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Design Help, Voltage Amplification

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tuurbo46
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Username: tuurbo46

Post Number: 41
Registered: 09-2006

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Posted on Thursday, 22 March, 2012 - 02:28 pm:   Edit Post Delete Post Print Post    Move Post (Moderator/Admin Only)

Hi All,

Im currently reading a device that pulses. I want to read these pulses but the logic level and shape of the pulse is wrong to read into my micro at TTL levels.

So im using a 2N4401 NPN transistor to amplify my signal and a schmitt trigger to square it up:

*Collector - is connected to 12v
*Base - is connected to signal with a 10Mohm to ground
* Emitter is connected to a double nand (schmitt trigger) in series, with the emmitter being tied to ground with a 1Mohm resistor in paralell with a 500pf capacitor.

My problem is on the output of the of the schitt trigger the voltage level is 1 volt, which is to low for TTL.

How would i ramp this voltage up?

* use a darlington pair on input of schmitt instead on one transistor?
* I cannot increase voltage level going in on input of base on current transistor.
* Or would it be best to use a 555 timer on output of schmitt to ramp voltage up?

Look forward to your replies

Cheers

Tuurbo46
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vlf
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Username: vlf

Post Number: 312
Registered: 10-2005


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Posted on Thursday, 22 March, 2012 - 03:47 pm:   Edit Post Delete Post Print Post    Move Post (Moderator/Admin Only)

Based on your using the transistor as a low level impedance follower, the emitter resistor should be around 4k7 for 12 volts, this would produce a current of just under 2.5mA; the resistor values your using will-not sink current, enough for the transistor the conduct fully... also, seems you are level DC biasing the transistor base from the TTL; if so, the emitter follower would give level voltage a little less than the TTL input, but the emitter will have current buffering with high impedance input at the base; the emitter capacitor should-not be required.

To keep the base turned-off between 1-High, a resistor value of 1M or less in the 100K would do.

The Schmitt, is the input through a capacitor coupled !.. if so, you need to ensure a resistor value to the input of the schmitt to either oV or Vs, depending on the data sheet of this device.

A 555 via a 1N capacitor input to pin 2 and 22K resistor to Vs, would allow you to ramp-up the voltage to around 12; and also add timing to pins 6-7 for a longer duration ? as required.

Regards, Brian L.
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tuurbo46
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Username: tuurbo46

Post Number: 42
Registered: 09-2006

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Posted on Thursday, 22 March, 2012 - 08:28 pm:   Edit Post Delete Post Print Post    Move Post (Moderator/Admin Only)

Hi Brian,

Thanks for your help.

Ok, i did not realise the transistor had a minimum current to enable it to work correctly. So i understand the math how you got the 2.5ma - thanks

As for the base of the transistor, i added a 10M resistor to ground thinking it was like a potential diveder and therefore putting the highest possible voltage to the base pin - was the thinking wrong? And the pulse is comming from a GM tube which is rated at 10M.

Im a bit lost on this bit "if so, you need to ensure a resistor value to the input of the schmitt to either oV or Vs, depending on the data sheet of this device"

Are you meaning connect resistor to Vs = Vdd, or oV = Vss?

And lastly my 555 on the output, was just a guess, is this the correct way of doing it or a bit of a heath robinson method?

When replying just copy my post into your post and put yes no etc to save you typing it all out.

Thanks for your help.

Regards,

Tuurbo46
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vlf
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Username: vlf

Post Number: 313
Registered: 10-2005


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Posted on Thursday, 22 March, 2012 - 10:14 pm:   Edit Post Delete Post Print Post    Move Post (Moderator/Admin Only)

The situation will change based on your detecting the pulse from a GM tube, onto the transistor base... their are so many ways to design a detection circuit that needs a high resistance input, to a low impedance output; to save a lot of confusion, would think it's best to have a look at this web-site... see the third circuit down from the top:

http://www.techlib.com/science/geiger.html

It shows a basic input design one transistor, and then pulse shaping gates are used; however any other design would provide the same result, even two or more transistors shown further down the page. A 555 timer as a schmitt, provides the same pulse clicks as the gate circuit; all depends on what you want to hear or see via a meter ?

Also, here is another web-site page; go down a bit to see GM circuit design, the top circuit shows the detector stage, all with transistors:

http://www.circuitsarchive.org/index.php/Geiger_counter

oV = half rail between +/- supplies or negative/ground.
Vs = your positive supply rail... better use V+

Their are other ways to confuse power supply and circuit connections, but I have normally used these terms.

Regards, Brian L.
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tuurbo46
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Username: tuurbo46

Post Number: 43
Registered: 09-2006

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Posted on Friday, 23 March, 2012 - 06:58 pm:   Edit Post Delete Post Print Post    Move Post (Moderator/Admin Only)

Brain,

Thanks for your links. Im actually using the third circuit down on the below link. The small part of circuit after the GM tube, which consists of the transistor and two nand gates, minus the parts after the last nand gate.

I have tried the 555 timer but it still has an output voltage of 1v. I think i must have an impedence problem because i have to add a 10M resistor in series with the output to the nand to get a reading on my analouge oscilloscope.

One last question, if i were to use a non inverting amplifier op amp (741) to ramp my voltage up. Can i connect the negative power supply on the op amp to zero volts, and the positive power supply to +12V if im only after positive volts on the output?

http://www.techlib.com/science/geiger.html

Regards,

Tuurbo46
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vlf
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Username: vlf

Post Number: 314
Registered: 10-2005


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Posted on Saturday, 24 March, 2012 - 09:51 pm:   Edit Post Delete Post Print Post    Move Post (Moderator/Admin Only)

The Op-amp 741 like most others, have two inputs; inverting and non-inverting; as you may know, to get the same pulse in and out, with more gain, you need to use the non-inverting + input-put... don't forget to provide a feed-back resistor maybe 200K inverting input to output; here is a circuit or two that might help:

http://talkingelectronics.com/projects/OP-AMP/OP-AMP-1.html

http://www.sentex.ca/~mec1995/gadgets/741/741.html

Yes, your power supply to the op-amp, is as you say.

I am not sure why your having trouble with the NAND or 555, if these IC's are connected to a 9 volt supply, and biased correctly, the output should trigger between the +/- rail.

Regards, Brian L.
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alec_t
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Username: alec_t

Post Number: 763
Registered: 03-2009

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Posted on Saturday, 31 March, 2012 - 03:55 pm:   Edit Post Delete Post Print Post    Move Post (Moderator/Admin Only)

As a mod of the 'third circuit down' you could try omitting the NPN altogether (it seems pointless and will only attenuate your signal) and applying the Geiger tube output via a 10 megohm resistor directly to the Schmitt NAND gate input. The gate must be a CMOS type (CD4093 or CD40106).
If you're only getting 1V out from a CMOS gate or a 555 you have a short-circuit problem elsewhere.
I wouldn't consider using a 741; it's ancient and there are much better opamps available.
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tuurbo46
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Username: tuurbo46

Post Number: 44
Registered: 09-2006

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Posted on Monday, 02 April, 2012 - 10:08 am:   Edit Post Delete Post Print Post    Move Post (Moderator/Admin Only)

Brian L & alec t

Thanks for your help and input.

Yeah im a bit out of the loop on the newer technologies. As for the 741 i just went through the search filter on farnell and entered general purpose op amp and the 741 was displayed. I will have to sign up to practical electronics again to brush my skills up.

Im currently working back through my circuit on the bread board and i seem to have alot of noise on the scope so i will cure that first and then sort out my voltage level problem.

Thanks again.

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