Another, even more bootstrap, reference circuit - Page 1
Forum section: electronics / general

author: Victorman222
author_rank: Regular Contributor
timestamp: June 19, 2025, 11:34:20 pm
content: Hello everyoneRecently i was repairing a piece of equipment and it had an interesting power supply configuration  — very similar to the bootstrap reference circuit that we know, but with the opamp powered by the output voltage. This circuit probably was chosen to combat LM709s horrible PSRR (200uV/V), since the supplies were somewhat used as references.So i decided to try and draw up a circuit for a reference using this idea. Circuit description: Q1 is the output series pass transistor. It is turned on by the R7, D1, R6 divider that puts about 10.7 volts at the base of transistor, so the emitter outputs 10V. The emitter of Q1 is the reference output net "+10VREF". Opamp, LM399 heater and zener diode are powered from the +10VREF supply. Once the transistor turned on, and the +10VREF supply starts rising, opamp and LM399 start turning on. Opamp regulates the output voltage by changing its output voltage, which changes current thru R5, thus the voltage at the junction R5-R6-D1 and on the Q1 base, therefore controlling the emitter voltage. Zener diode D1 is used so the output voltage of the opamp is approximately between its supplies (5V).Then i built a breadboard of it and it worked! [3-7] Next step is to put in a box and build a bunch of references to compare between. The schematic is not complete as its missing some bypass caps and the 15V regulator.What do you think about this circuit? Is this just overkill and unnecessary with the PSRR of good opamps (OP07 has 32 uV/V) and a pre regulator?Is there some problem with this circuit that i have missed?Would appreciate any feedback

author: David Hess
author_rank: Super Contributor
timestamp: June 20, 2025, 12:35:04 am
content: The zener level shift is common in old regulator designs where the voltage to be regulated is higher than the operational amplifier's supply voltage, whether the operational amplifier is bootstrapped or not. It's advantage is that it does not increase gain within the feedback loop preserving stability.A bypass capacitor should be placed around the zener diode and resistor from the operational amplifier output to the power pass element.

author: Victorman222
author_rank: Regular Contributor
timestamp: June 20, 2025, 03:27:13 am
content: Quote from: David Hess on June 20, 2025, 12:35:04 amThanks for the tip! I tried modelling circuit with the capacitor, looks like it made the system more damped. There is a lot less overshoot (no overshoot at startup), but it takes longer to settle after loadstep (with 4u7 cap 9ms to 1ppm). Tried making bode plots for it (not sure if correctly) and it went from 96 degree with C=0 to 114 degree with C=4u7 phase margin, and from 55 dB gain margin with C=0 to 39dB with C=4u7.So, a 4u7 tantalum goes thereWill have to play on real circuit when get the time!Not sure how relevant this is for DC reference, but still very interesting

author: zlymex
author_rank: Frequent Contributor
timestamp: June 20, 2025, 06:40:47 am
content: Similar to Fluke 731B starting from 1974 with added current limiting function by Q2 and R28.Modern 732B/C is virtually the same.

author: Victorman222
author_rank: Regular Contributor
timestamp: June 20, 2025, 07:38:31 am
content: Quote from: zlymex on June 20, 2025, 06:40:47 amThanks for showing the schematics!It is interesting that the LM308 in the 731B has its supply powered by the reference output but the OP97 in 732B is powered from the power supply. Looking at the ICs datasheets it kind of makes sense: LM308 has worst case PSRR of 80 dB (100 uV/V) and OP97 has minimum PSRR spec of 110 dB (3.2 uV/V)So maybe for something like an OP07 (20 or 32 uV/V) it makes sense to use this circuit

author: iMo
author_rank: Super Contributor
timestamp: June 21, 2025, 07:02:40 am
content: I do not see much difference to this traditional version people use since everhttps://www.eevblog.com/forum/metrology/lm399-based-10-v-reference/msg5951613/#msg5951613The version with zener at the opamp's output (and I added a diode in series with the zener and the 100ohm resistor in the base of the current limiter there as well) is there to get the opamp's output voltage lower (say 7V) than it would be with the simple resistor (10.5V) version.PS: ok, I see the diff - the powering the opamp from the Vref's 10V output..Also mind the CR1, CR101, CR102 zeners in zlymex'es schematics - perhaps to allow a safe start of the regulator when the opamp's power is bootstrapped?

author: Victorman222
author_rank: Regular Contributor
timestamp: June 21, 2025, 12:02:55 pm
content: Quote from: iMo on June 21, 2025, 07:02:40 amFrom the symbol and 5.5 mA marking i think those are constant current diodes to bias the transistor on.The opamp should be safe, it will never see more than its supply voltage on its input because of the zener.(the supply voltage is the voltage on the base of the transistor -0.7V, and the zener is more than 0.7V)

author: zlymex
author_rank: Frequent Contributor
timestamp: June 21, 2025, 02:10:59 pm
content: Quote from: iMo on June 21, 2025, 07:02:40 amThats right there is a bug in the Fluke 732B(and 731A/732A) circuit and I added a diode too to avoid too much current when output shorted.And I add this diode inseries with the Zener in all of my 10V circuits.

author: dietert1
author_rank: Super Contributor
timestamp: June 22, 2025, 03:38:39 pm
content: I am also using that "over the head" regulation for several circuits. Instead of using the zener i made Darlingtons of a JFET and a bipolar transistor. Also i added a small circuit to guarantee startup and there is the separation of the supply voltage regulator inside the oven and the Darlington outside of the oven. In my case it's not so much about the supply voltage rejection but about running the circuit inside the oven at constant power consumption and thus constant temperature.Image shows test board i am using for LiFePo4 reference experiments. It has two separate supply circuits, each one with a JFET, the output transistor and third bipolar transistor for output current limiting.Regards, Dieter

author: iMo
author_rank: Super Contributor
timestamp: June 22, 2025, 04:42:51 pm
content: Quote from: Victorman222 on June 20, 2025, 07:38:31 amSome time back I made simulation of PSRR vs. the feedback capacitor in the same design (wired between -IN and OUT of the opamp). The higher the capacitor value the lower the PSRR (as the opamp gets slower, imho). I have to find the post with the table..https://www.eevblog.com/forum/metrology/adr1399-reference/msg4733726/#msg4733726Also look at the posts there - ie. the jfet based current source instead of the 10k resistor in the output buffer, etc..Here is the bootstrapped opamp version incl the current source there..https://www.eevblog.com/forum/metrology/adr1399-reference/msg4734137/#msg4734137