Influence of resistor ratios in bootstrapped zener reference - Page 1
Forum section: electronics / general

author: cellularmitosis
author_rank: Unknown
timestamp: June 13, 2017, 10:05:31 pm
content: Consider a typical "bootstrapped zener" reference circuit (figure-1.png).If all we care about is the zener voltage, we can pick different values for R2 and R1. In figure-2, we've knocked them down by a decade, but we still have the same current through the zener and should have the same zener voltage.My question is, are there any pitfalls to knocking these values up or down a few decades? To push this to an extreme, we could choose 10R and 1R and still have the same zener current (see figure-3). Similarly, we could knock them up a decade (100k / 10k, see figure-4).If we go up by a decade, maybe that would increase the Johnson noise from the resistors? Going down in decades would reduce the op-amp's output voltage, which might be useful if you are constrained (i.e., trying to run from a 9V battery, etc).What I'm more curious about is if there are any practical gotcha's here -- i.e. a manufacturers claims about tempco etc are valid at 1k but maybe not at 1R? Or maybe there are issues from changing the feedback resistors from a 6.8:1 ratio to a more extreme 6800:1 ratio? I.e. maybe the op-amp accuracy or drift might affected?Anyway, just thought I'd ask. This is one of those cases where in theory it seems there's no difference, but I'm sure there's practical difference.

author: Alex Nikitin
author_rank: Super Contributor
timestamp: June 13, 2017, 10:31:14 pm
content: Strictly speaking, the larger is the resistor value the better in this case.CheersAlex

author: chuckb
author_rank: Frequent Contributor
timestamp: June 14, 2017, 12:04:54 am
content: The good 2DW23X Zeners seem to have an internal resistance (ESR) of 5-10 ohms. If the bias current and zener temperature were perfectly steady then the ESR just causes a voltage offset in the system. It can be calibrated out.An LM399 with internal active feedback circuitry has an effective ESR of less than 1 ohm.In the real world R1 has a temperature coefficient and there may be other noise or drift from the power supply, the opamp, or R2 and R3. Lets say we have a system with R1 at 1000 ohms and the zener ESR is 10 ohms. Now any variation in the voltage supplying R1 or the value drift of R1 will be reduced by a factor of 100 at the 6.8 V node. If R1 had a temp co of 100 ppm / deg C then it's influence on the 6.8 V node would be 1 ppm / deg C.A low ESR zener, a high resistance for R1 or a stable active current source in place of R1 (1 Megohm output Z) will minimize bias current induced errors in the 6.8 V node voltage.

author: Kleinstein
author_rank: Super Contributor
timestamp: June 14, 2017, 04:01:28 pm
content: The error (e.g. Offset) of the OPs enters mainly relative to the voltage over R1. So a very small voltage at R1 would increase those errors. It is also that resistors of less than about 100 Ohms might be less accurate and wire resistance gets more important. So a practical lower Limit for R2/R3 is something like 1/10.If the resistance of R1 gets very large, power dissipation and thus self heating might become a problem. So it usually does not make much sense to have R2 much lager than R3.Noise is usually not a problem, as most zeners are much more noisy than the resistors.The absolute values for R2/R3 should consider the input bias of the OP. Depending on the OP one might not want to high impedance, as this would increase errors due to bias current.

author: cellularmitosis
author_rank: Unknown
timestamp: June 15, 2017, 11:26:49 pm
content: Thanks for the advice guys!