Electronics > Projects, Designs, and Technical Stuff
help with Class AB amplifier
magic:
I don't think there is any "another function" :)
Yes, it's the bias spreader which determines, and hopefully thermally compensates, the output stage standing current.
David Hess:
--- Quote from: rex1232 on November 23, 2019, 07:33:47 pm ---Is another function of Q7 (R12, R14, R28) not biasing?
--- End quote ---
That is Q7's only function; Q7 is configured as a Vbe multiplier.
The resistor ratio multiplies Q7's Vbe, including the roughly -2mV/C temperature coefficient of Q7's Vbe, to produce a higher voltage which in this case is about 4 x Vbe + -8mV/C from the driver and output transistors. Q7 should be mounted on the same heatsink as the output transistors so that they track in temperature.
bson:
--- Quote from: David Hess on November 23, 2019, 06:27:42 pm ---For a given current, bipolar transistors have a greater gm than FETs.
--- End quote ---
What current is that, the DC bias current? Why would that matter - you'd bias for linearity and range, the bias current then is what it is - within reason of course. In other words, BJTs and FETs will obviously have different bias currents.
rex1232:
Hey, the current mirrors I have seen always have 2 transistors where they are connected by the base and also to their collector.
How does it work when there only is when transitor? (Q6)
Does it have something to do with D10's temperature compensation and Q16, Q17 pulling the Current mirror up and down?
Thank you in advance :)
magic:
The diode is an approximation of that missing transistor ;)
I'm kinda too lazy to write a whole post explaining how mirrors work, you should be able to find info on the net.
Recently W2AEW advertised his video on this topic so what the heck, I will link it, his videos tend to be informative and easy to digest.
I pray that he explained both the transistor and the diode version :-DD
--- Quote from: bson on November 24, 2019, 06:52:36 pm ---What current is that, the DC bias current? Why would that matter - you'd bias for linearity and range, the bias current then is what it is - within reason of course. In other words, BJTs and FETs will obviously have different bias currents.
--- End quote ---
In my (very limited and somewhat biased towards fancy high-spec designs) experience it's not that unusual to see BJT inputs running at a milliamp or more of current per leg, which gives each transistor a gm in excess of 30mS. That's more than most JFETs can do at full IDSS. Many BJT designs degenerate the input pair to keep gm down.
Navigation
[0] Message Index
[#] Next page
[*] Previous page
Go to full version