putting comparators in parallel - Page 1
Forum section: electronics / projects

author: Simon
author_rank: Unknown
timestamp: April 09, 2012, 08:23:08 am
content: is it wise to completely parallel all 4 comparators in an LM339 to boost the current sinking capability ?

author: Bored@Work
author_rank: Super Contributor
timestamp: April 09, 2012, 08:37:38 am
content: The outputs are open collector, so you can parallel them. But you won't get what you want. Not all comparators will switch at the same time, and unless all outputs transistors are turned on you won't get four times the normal sink current.Further, the LM339 has the nasty habit of rising the output when sinking too much current while the output is supposed to be low.So there is a good chance you get some rather phunny switching behavior.Use a separate transistor if you need more sink current.

author: Mechatrommer
author_rank: Super Contributor
timestamp: April 09, 2012, 08:39:27 am
content: instead of using comparator as power source, why dont use that small bjt driven?

author: Simon
author_rank: Unknown
timestamp: April 09, 2012, 08:48:34 am
content: right thought it might be a bad idea I guess the comparators will have slightly different offsets. I tried the output transistor but with a 2K load on 23V I got 3V !!! on the output transistor. so my 2N7000 mosfet just stayed on, 20K load improved things to .3V and a very odd ouput and a lot of ringing. As I'm driving a high side P mosfet I wanted fast switching and got better results coupling it straight to the lm339 output but the rise and fall times on the gate were a bit nasty.Basically I'm just playing around making an uber basic SMPS converter. I suppose a proper mosfet driver is the best bet but this is just for kicks,

author: SeanB
author_rank: Super Contributor
timestamp: April 09, 2012, 09:12:21 am
content: ry using a transistor connected as a cascode to provide better drive to the mosfet. If you bias it to about 2V it should give a good fast swing and do so cleanly.

author: Simon
author_rank: Unknown
timestamp: April 09, 2012, 09:27:48 am
content: ok so what is a cascode ? wikipedia shows me what I did but with another fet in series with a grounded gate and so permanently closed ?

author: SeanB
author_rank: Super Contributor
timestamp: April 09, 2012, 09:58:34 am
content: Basically a transistor in the collector line of another. The bottom one only has a low voltage shift in the collector, while the upper one has a high swing. Current is the same in both, but the top one has a high voltage swing without causing problems to the lower one. For your use biasing the transistor to around 3V will cause it to either turn on full or switch off fast, while the bottom will only ever see around 2.4v swing, so will switch fast.The FET does the same, but using the gate threshold voltage to provide the switching action from a junction FET. Saves the 2 resistors, but has a wide range from unit to unit so needs a supply of around 10V for reliable operation.

author: Simon
author_rank: Unknown
timestamp: April 09, 2012, 10:35:11 am
content: am I right in saying that this circuit would not pass a "DC" signal but only a variable one because the capacitance of the higher mosfets gate to source junction comes into play else the second transistor is constantly turned off ?

author: SeanB
author_rank: Super Contributor
timestamp: April 09, 2012, 10:58:37 am
content: No, it passes DC, but the top transistor isolates the bottom one from capacitance, and increases the voltage swing available. Useful to drive something from logic directly, but where the second rail is higher.

author: Simon
author_rank: Unknown
timestamp: April 09, 2012, 12:26:39 pm
content: I must be really dum. the top transistor has the gate tied to ground, so the transistor is perpetually off ? how is it turned on at all ? the emiter/source is always higher voltage than the base/gate ?

author: Zero999
author_rank: Super Contributor
timestamp: April 09, 2012, 12:35:08 pm
content: To answer the original question. It depends on what you're doing. If all you're doing is switching LEDs at lower frequencies, then I'd say yes, it will work and you can get reasonable current sharing. In this case though, no it won't work.I notice you're switching a MOSFET and the supply voltage is 23V; you do know the maximum gate source voltage rating of the 2N7000 is 20V?A couple of BJTs can be used as a MOSFET driver.

author: SeanB
author_rank: Super Contributor
timestamp: April 09, 2012, 12:40:43 pm
content: The first diagram is using Jfets which need a negative gate voltage, and the biasing is omitted. Look further down at the bipolar ones, the comparator will replace Q2 and Ib2 in Fig 2 ( and you select the bias voltage to provide switching instead of linear operation), and you will use a resistive divider to provide Ib1. You can use a resistor as collector load, and if the gate voltage is too high, use a resistive divider or a zener diode to keep it in limits.

author: T4P
author_rank: Super Contributor
timestamp: April 09, 2012, 01:10:20 pm
content: But sadly JFETs have poor demand and are quite expensive now compared to MOSFETs .In the past yes but not now ... *Back to the drawing board*

author: SeanB
author_rank: Super Contributor
timestamp: April 09, 2012, 01:20:57 pm
content: I have used a RF dual gate FET as an and gate before, running off 30V from a VFD to decode a segment and a digit to switch a relay ( I decoded if the channel was any one with the centre bar lit) to do audio switching. Worked pretty well, even if the FEt itself was 20 odd years old then.

author: Simon
author_rank: Unknown
timestamp: April 09, 2012, 02:55:18 pm
content: Quote from: Hero999 on April 09, 2012, 12:35:08 pmYes i suspected it might be a bad idea, yes the 2N7000 does not work to 23V so I used a divider of 2 1K or 10K resistors so that it is 11.5V.I like your driver, I was thinking of something like that but worried about shoot though but i think the way round you have done it will work quite well

author: SeanB
author_rank: Super Contributor
timestamp: April 09, 2012, 03:45:15 pm
content: Look at jim Williams app note AP70 for a little more info about driving mosfets with a cascode.

author: Simon
author_rank: Unknown
timestamp: April 09, 2012, 05:16:28 pm
content: The BJT buffers has helped lower the heat in the mosfet but I still get a lot of ringing on the switching. See photo, I'll try puting a "window" in the comparator levels as it is going fast like 2+ MHz with a 220 ohm load

author: Simon
author_rank: Unknown
timestamp: April 09, 2012, 05:28:56 pm
content: sorry forgot to add. I tried a 100K feedback resistor and that produce a higer frequency yet (well acording to the scope) but a more sloping signal. I tried a 1M resistor and it went back to like the photo.