Transistor biasing techniques

[keantoken]

oops... I wasn't thinking when I pulled this circuit out of memory... I don't know what I was thinking when I put C1 there. Maybe I was thinking that as a way of feedback or something. I have kindof forgot about positive and negative feedback, but as I do recall, negative feedback goes against the input signal and if there is a phase shift or a delay, the circuit will go into oscillation... I was seeing if I could use the negative feedback from Q2 to keep the current from overdriving the output transistors. Though I guess I wouldn't want do do that because I don't want an oscillator, especially with the low semiconductor capacitances making it go into microwave oscillation and burn the house down (okay, maybe not that bad...). I remember something on wikipedia about a sziklai pair or something to that effect and That is what this arrangement is. I faintly remember something about connecting a resistor of about 100k across the emitter and base of Q2 to improve switching speeds, although a resistor would be required to bias it... I know that my knowledge (or maybe more my understanding) is quite feeble, but I will try to understand.

[TGRANT]

Check out the following link: It’s a free online chapter from a textbook. Its ment for technicians but covers the basics of biasing and calculations for current, saturation and cutoff rather well without any advanced math.

Also try a search using “bipolar transistor biasing” and limit to pdf document types. You’ll get a bunch of online references that will help understand it.

Hope this helps



http://webtools.delmarlearning.com/s...etype%3Apdf%22

[lineup]

.
Refering to my ACTIVE BIAS CIRCUIT in this post
http://www.diyaudio.com/forums/showt...431#post997431


> nice touch and on the face of it this will work

thanks for this info
this confirms my finding in practical circuits


> currents are strongly temp dependent

post one of your circuits you have actually built
where some drift in current does not occur at all


> you may adjust the pot for a specific point but it will then drift

set up a LTP pair with 'my' bias technique
you might be surprised to find, that in pratical appplication
currents will settle nicely after some minutes of warmup

even in a lab board ( the plastic type where you stick in your components ) without soldering and without any keeping associated transistors in body contact for temp compensation
I have had to use base resistors in the magnitude of 220k - 1M
to get safe readings of offset currents

( now I have ordinary DIY multimeter with no greater accuracy than 0.1 mV and as my amplifiers do not suffer very much from less than 0.0001 V offsets I am perfectly happy with my multimeter )


I am very happy to use this ACTIVE BIAS TECHNIQUE
when sometimes I build amplifiers with very high voltage gain.

To all others in this topic:
Do not let comments of negatives and those who havent tested for real
discourage you and keep you from try this at home!
if you have an application that needs very high precision.


lineup
would not post a bad circuit to you ... and call it good
would he?