1,004
edits
Changes
Jump to navigation
Jump to search
Line 1:
Line 1: + + + + + + + + + + + + + + Line 15:
Line 29: + + + + + + + + + + +
Amplifier board debugging notes (view source)
Revision as of 15:31, 13 September 2011
, 15:31, 13 September 2011no edit summary
This page contains a list of all currently known problems with the amplifier board and the solutions implemented
This page contains a list of all currently known problems with the amplifier board and the solutions implemented
= Issue List =
* check that the summing circuit is up to date
* finalize DC plane "fingers"
* settle on switch circuit design
* check voltage references
* check impedance matching, where relevant (possible on internal layers?)
* ensure sufficient bypass
= Bug List =
== Improper DC levels from voltage regulators ==
== Improper DC levels from voltage regulators ==
* '''modifications of the PCB layout is pending'''
* '''modifications of the PCB layout is pending'''
== Change of reference for summing circuit input stage DC level ==
[[Image:VrefSum.png|thumb|136px|Labels in diagram parallel those in the NCP100SNT1G unit manual.]]
The new PNP input stage of the summing circuit, in order to be placed far from cutoff and saturation, is very sensitive to its emitter current, which, with RE=154Ω requires transistor base control on the order of 10mV. Basically, we need the set the base for the desired emitter voltage and accounting for the diode drop ''referencing Vcc, not ground, as it was with NPN''. The shunt voltage regulators used so far can be employed for this, but the ~0.9V offset from Vcc cannot be done with most regulators as they use a double diode drop - 1.24V as internal reference and allow settings above this value. A reference NCP100 SNT 1G (On semiconductors) has been chosen because of it's lower possible settings, making reference to just one diode drop: 0.698V
The adjacent diagram shows the necessary circuit. The component values are set as follows.
* Recommended values for V<sub>cc</sub> - V<sub>B4</sub> = 0.9V - {R1,R2} = {30kΩ,100kΩ}
* Good value for I<sub>k</sub> is 2mA. Current through R1, R2 is negligible, as is the current going to the load (6 summers x 10uA base current) Thus I<sub>k</sub> ~ I, so R ~ 2kΩ (but anywhere 1-5kΩ works)
* 154Ω worked well for the emitter resistor (R<sub>f</sub>) on the summer input stage.
== Wrong voltage on R<sub>C</sub>/R<sub>D</sub> junction ==
== Wrong voltage on R<sub>C</sub>/R<sub>D</sub> junction ==