Super Contributor
timestamp: July 10, 2025, 07:29:20 am
content: Quote from: jdutky on July 10, 2025, 07:05:58 amI suggest you think it through.Is there a configuration of amplifier when the collector is the input?

author: Doctorandus_P
author_rank: Super Contributor
timestamp: July 10, 2025, 10:44:52 am
content: First, this is a weird requirementand as Zero999 mentioned, have you thought this through yourself?Quote from: jdutky on July 10, 2025, 07:05:58 amFirst thing that comes to mind here is to use a "standard" emitter follower, but instead of considering the base as the "input", you take the emitter as the input. As long as a BJT is within it's operating range, the Base-Emitter junction will always be around 600mV (Depending on current and temperature), but it is a diode after all.But I'm guessing this is an XY problem. See:https://en.wikipedia.org/wiki/XY_problemWhat is the goal you want to achieve? Hfe (Beta) of a BJT is not a constant. It has a big production tolerance, and it also varies with the temperature, voltages and currents that are applied to a BJT.Maybe a current mirror or a cascode (See:https://en.wikipedia.org/wiki/Cascode) has some overlap with your goals.

author: chilternview
author_rank: Frequent Contributor
timestamp: July 10, 2025, 11:21:29 am
content: If you just want to produce a current that is a fraction of another, have you thought of a current mirror?

author: jdutky
author_rank: Regular Contributor
timestamp: July 11, 2025, 03:22:42 am
content: Yes, I started thinking about this while designing a pico current source. I started with a PNP current mirror and added resistive dividers to get down to the pico amp level, after reading the schematics for a Keithley pico current source. Before that, however, I was wondering if I could run a transistor "in reverse" to get a lower current from a larger one.I'm not exactly a beginner, but I'm just a hobbyist and am still struggling with transistors.

author: MariuszD
author_rank: Regular Contributor
timestamp: July 11, 2025, 06:36:07 am
content: It is possible. I use this method to measure the beta of power transistors. I set the power supply to, for example, 2A, and 50mA flows through the meter in the base circuit. This method will not work with currents at the picoampere level.Use a very large resistor in the gigaohm range and create a voltage regulation circuit on it using op-amps with low bias currents.

author: chilternview
author_rank: Frequent Contributor
timestamp: July 11, 2025, 06:37:16 am
content: There's a picoamp source schematic described in a LTC datasheet:https://www.analog.com/media/en/technical-documentation/data-sheets/60812fd.pdf

author: David Hess
author_rank: Super Contributor
timestamp: July 11, 2025, 07:02:21 am
content: Quote from: jdutky on July 10, 2025, 07:05:58 amYes, but I do not know that it has a specific name. The configuration is the same as with a cascode but the purpose is different, although it may serve as a cascode as well.That configuration is used to create a compensating current for base current bias cancellation in another transistor. It is commonly found in operational amplifiers which implement input bias current cancellation. Some old integrated JFET operational amplifiers also use it for the same reason but with a JFET insteads of a bipolar transistor.Check out the OP-07 datasheet schematic where Q3 and Q4 are configured that way by being placed in series with the collector of the compensated transistor.

author: jwet
author_rank: Frequent Contributor
timestamp: July 11, 2025, 04:00:22 pm
content: The Wilson current mirror runs a transistor this way. Refer to the wiki, the wilson mirror corrects for the base current error on the shorted CB side of the mirror by having a transistor in the opposite leg. It is cascoded to conduct the right mirror current and is forced to generate a base current to correct for the imbalance in and correct beta +1 = beta error.In IC design, its common to have scaled current mirrors where different sized devices are used in mirrors. This isn't so easy in discrete design but you can approximate the circuits with op-amp circuits.Remember beta is a pretty loosey goosey parameter. Its more useful in IC's where matching is straightforward.Good thinking.

