Attila Kinali wrote:
On Sun, 23 Dec 2012 22:45:40 +0100
Fabio Eboli<fabi...@quipo.it> wrote:
Il 2012-12-23 07:42 Bruce Griffiths ha scritto:
The classic TAC using current mode switching is similar to the
attached circuit schematic.
http://pastebin.com/EkgqmgfE
I have a couple of small questions about this circuit.
Why are Q2 and Q13 driven by Vth? As there are inverted versions
of the Discharge and Ramp signals available, wouldn't it make sense
to use those to drive Q2 and Q13? I guess it would enhance switching
speed.
Only if the 2 complementary signals have closely matching propagation
delays.
This is usually true with ECL logic but not necessarily true with CMOS
logic.
Also the reverse emitter base breakdown voltage of faster transistors
will be exceeded with 10V pp differential drive.
What is the reason behind the emitter followers Q1 and Q9?
Respecitvely, why shouldnt R3/R4, R7/R8 be connected directly to V+/V-?
Avoidance of saturation and ensuring sufficent headroom for the current
sources/sinks.
Is there a special reson why the current source around Q9 is set to 20mA
and the one around Q4 to 10mA? Is it because Q14/Q15 are driven by a 20mA
current source while Q18/Q19 by a 10mA source?
No special reason although if the long tailed pair driving a subsequent
longtailed pair has a significantly lower tail current than the driven
pair the second pair will switch more slowly.
Am i correct, that the only current source whos value really matters is
the one around Q11? If so, wouldn't it be beneficial to use a stable
reference voltage (probably coupled with the ADC reference) to be used
in an opamp based current source against GND or V- and a current mirror
(cascode or wilson) to drive Q18/Q19 (while leaving the other LED based
current sources as they are, including Q16)?
Ideally the discharge switch current source should equal twice the
charge switch current source to ensure equal currents (and ideally equal
voltage drops across) in the clamp diodes when the ramp capacitor is
fully discharged..
A mirror uses extra matched transistors that can be avoided if an opamp
and reference is used to replace the LED's together with resistor
isolated feedback from the current source emitter.
The ADC reference isn't always accessible particularly with an ADC
embedded within a microprocessor.
Attila Kinali
Bruce
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