Boosting weak devices

Discussion in 'General Electronics Chat' started by Wendy, Apr 4, 2008.

  1. Wendy

    Thread Starter Moderator

    Mar 24, 2008
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    The subject has come up several times concerning drive currents. This is something I've puzzled over for a long time, I have the text book examples, but I'm interested in any other ways that might be used to go about this. Here are 3 classic examples.

    [​IMG]

    Figure 1 is a method for boosting a 555 output. It is quick and dirty, and doesn't have any major drawbacks other than switching current that I am aware of.

    Figure 2 is somewhat similar, but in analog mode. There is some crossover distortion, but the op amp will try to minimize it to an extent.

    Figure 3 is an effort to compensate for the crossover distortion, but has some major problems. The biggest is the possibility of thermal runaway, because it is possible that the transistors will both be conducting at the same time. The second is if the drive required by the transistors exceeds the current flowing through the diodes there will be a cut off of current.

    I'm just interest in other possible solutions to this kind of problems. Thanks.
     
  2. Audioguru

    New Member

    Dec 20, 2007
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    When the government allowed people to buy and use their own telephones (instead of renting from Bell), the company I worked for started selling to businesses the cheapest telephone system available.
    It had horrible crossover distortion which caused complaints from customers.
    I added a resistor so that the opamp drove the speaker directly when the transistors were cutoff. The speaker was 45 ohms, not 8 ohms.
    Word got around that my modification sounded great and I had to mod them all.

    In the circuit with forward biased diodes that allow the transistors to conduct a little, low value emitter resistors are used to iron out small variances in the base-emitter voltage and diodes voltage.
    Then the diodes are attached to the heatsink so that they change their voltage the same amount as the transistors when they heat.
     
  3. Wendy

    Thread Starter Moderator

    Mar 24, 2008
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    Here is my latest attempt (as yet untested) for increasing 555 drive, and making it rail to rail. I think you could change the transistors for Darlington's, which would both boost the output drive and reduce the current required.

    [​IMG]

    Yes SgtWookie, I know it will surge. I'm always trying to see what I can get by with. :p
     
    Last edited: Oct 17, 2008
  4. DickCappels

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  5. studiot

    AAC Fanatic!

    Nov 9, 2007
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    If crossover distortion is your concern, why use push-pull?

    A single ended power device (BJT, FET, Darlington) will exhibit no crossover distortion.
     
  6. Wendy

    Thread Starter Moderator

    Mar 24, 2008
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    Interesting, I will freely admit MOSFET are a weakness of mine. I am missing some details, it has elements of a common base, but I'm not sure what the current mirrors are doing, other than something to do with bias.
     
  7. SgtWookie

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    Jul 17, 2007
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    Sorry Bill, it's worse than that.

    Since the PNP base resistor is tied to the base of the NPN output transistor, it'll never rise above the Vbe of the NPN transistor; so the PNP transistor will always be conducting - even with the NPN turned on.

    I doubt that the NPN's Vbe would rise above 0.8v.
    [eta]
    If Vcc=15 and the first NPN were off, it's collector resistor would have about 14.4mA of current through the base of the 2nd NPN; and slightly less current (due to the Vbe of the PNP) through the PNP's base resistor through the NPN's base, for roughly 28mA total current through the NPN's base. This would be more than enough to put most small signal or switching transistors such as a 2N2222 or 2N3904 into saturation; so Vbe may rise well past a volt.
     
    Last edited: Oct 18, 2008
  8. Wendy

    Thread Starter Moderator

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    IF the first NPN conducts, the PNP will be full on, and the base of the second NPN transistor is ground. If the NPN is off, the second NPN will be getting forward bias through the resistor going to Vcc, the same bias will turn the PNP off. It's current, not voltage that matters here. I'm going to redraw this and put designators on it. Visualize the circuit with the first transistor as a short or open.

    Opps, saw what your talking about. Back to the drawing board.
     
  9. Wendy

    Thread Starter Moderator

    Mar 24, 2008
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    OK, here is my latest attempt. Too many parts for my taste, the problem is I need it to be noninverting. The first drawing will definately invert, the second compensates.

    Why is this important? I have some other projects I want to use this on. The only other thing I can think about is another chip that has high current output and rail to rail capabilities.

    Anyone have other ideas?

    [​IMG]

    Wait a minute, the first design might be what I'm looking for (if it works).
     
  10. studiot

    AAC Fanatic!

    Nov 9, 2007
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    I personally don’t like the 555 as it rarely reforms as predicted, for me at any rate.
    Judging from the literature many others have had similar experiences.

    Having said that I am attaching a couple of sketches to add to the discussion.

    It should be realised that the 555 is analog only in that it derives its timing information from current charging and discharging a capacitor. The rest of the circuitry is really meant to be digital in nature.

    In particular the output transistor, feeding pin 3, is like a TTL output. It is meant to be either in cut off or saturation. Like a TTL output it can be persuaded to run in a linear fashion, but not reproducibly.

    My first sketch describes conditions relevant to the pin 3 output. This can either source or sink 50 to 200 mA depending upon the circuit voltages and particular 555.

    The output is asymmetric. It will swing to around 0.5 volts below Vcc but get to within 0.1 volts of ground..
    Driving this into a darlington will further increase the distance from the rails.

    For this reason additional rails are advised if you want to get the output to Vcc or ground.

    The closest approach will without these will be achieved by use of a relay as described in the bottom sketches. Modern relays can operate as fast as any 555.
    Modern switching FETs are next best, followed by transistor drivers as in the ULN line driver series.
    If you are operating at TTL or CMOS levels you can also increase the current drive by paralleling several gates.

    With regards to pin 7, if your charge / discharge currents are too large to handle then you can increase these with FET or relay. Voltage levels here are not between zero and ground but depend upon the application and may well be outside logic or line driver levels. The second sketch should help model a suitable add on.
     
  11. Wendy

    Thread Starter Moderator

    Mar 24, 2008
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    The thing of it is, a 555 isn't rail to rail. Took me a while to realize this, but it is so. Check the functional diagram I've made for it out.

    [​IMG]

    That Darlington pair could be some of the source of inconsistancy I suspect. To date it has been very predictable for me, but I find I'm still learning things about it.

    *********************************

    Just looked at my schematics again, I probably ought to have 3 diodes in series instead of 2.

    [​IMG]
     
    Last edited: Oct 19, 2008
  12. SgtWookie

    Expert

    Jul 17, 2007
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    Bill, take a look at this page:
    http://www.solarbotics.net/library/circuits/driver_4varHbridge.html
    The problem with the top circuits in your last two posts is that whenever the voltage on the bases is between about 0.6v and Vcc-0.6v, both transistors will be conducting.

    Steve Bolt's H-bridge uses the 74HC14 Schmitt trigger inverter. There will still be a brief moment when both transistors are turned on, but the Schmitt trigger helps minimize that.

    However, the 74HC14 is limited to a Vcc from 2v to 6v.
     
  13. Wendy

    Thread Starter Moderator

    Mar 24, 2008
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    Yep, but like Steve Bolt's design it's on a digital output, so it should be minimized much the same way. If it weren't for those pesky Darlington's it wouldn't be needed, for most applications they come close enough to rail to rail.

    I'll eventually get back with this project, and I want to use a 555 as much as stubborness as anything, the op amp isn't important.

    http://forum.allaboutcircuits.com/showthread.php?t=12405
     
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