author: TimFox
author_rank: Super Contributor
timestamp: July 11, 2025, 04:15:40 pm
content: In discrete circuit design, it is unwise to rely on beta as a parameter: it depends on the collector current and varies greatly from one 2N3904 to the next one.Monolithic designs can exploit matching between two transistors built simultaneously, as discussed above.Historical note: I have seen ultra-high input impedance amplifiers using a triode tube, where the input voltage is applied to the plate (otherwise unloaded) and the grid current is the output.(I don’t remember the bias conditions; it was for AC measurement.)A professor measuring atmospheric fields with a rotating chopper used a 955 “acorn” tube with a carefully-cleaned glass envelope and no socket.

author: David Hess
author_rank: Super Contributor
timestamp: July 11, 2025, 05:59:54 pm
content: Useful results are possible if discrete devices are matched and thermally coupled.

author: jdutky
author_rank: Regular Contributor
timestamp: July 12, 2025, 04:23:13 am
content: I saw that third transistor in the Wilson current mirror but did not understand what it was doing. I recognized the cascode configuration, but it didn't make sense to me in that application, as I learned about cascode as a bandwidth boost.The LTC6081/82 datasheet looks like just the thing I need, as I'm pretty confident I can build a precision resistive divider to turn nano amps into pico amps.

author: David Hess
author_rank: Super Contributor
timestamp: July 12, 2025, 08:13:43 am
content: Quote from: jdutky on July 12, 2025, 04:23:13 amWhat is it that you are trying to do? You may have asked the wrong question.The old style "ohms converter" circuit in multimeters uses a single operational amplifier and two resistors, to multiply or divide a reference current. In principle it can divide by 1000 and produce a picoamp output limited by the input bias current error of the operational amplifier and the need for high value resistors.There is an alternative circuit which uses a pair of matched transistors to do the same thing without any high value resistors.

author: MrAl
author_rank: Super Contributor
timestamp: July 12, 2025, 11:44:15 am
content: Quote from: jdutky on July 10, 2025, 07:05:58 amI am not sure what your application is, but something like what you are referring to is called "Reverse Beta". You can look that up for more info as it may not be exactly what you mean.The thing is, in a circuit the base junctions will experience a change in impedance related to the other two terminals, so there will be some interaction. If you bias the transistor normally in common emitter form with external emitter resistor for example, then power the emitter with an external current, the characteristics related to the base terminal are going to change.You should be able to easily model this in some spice environment (LT Spice) and see if the results fit your needs.The spice Reverse Beta happens when the collector base junction is forward biased. That would happen when the transistor first starts to enter saturation for one case.

author: MariuszD
author_rank: Regular Contributor
timestamp: July 12, 2025, 05:26:34 pm
content: Quote from: jdutky on July 12, 2025, 04:23:13 amReplace 10Mohm resistor with 10Gohm resistor.This circuit works with 10mV but will not work with 10uV input

author: Perrold
author_rank: Contributor
timestamp: July 12, 2025, 09:42:46 pm
content: Quote from: TimFox on July 11, 2025, 04:15:40 pmThe triode trick is neat. It'll work with low level AC signals though the effective transconductance is very low. It also works with very high negative DC inputs. Saw a note somewhere about using one of the TV HV regulator triodes as a HV meter. Plate could go up to -30kV and a meter in the grid circuit showed the measured voltage. Zero loading. I'm sure the scale is fairly nonlinear.

author: TimFox
author_rank: Super Contributor
timestamp: July 12, 2025, 10:54:52 pm
content: Yeah, the 6BK4 and similar HV triodes have a plate cap with almost 4" of glass from the cap to the base.They are normally operated with +25 kV or so on the plate, at low plate current (roughly 1.5 mA) , in a shunt regulator.

author: jwet
author_rank: Frequent Contributor
timestamp: July 13, 2025, 03:22:07 pm
content: The third transistor in the Wilson Mirror-Base current cancellation is one of its functions but perhaps not the main one. Its also a cascode and forces the mirror transistors to operate at more identical conditions of Vce. This corrects for early effect errors. Early effect is a small change in Vbe with Vce changes. Its one of many subtleties that IC designers deal with. A bunch of really smart guys like Widlar, Wilson, Gilbert, Gray and Meyer, Fullagar, Brokaw, Dobkins and many others sorted this all out. I worked with IC designers as an Apps/Business guy, they are pretty brilliant.