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Line 17: + + − [[Image:VrefSum.png|thumb|136px|Shunt voltage reference circuit for the summing circuit, similar to that supplying the input stage of the amplifier. Labels in diagram parallel those in the NCP100SNT1G unit manual.]] − + + +
Amplifier board debugging notes (view source)
Revision as of 16:28, 30 September 2010
, 16:28, 30 September 2010→Change of reference for summing circuit input stage DC level
== Change of reference for summing circuit input stage DC level ==
== 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 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 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> 2mA. Current through R1, R2 is negligible, as is the current going to the load (6 summers x 10uA base current) This I<sub>k</sub> ~ I, so R ~ 2kΩ (but anywhere 1-5kΩ works)
== Wrong voltage on R<sub>C</sub>/R<sub>D</sub> junction ==
== Wrong voltage on R<sub>C</sub>/R<sub>D</sub> junction ==