Not OT: BC850 as a diode clamp?

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Ricky

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Jan 29, 2023, 12:41:49 AMJan 29
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A differential input amp is using BC850 transistors with the base tied to the collector as a clamp to ground. What is better about this than a diode? I guess it can handle a lot more current before the voltage starts to rise?

https://html.scribdassets.com/9r5y0a5pxc35p3zy/images/4-1adb6b3466.jpg

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Anthony William Sloman

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Jan 29, 2023, 1:10:30 AMJan 29
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On Sunday, January 29, 2023 at 4:41:49 PM UTC+11, Ricky wrote:
> A differential input amp is using BC850 transistors with the base tied to the collector as a clamp to ground. What is better about this than a diode? I guess it can handle a lot more current before the voltage starts to rise?
>
> https://html.scribdassets.com/9r5y0a5pxc35p3zy/images/4-1adb6b3466.jpg

It might just be that Vbe is more tightly specified than forward voltage of a regular diode.

https://assets.nexperia.com/documents/data-sheet/BC849_BC850.pdf

It is shown as min 580mV, typ 660mV and max 700mV at 2mA and 25C, which is pretty tight, and unusually detailed. there's no minimum voltage at 10mA.

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piglet

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Jan 29, 2023, 5:34:51 AMJan 29
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On 29/01/2023 05:41, Ricky wrote:
> A differential input amp is using BC850 transistors with the base tied to the collector as a clamp to ground. What is better about this than a diode? I guess it can handle a lot more current before the voltage starts to rise?
>
> https://html.scribdassets.com/9r5y0a5pxc35p3zy/images/4-1adb6b3466.jpg
>

Low leakage and lower dynamic resistance due to transistor action.

piglet

Gerhard Hoffmann

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Jan 29, 2023, 6:56:21 AMJan 29
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Am 29.01.23 um 11:34 schrieb piglet:
Someone at NIST built a complete ring mixer around that structure.

< https://tf.nist.gov/general/pdf/2556.pdf >

Gerhard



Mike Monett VE3BTI

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Jan 29, 2023, 7:35:02 AMJan 29
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Gerhard Hoffmann <dk...@arcor.de> wrote:

> Someone at NIST built a complete ring mixer around that structure.
>
> < https://tf.nist.gov/general/pdf/2556.pdf >
>
> Gerhard
>
+1. Thanks


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John Larkin

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Jan 29, 2023, 10:31:15 AMJan 29
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On Sun, 29 Jan 2023 10:34:43 +0000, piglet <erichp...@hotmail.com>
wrote:
The be-junction is in parallel with two BAV99 (? illegible) diode
junctions, so there is no leakage advantage.

That's a really weird circuit.

Lasse Langwadt Christensen

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Jan 29, 2023, 10:41:24 AMJan 29
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it sorta works like a zener diode

https://youtu.be/BGcKjy_UNQ4?t=18


piglet

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Jan 29, 2023, 11:00:56 AMJan 29
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I never said it was appropriately used in that circuit! Probably
designed by a physics student who thought it looked clever?

piglet

John Walliker

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Jan 29, 2023, 11:15:42 AMJan 29
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It was probably intended to protect the person or animal to which the electrodes
are connected from being injured by excessive dc in the event of a failure of
one of the other components. I'm assuming it was intended as an ECG
or EMG amplifier.

John

Fred Bloggs

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Jan 29, 2023, 12:41:46 PMJan 29
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https://www.researchgate.net/figure/Practical-ECG-amplifier-circuit_fig2_258847810

https://pubmed.ncbi.nlm.nih.gov/16594298/

The details are behind registration walls- so I'm not going to bother going any further- it's a "pest" post.

It's a high impedance difference input amp running off 1.8V, probably floating, so they're really concerned about rejecting pickup on the electrode leads, in addition to the precision. I notice TI calls out the OPA333 for it's extraordinary CMRR out to 250MHz, so there's a concern regarding RF reception as well. Looks like it's a self-correcting offset RRIO amplifier.

As for the exact choice of BC850, you tell us.

>
> --
> Bill Sloman, Sydney

Fred Bloggs

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Jan 29, 2023, 12:41:58 PMJan 29
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Fred Bloggs

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Jan 29, 2023, 12:42:44 PMJan 29
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Biomedical research by pros.

>
> piglet

Ricky

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Jan 29, 2023, 12:48:39 PMJan 29
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I thought of that, but this is preceded by a 36 kohm/470 pf filter and followed by a 5.1 kohm resistor and a pair of BAV99 diodes to the power rails and on to differential inputs of an op amp, INA321. I don't get the need to clamp both inputs to ground on the negative excursion and not the positive. Oh, wait, I may have found it. The INA321 is not connected to V-, rather the circuit ground. The circuit on the input side of the INA321 is referenced to ground, while the output side is reference to a midpoint reference formed by an op amp. The input side has a pair of op amps in a "bootstrap" arrangement with a separate virtual ground slightly below that midpoint. There's no DC blocking caps on the input, so I guess it's ok to provide a low voltage DC to the test subject through the input electrodes.

This author circuit has a number of circuits for amplifying "biosignals" meaning very low amplitude. I'm not clear on the advantages of many of these circuits.

