op amp feedback loop by a  open collector BJT transistor? - Page 1
Forum section: electronics / projects

author: awang7
author_rank: Newbie
timestamp: June 30, 2021, 08:18:25 am
content: HI All,I'm working on a current clamp project and a schematic makes me confused. the opamp has a inverting feedback by a open drain BJT transistor parallel with a capacitor.I assume the BJT is used as a diode to cut negative feedback but i'm not sure. why not just use a diode?Any idea how this worked? Thanks.attachment 1 is the circuit.attachment 2 is the function diagram.

author: fcb
author_rank: Super Contributor
timestamp: June 30, 2021, 09:16:20 am
content: Any number of reasons. Could be available on the BOM (so's the 1N914 so perhaps not), could be part of matched pair with another part of the circuit, could be part of a CA3086 or similar?Do they list the manufacturers part number for Q7?

author: awang7
author_rank: Newbie
timestamp: June 30, 2021, 09:27:28 am
content: HI fcb,Thank you for the reply. The part number of Q47 is MPSH17 from On Semi. It looks just like a normal BJT transistor.

author: richard.cs
author_rank: Super Contributor
timestamp: June 30, 2021, 10:00:56 am
content: It's used as a diode with a very low reverse leakage. This is fairly common, the leakage is good (pA), but the reverse breakdown is only 6 V or so.

author: fcb
author_rank: Super Contributor
timestamp: June 30, 2021, 11:30:19 am
content: Richard.cs makes a good point w.r.t. potentially lower leakage.What's the part number for the diode-thing to the right of the 1N914?

author: harerod
author_rank: Frequent Contributor
timestamp: June 30, 2021, 12:14:00 pm
content: awang7, welcome to the forum.Would you mind changing the thread title into "open collector BJT transistor", please?Bipolar Junction Transistors have Collector, Basis, Emitter, while Field Effect Transistors have Drain, Gate, Source.

author: David Hess
author_rank: Super Contributor
timestamp: June 30, 2021, 01:06:55 pm
content: The transistor is operating as a fast low leakage diode to clamp the positive swing.

author: TimFox
author_rank: Super Contributor
timestamp: June 30, 2021, 01:43:05 pm
content: Also, often one will see C-B junctions (base shorted to emitter) used as low-leakage diodes with higher reverse rating than the B-E junction.

author: awang7
author_rank: Newbie
timestamp: June 30, 2021, 02:24:13 pm
content: it is a current regulator CR430, I assume it is used for decrease current.

author: awang7
author_rank: Newbie
timestamp: June 30, 2021, 02:28:03 pm
content: harerod, thank you for your advise. the typo is corrected.

author: David Hess
author_rank: Super Contributor
timestamp: June 30, 2021, 02:57:28 pm
content: Quote from: TimFox on June 30, 2021, 01:43:05 pmUsually that would be my preference but the collector-base junction is slower. I thought they might be using the base-emitter junction to also clamp the opposite polarity but that does not appear to be the case.Quote from: awang7 on June 30, 2021, 02:24:13 pmI think that is for limiting the output current but not the current to the integration capacitance because it is so small.

author: TimFox
author_rank: Super Contributor
timestamp: June 30, 2021, 03:18:02 pm
content: I used CB diodes from low-noise audio BJTs (e.g., 2N2484 and 2N4250) in DC high-impedance analog circuits (e.g., output bounds on op amps), but I had reasonable results with CB diodes of RF BJTs (e.g., 2N918) in high impedance circuits between 5 and 10 MHZ, where the breakdown voltage of the BE junction was not sufficient for the application.

author: David Hess
author_rank: Super Contributor
timestamp: July 01, 2021, 02:28:31 am
content: Quote from: TimFox on June 30, 2021, 03:18:02 pmI have used the base-emitter and base-collection junction of all kinds of small signal transistors for low leakage, less than 10 picoamps, diodes.