spec out low power bjt

[panfilero]

I'd like to make a low power one shot circuit, I'm planning to use a monostable oscillator to get my one shot functionality, but I was wondering what NPN transistor specs do I need to look for in order to get the lowest powercircuit out of this? The circuit I'm building is below

link:
http://pcbheaven.com/scripts/imagep...ages/images/transistorcircuits_1235488122.jpg


thanks!

 

[panfilero]

High beta at low current. BCX70K maybe. Make sure you don't zener the e-b

junctions.



Maybe you can use the UniShot circuit. It's like a 1-transistor monostable, sort

of. You can build long strings of them. Fun.



https://dl.dropboxusercontent.com/u/53724080/Circuits/Timing/UniShot.JPG





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John Larkin Highland Technology Inc

www.highlandtechnology.com jlarkin at highlandtechnology dot com



Precision electronic instrumentation

Picosecond-resolution Digital Delay and Pulse generators

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Multichannel arbitrary waveform generators

thank you sir,

so on my circuit, looks like my only current flow will be through R3, whichif I pull it up to 3.3V and use a 10k gives me about 330uA going through it, where do I start to push my limits here, if I have a 100k I end up at 33uA, is this pushing it... I'm just wondering how to evaluate this... 10k isprobally fine for what I'm doing but I'm just curious about how low I could bring my current... does the high beta take care of this?

Is the UniShot non-retriggable?

 

[Bill Sloman]

I'd like to make a low power one shot circuit, I'm planning to use a monostable oscillator to get my one shot functionality, but I was wondering what NPN transistor specs do I need to look for in order to get the lowest power circuit out of this? The circuit I'm building is below

link:

http://pcbheaven.com/scripts/imagep...ages/images/transistorcircuits_1235488122.jpg

Years ago (July 2008), we had a discussion here on making a low power oscillator -"micro power square wave oscillator". The base-emitter capacitance of the transistor involved turned out to be the feature that determined the current drawn at any sensible frequency, and the palm went to wide-band transistors with Ft's of 5GHZ and higher.

I think I posted an LTSpice circuit using a pair of BFR92 transistors to create a 100kHz oscillator. Jeorg went one better with BFP620's (which are more expensive and even more prone to oscillate, but faster). You need a "base-stopper" - something of the order of 33R in series with the base connection - to stop this kind of part from oscillating at few GHz, and you need toremember that base-emitter break-down voltage is only 2V in this kind of transistor.

 

[panfilero]

Years ago (July 2008), we had a discussion here on making a low power oscillator -"micro power square wave oscillator". The base-emitter capacitanceof the transistor involved turned out to be the feature that determined the current drawn at any sensible frequency, and the palm went to wide-band transistors with Ft's of 5GHZ and higher.



I think I posted an LTSpice circuit using a pair of BFR92 transistors to create a 100kHz oscillator. Jeorg went one better with BFP620's (which are more expensive and even more prone to oscillate, but faster). You need a "base-stopper" - something of the order of 33R in series with the base connection - to stop this kind of part from oscillating at few GHz, and you need to remember that base-emitter break-down voltage is only 2V in this kind oftransistor.

thank you sir, I'll look into these

 

[Jasen Betts]

On Thu, 09 May 2013 14:08:07 -0700, Jim Thompson

[snip]

http://www.analog-innovations.com/SED/Oooops.pdf

The above link has been re-posted, with a fix for the glitch... easy
for those of us who actually understand transistors >:-}

...Jim Thompson

Those without "limited skill set" are invited to muse over WHY the
base resistors I added get rid of the glitch sensitivity.

It forms a low pass filter with the base capacitance


Another way to fix it is to increase the collector resistors a bit.
if the transistors aren't saturating you've got a high pass filter
with unconstrained gain if they are you have sub-unity AC gain,
and thus should be safe.


I had to reduce the collector resistors in LTSpice else I didn't get
the glitches you were seeing

 

[Jasen Betts]

On Thu, 09 May 2013 15:14:49 -0700, Jim Thompson

On Thu, 09 May 2013 14:08:07 -0700, Jim Thompson

[snip]

http://www.analog-innovations.com/SED/Oooops.pdf


The above link has been re-posted, with a fix for the glitch... easy
for those of us who actually understand transistors >:-}

...Jim Thompson

Those without "limited skill set" are invited to muse over WHY the
base resistors I added get rid of the glitch sensitivity.


It forms a low pass filter with the base capacitance

Nope. It has to do with the base input characteristics.

Another way to fix it is to increase the collector resistors a bit.
if the transistors aren't saturating you've got a high pass filter
with unconstrained gain if they are you have sub-unity AC gain,
and thus should be safe.

Are you suggesting suicide bias? But you idea _will_ work, IF you pad
the emitter with a resistor and then use a divider to set the bias
rather than a single resistor... then you basically have a class-A
stage.

I'm not familiar with that term ( googling "bipolar suicide bias" was
no help :^) )

I'm seeing a failure mode where the voltage source beeing is pushing a pulse
through tee B-C junctions of the transistors when it goes high, adding
emitter resistors but leaving the base connected to the capacitor will
likely make that glitch worse.

I'm seeing it run slightly faster. with increased input impedance of
each stage the collector waveform output of the intermediate stages
is squarer. that's about all.

I ran it with TStepMax = 10ns

10ns made no diference. neither did 1ns, except for increasing the run
time.

I dont see the glitches like in your PDF until I reduce the collector
resistor to about 1K



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