op amp feedback loop by a open collector BJT transistor?

[awang7]

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.

[fcb]

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?

[awang7]

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.

[richard.cs]

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.

[fcb]

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?

[harerod]

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.

[David Hess]

The transistor is operating as a fast low leakage diode to clamp the positive swing.

[TimFox]

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.

[awang7]

it is a current regulator CR430, I assume it is used for decrease current.

[awang7]

harerod, thank you for your advise. the typo is corrected.

[David Hess]

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.

Usually 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.

it is a current regulator CR430, I assume it is used for decrease current.

I think that is for limiting the output current but not the current to the integration capacitance because it is so small.

[TimFox]

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.

[David Hess]

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.

I have used the base-emitter and base-collection junction of all kinds of small signal transistors for low leakage,  less than 10 picoamps, diodes.