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.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.
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 3:29:58 PM UTC-5, lang...@fonz.dk wrote:> søndag den 29. januar 2023 kl. 21.11.31 UTC+1 skrev Fred Bloggs: > > 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. > > I'm pretty sure the BAVs are there for ESD protection. Now the hunt is on for the rationale behind BC850 clamp. > it works like a zener, together with the 5.1k it limits how much current can be pushed into the supplyPossible but not likely. It's there for offset reduction/ elimination.
Reply by ●January 29, 20232023-01-29
On Sunday, January 29, 2023 at 3:29:58 PM UTC-5, lang...@fonz.dk wrote:> søndag den 29. januar 2023 kl. 21.11.31 UTC+1 skrev Fred Bloggs: > > 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. > > I'm pretty sure the BAVs are there for ESD protection. Now the hunt is on for the rationale behind BC850 clamp. > it works like a zener, together with the 5.1k it limits how much current can be pushed into the supplyThe 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.
Reply by ●January 29, 20232023-01-29
On Sunday, January 29, 2023 at 4:06:37 PM UTC-4, lang...@fonz.dk wrote:> søndag den 29. januar 2023 kl. 20.04.31 UTC+1 skrev Ricky: > > On Sunday, January 29, 2023 at 2:56:15 PM UTC-4, lang...@fonz.dk wrote: > > > søndag den 29. januar 2023 kl. 19.15.22 UTC+1 skrev John Larkin: > > > > 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. > > > > > > > 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 > > I typically use Schottky diodes for this, to get the lower Vf. > 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 diodesWhy would anyone care about the leakage current? This design is AC coupled. DC is not being measured. Let 'er leak! -- Rick C. -++ Get 1,000 miles of free Supercharging -++ Tesla referral code - https://ts.la/richard11209
Reply by ●January 29, 20232023-01-29
In one hand-wired proto recently,(*) I was using an LM319N open-collector dual comparator to drive the !set and !clear inputs of one section of a 74VHC74A dflop. The 319 was running off +-15V. By mistake, I wired the pull-up resistor on one section to +15 rather than +5. The circuit worked perfectly--I only noticed the blunder when I went to make a slight change to the ramp current source. Abs max on both VDD and any input is +7V, which appears to be a fairly absurd underestimate of what the !clear input can comfortably manage, at least for a day or two. It didn't seem to be drawing any input current, either--at most a few dozen microamps. Cheers Phil Hobbs (*) It was that highly linear ramp generator I was talking about in another thread. Swapping out the mylar cap for a polypropylene reduced the soakage tail on the ramp, but didn't eliminate it. -- Dr Philip C D Hobbs Principal Consultant ElectroOptical Innovations LLC / Hobbs ElectroOptics Optics, Electro-optics, Photonics, Analog Electronics Briarcliff Manor NY 10510 http://electrooptical.net http://hobbs-eo.com
Reply by ●January 29, 20232023-01-29
Lasse Langwadt Christensen <langwadt@fonz.dk> wrote:>> That's a really weird circuit. > > it sorta works like a zener diode > > https://youtu.be/BGcKjy_UNQ4?t=18That circuit has a big advantage. The capacitance of a reverse-biased BE junction is very low, so it should be much faster than a Zener. However, the power capability is extremely low since the junction area is so small. The circuit may be useful for transient events, but a sustained overload may short the BE junction, rendering the circuit useless. The operating voltage is limited to around 6 volts, which is the breakdown voltage of most common NPN transistors. There is no information on the temperature coefficient of the reverse-biased BE junction, so the circuit has limited application in precision regulators. In addition, breaking down the BE junction will probable damage the junction and degrade the noise figure of the transistor, rendering it useless for low level amplification. -- MRM
Reply by ●January 29, 20232023-01-29
On 1/29/2023 4:34 PM, Phil Hobbs wrote:> In one hand-wired proto recently,(*) I was using an LM319N > open-collector dual comparator to drive the !set and !clear inputs of > one section of a 74VHC74A dflop. > > The 319 was running off +-15V. By mistake, I wired the pull-up resistor > on one section to +15 rather than +5. The circuit worked perfectly--I > only noticed the blunder when I went to make a slight change to the ramp > current source. > > Abs max on both VDD and any input is +7V, which appears to be a fairly > absurd underestimate of what the !clear input can comfortably manage, at > least for a day or two. It didn't seem to be drawing any input current, > either--at most a few dozen microamps. > > Cheers > > Phil Hobbs > > (*) It was that highly linear ramp generator I was talking about in > another thread. Swapping out the mylar cap for a polypropylene reduced > the soakage tail on the ramp, but didn't eliminate it. >you ever try them xicon polystyrenes: <https://www.mouser.com/c/passive-components/capacitors/film-capacitors/?q=xicon%20polystyrene&voltage%20rating%20dc=50%20VDC&sort=pricing%7C1> They're real nice, glad they haven't <knock on wood> discontinued 'em yet
Reply by ●January 29, 20232023-01-29
On Sunday, 29 January 2023 at 20:11:31 UTC, Fred Bloggs wrote:> > That's a really weird circuit. > I'm pretty sure the BAVs are there for ESD protection. Now the hunt is on for the rationale behind BC850 clamp.The BC850 clamps in conjunction with the 22k input resistors are almost certainly there to protect the patient from excessive dc under single fault conditions. Direct current flowing through electrodes attached to the skin can cause severe burns if it is sustained for a long time - which could easily happen in some medical situations. Nobody would put 22k resistors in series with the input of a low-noise amplifier if they didn't need to. Their purpose is to limit the current that can flow when driven by the clamping voltage. Without the clamps the fault current through the electrodes would be around 10 times greater if, for example, one of the BAV99s developed a short to Vcc. John
Reply by ●January 29, 20232023-01-29
On 30/01/23 08:34, Phil Hobbs wrote:> In one hand-wired proto recently,(*) I was using an LM319N > open-collector dual comparator to drive the !set and !clear inputs of > one section of a 74VHC74A dflop. > > The 319 was running off +-15V. By mistake, I wired the pull-up resistor > on one section to +15 rather than +5. The circuit worked perfectly--I > only noticed the blunder when I went to make a slight change to the ramp > current source. > > Abs max on both VDD and any input is +7V, which appears to be a fairly > absurd underestimate of what the !clear input can comfortably manage, at > least for a day or two. It didn't seem to be drawing any input current, > either--at most a few dozen microamps.Possibly such high-speed parts dispense with ESD diodes to reduce input capacitance? Clifford Heath
Reply by ●January 29, 20232023-01-29
mandag den 30. januar 2023 kl. 00.34.59 UTC+1 skrev Clifford Heath:> On 30/01/23 08:34, Phil Hobbs wrote: > > In one hand-wired proto recently,(*) I was using an LM319N > > open-collector dual comparator to drive the !set and !clear inputs of > > one section of a 74VHC74A dflop. > > > > The 319 was running off +-15V. By mistake, I wired the pull-up resistor > > on one section to +15 rather than +5. The circuit worked perfectly--I > > only noticed the blunder when I went to make a slight change to the ramp > > current source. > > > > Abs max on both VDD and any input is +7V, which appears to be a fairly > > absurd underestimate of what the !clear input can comfortably manage, at > > least for a day or two. It didn't seem to be drawing any input current, > > either--at most a few dozen microamps. > Possibly such high-speed parts dispense with ESD diodes to reduce input > capacitance?usually so they can be 5V tolerant and/or handle input while not powered
