On Sunday, January 29, 2023 at 1:10:30 AM UTC-5, bill....@ieee.org wrote:> 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.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
Not OT: BC850 as a diode clamp?
Started by ●January 29, 2023
Reply by ●January 29, 20232023-01-29
Reply by ●January 29, 20232023-01-29
On Sunday, January 29, 2023 at 11:00:56 AM UTC-5, erichp...@hotmail.com wrote:> On 29/01/2023 15:31, John Larkin wrote: > > On Sun, 29 Jan 2023 10:34:43 +0000, piglet <erichp...@hotmail.com> > > wrote: > > > >> 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 > > > > The be-junction is in parallel with two BAV99 (? illegible) diode > > junctions, so there is no leakage advantage. > > > > That's a really weird circuit. > > > I never said it was appropriately used in that circuit! Probably > designed by a physics student who thought it looked clever?Biomedical research by pros.> > piglet
Reply by ●January 29, 20232023-01-29
On Sunday, January 29, 2023 at 6:34:51 AM UTC-4, erichp...@hotmail.com wrote:> 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. > > pigletI 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. -- Rick C. + Get 1,000 miles of free Supercharging + Tesla referral code - https://ts.la/richard11209
Reply by ●January 29, 20232023-01-29
On Sunday, January 29, 2023 at 11:41:24 AM UTC-4, lang...@fonz.dk wrote:> søndag den 29. januar 2023 kl. 16.31.15 UTC+1 skrev John Larkin: > > On Sun, 29 Jan 2023 10:34:43 +0000, piglet <erichp...@hotmail.com> > > wrote: > > >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 > > The be-junction is in parallel with two BAV99 (? illegible) diode > > junctions, so there is no leakage advantage. > > > > That's a really weird circuit. > it sorta works like a zener diode > > https://youtu.be/BGcKjy_UNQ4?t=18Not 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. -- Rick C. -- Get 1,000 miles of free Supercharging -- Tesla referral code - https://ts.la/richard11209
Reply by ●January 29, 20232023-01-29
On Sunday, January 29, 2023 at 12:00:56 PM UTC-4, erichp...@hotmail.com wrote:> On 29/01/2023 15:31, John Larkin wrote: > > On Sun, 29 Jan 2023 10:34:43 +0000, piglet <erichp...@hotmail.com> > > wrote: > > > >> 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 > > > > The be-junction is in parallel with two BAV99 (? illegible) diode > > junctions, so there is no leakage advantage. > > > > That's a really weird circuit. > > > I never said it was appropriately used in that circuit! Probably > designed by a physics student who thought it looked clever? > > pigletThe 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. -- Rick C. -+ Get 1,000 miles of free Supercharging -+ Tesla referral code - https://ts.la/richard11209
Reply by ●January 29, 20232023-01-29
On Sun, 29 Jan 2023 07:41:19 -0800 (PST), Lasse Langwadt Christensen <langwadt@fonz.dk> wrote:>s�ndag den 29. januar 2023 kl. 16.31.15 UTC+1 skrev John Larkin: >> On Sun, 29 Jan 2023 10:34:43 +0000, piglet <erichp...@hotmail.com> >> wrote: >> >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 >> The be-junction is in parallel with two BAV99 (? illegible) diode >> junctions, so there is no leakage advantage. >> >> That's a really weird circuit. > >it sorta works like a zener diode > >https://youtu.be/BGcKjy_UNQ4?t=18 >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.
Reply by ●January 29, 20232023-01-29
On Sun, 29 Jan 2023 09:41:54 -0800 (PST), Fred Bloggs <bloggs.fredbloggs.fred@gmail.com> wrote:>On Sunday, January 29, 2023 at 10:31:15 AM UTC-5, John Larkin wrote: >> On Sun, 29 Jan 2023 10:34:43 +0000, piglet <erichp...@hotmail.com> >> wrote: >> >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 >> The be-junction is in parallel with two BAV99 (? illegible) diode >> junctions, so there is no leakage advantage. >> >> That's a really weird circuit. > >https://pubmed.ncbi.nlm.nih.gov/16594298/$40 to see the PDF. No thanks.
Reply by ●January 29, 20232023-01-29
On Sunday, January 29, 2023 at 1:41:58 PM UTC-4, Fred Bloggs wrote:> On Sunday, January 29, 2023 at 10:31:15 AM UTC-5, John Larkin wrote: > > On Sun, 29 Jan 2023 10:34:43 +0000, piglet <erichp...@hotmail.com> > > wrote: > > >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 > > The be-junction is in parallel with two BAV99 (? illegible) diode > > junctions, so there is no leakage advantage. > > > > That's a really weird circuit. > https://pubmed.ncbi.nlm.nih.gov/16594298/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. -- Rick C. +- Get 1,000 miles of free Supercharging +- Tesla referral code - https://ts.la/richard11209
Reply by ●January 29, 20232023-01-29
On Sun, 29 Jan 2023 09:41:41 -0800 (PST), Fred Bloggs <bloggs.fredbloggs.fred@gmail.com> wrote:>On Sunday, January 29, 2023 at 1:10:30 AM UTC-5, bill....@ieee.org wrote: >> 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. > >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. >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?
Reply by ●January 29, 20232023-01-29
On Sunday, January 29, 2023 at 2:15:22 PM UTC-4, John Larkin wrote:> On Sun, 29 Jan 2023 07:41:19 -0800 (PST), Lasse Langwadt Christensen > <lang...@fonz.dk> wrote: > > >søndag den 29. januar 2023 kl. 16.31.15 UTC+1 skrev John Larkin: > >> On Sun, 29 Jan 2023 10:34:43 +0000, piglet <erichp...@hotmail.com> > >> wrote: > >> >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 > >> The be-junction is in parallel with two BAV99 (? illegible) diode > >> junctions, so there is no leakage advantage. > >> > >> That's a really weird circuit. > > > >it sorta works like a zener diode > > > >https://youtu.be/BGcKjy_UNQ4?t=18 > > > 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.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. -- Rick C. ++ Get 1,000 miles of free Supercharging ++ Tesla referral code - https://ts.la/richard11209
