BJT VBE auto adjust?

Find an LED that has the reverse temperature dependency and derive bias from it.

Sounds like a good idea

I could maybe use a dual BJT, and then use the second BJT to set the bias (VBE amplifer style, so the second BJT creates a little higher voltage)

Cheers

Klaus

AOE rev2 figure 2.41 has a circuit like this

Sounds risky. You could add some gain ahead of the detector for a few cents more.

This is pleasingly weird:

(hope the pic works. Dropbox has again chosen to change everything just to annoy their paying customers.)

Can your digital thing catch moderate duty cycle things? If so, eliminate Q2.

--

John Larkin         Highland Technology, Inc 

Science teaches us to doubt. 

  Claude Bernard

I've used the collector voltage, through an RC filter, to bias the base. If you attenuate it, down to circa 0.6V for 2.5V at the collector, your 5V square wave stabilizes at about 50% duty cycle, and tracks that duty cycle without much temperature drift.

you're describing a data slicer.

If the 1Mhz signal is always present you can just take bias from the collector

-----+-- +5 | 10K | --||--+-[100k]---+ | |/ +-[10K]--| | |e 33K | | | +----------+ --- ///

somethig like that,

there will be a slight temperature-to-PWM effect at the output and the

100k feeds that back to base bias.

--
  Jasen.

It works fine. (Dropbox has a funny way to annoy their customers)

I am working on your idea, needs some more work. Will post back if it works

It's a UART input on a micro, so it needs pretty accurate timing

Cheers

Klaus

uart has DC is that an issue?

anyway how about a dual transistor and use one of them to set the bias, sorta like a current mirror?

3Peak comparators from LCSC cost as low as 5 cents.

--
 Thanks, 
    - Win

Normal method is to use a diode or vbe in the biasing network.

RL

100 mV peak, 200 p-p, should work with healthy margins. Run Q1 saturated with a forced beta around 40 maybe. I've spiced that in the past and it works.

Oh, you must be modulating serial data on a carrier. I think my circuit will work for that if the carrier/baud ratio is reasonably high.

--

John Larkin         Highland Technology, Inc 

Science teaches us to doubt. 

  Claude Bernard

I don't believe you. A 100 Mhz ft transistor is a slow transistor, there are plenty of cheap comparators. As soon as one states adding any components, such as diodes, it all adds up.

Even making a diff pair would probably be better option. I would research strandad solutions a bit harder.

-- Kevin Aylward

- SuperSpice

I think he may be sending async serial data modulated OOK on a 1 MHz carrier of varying amplitude. ?. That will take more than just a comparator to translate.

--

John Larkin         Highland Technology, Inc 
picosecond timing   precision measurement  

jlarkin att highlandtechnology dott com 
http://www.highlandtechnology.com

I know. Sadly, no way I can get such a part in a design, purchase would crucify me :-)

voltage (1Vpeak) sine wave into a digital signal

5V from the collector. The signal is 1MHz, so I have added a baker clamp sc hotkky to avoid the BJT storage time

to keep it biased just below the point where the BJT knee kicks in (600mV)

ure, 2mV/K

ng to the actual VBE knee (maybe check the digital side, if no signal, then I need to shift the bias up, and if it is clamped to GND, I need to shift the bias down)

d be the easy solution)

rks

I used forced beta at 30, and tried different values, but could not get it to work

Yes, correct. I am modulating a carrier with a UART TX signal. Then trying to pick it up at the other end with a sort of a AM modulation scheme. I hav e a cap on the output of the last BJT emitter, with a pull-up, so that I ha ve a hold function when the NPN is turned on a regular intervals

Cheers

Klaus

NPN or diodes are much below 0.01 EUR. A comparator with less than 100ns delay is close to 0.1 EUR

A diff pair could be a good idea to try out. If I can keep one transistor out of deep saturation

Cheers

Klaus

Yes, OOK it is :-)

It can be done with a comparator. Just need a hold capacitor on the output and a schmitt trigger input on the UART

An LVDS line receiver, or an opamp, can be a good comparator. I think there are cheap opamps.

I like the zoo sound. OOK! OOK!

--

John Larkin         Highland Technology, Inc 
picosecond timing   precision measurement  

jlarkin att highlandtechnology dott com 
http://www.highlandtechnology.com

AM is not a good idea for digital transmission. It was understood in radio transmissions decades ago and moved to FSK.

It is easy to detect the onset of the AM pulse, but the absence of the pulse (or low level) is difficult to relaibly detect.

--

-TV