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Ricky

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Jan 29, 2023, 1:03:34 PMJan 29
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Not exactly the same circuit. The schematic in question grounds the collector/base connection, so the two transistors work independently. I suppose it does however, give you the same effect, by clamping like a diode at 0.6V below ground, and some zener-like effect above ground. I didn't think of that, but it suits the circuits this guy designs I suppose. I need to remember this one. I don't think I can use it in my current design though. That is clamped to the power rails on the analog inputs, ±12V. I may need to rethink that circuit for other reasons though. The input switches are hard to find.

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Ricky

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Jan 29, 2023, 1:06:23 PMJan 29
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The problem with diodes to the power rails, is that it injects current into those rails. This circuit dumps the bulk of the energy into ground and the diode clamps deal with the rest. Looks like a good circuit to me. Amplifier inputs are not always about minimizing input capacitance. Only optimize what needs to be optimized. You save a lot of work and the design is more widely useful.

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John Larkin

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Jan 29, 2023, 1:15:22 PMJan 29
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But why? The BAV99 already clamps up and down. I assume the supply is
5 volts, split-rail at 2.5, so the BAV clamps at +-3 roughly.

Of course the split supply is soft.

Very weird circuit.

John Larkin

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Jan 29, 2023, 1:18:27 PMJan 29
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$40 to see the PDF. No thanks.

Ricky

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Jan 29, 2023, 1:21:47 PMJan 29
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You have the same guy, but this is the paper the circuit I posted came from. I was able to download the PDF file.

https://www.academia.edu/5202695/Increased_Power_Line_Interference_Rejection_by_a_Stray_Capacitance_Drive?email_work_card=view-paper

You may have to select it before you try to open the link since it is a bit long.

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Rick C.

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John Larkin

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Jan 29, 2023, 1:24:39 PMJan 29
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The opamps run single-ended so don't need much cmrr. CM will be
dominated by resistor and capacitor tolerances.

Horrible mess. Why not buy one good diffamp?

Ricky

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Jan 29, 2023, 1:25:56 PMJan 29
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Only to someone who doesn't understand it. The BAV diodes clamp to the power rails, which means a heavy surge can overload every part on the board. In this design the BC850 acts as a higher current path to ground for the bulk of the overvoltage. Grounds can typically handle that sort of thing better than power rails.

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Rick C.

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Ricky

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Jan 29, 2023, 1:26:33 PMJan 29
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Ricky

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Jan 29, 2023, 1:27:42 PMJan 29
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If you read a few of his articles, you might learn something. The guy is not just another pretty face.

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Fred Bloggs

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Jan 29, 2023, 1:31:11 PMJan 29
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They're talking about uV level stuff appearing right up on the OA inputs. If you're dealing with mV signal levels, uV is only 60dB down.

Fred Bloggs

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Jan 29, 2023, 1:33:16 PMJan 29
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On Sunday, January 29, 2023 at 1:24:39 PM UTC-5, John Larkin wrote:
The non-inverting configuration is self-corrupting with CMRR.

Lasse Langwadt Christensen

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Jan 29, 2023, 1:56:15 PMJan 29
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until you push too much current in to the supply

the BAV99s are not really needed the opamp is rated for +/-10mA into the input

Ricky

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Jan 29, 2023, 2:04:31 PMJan 29
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I typically use Schottky diodes for this, to get the lower Vf.

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Lasse Langwadt Christensen

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Jan 29, 2023, 3:06:37 PMJan 29
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sure, but they also have higher leakage current

the point was that according to the datasheet as long as the current is limited to +/-10mA
it is ok to use the ESD diodes



Fred Bloggs

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Jan 29, 2023, 3:11:31 PMJan 29
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On Sunday, January 29, 2023 at 10:31:15 AM UTC-5, John Larkin wrote:
I'm pretty sure the BAVs are there for ESD protection. Now the hunt is on for the rationale behind BC850 clamp.

Phil Hobbs

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Jan 29, 2023, 3:27:44 PMJan 29
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And much lower shunt resistance.

> the point was that according to the datasheet as long as the current is limited to +/-10mA
> it is ok to use the ESD diodes

(Responding in a new thread)

Cheers

Phil Hobbs


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Lasse Langwadt Christensen

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Jan 29, 2023, 3:29:58 PMJan 29
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it works like a zener, together with the 5.1k it limits how much current can be pushed into the supply

Fred Bloggs

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Jan 29, 2023, 3:31:55 PMJan 29
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On Sunday, January 29, 2023 at 10:31:15 AM UTC-5, John Larkin wrote:
Okay, you were close. The diode connected BC850 is there to divert the BAV leakage away from the OA input, which would produce a sizeable output offset with all that gain.

Fred Bloggs

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Jan 29, 2023, 3:33:30 PMJan 29
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Possible but not likely. It's there for offset reduction/ elimination.

Fred Bloggs

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Jan 29, 2023, 3:44:00 PMJan 29
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On Sunday, January 29, 2023 at 3:29:58 PM UTC-5, lang...@fonz.dk wrote:
The output difference signal is AC coupled, so a little bit of offset is okay. They just need to keep the offset well away from rail'ing the OAs.

Ricky

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Jan 29, 2023, 4:03:30 PMJan 29