Class AB Amplifier Input Impedance Question

Thread Starter

Jhon Paul Jaspe

Joined May 13, 2017
23
I'm working on the output stage of an amplifier and i'm wondering. Is the Input Impedance for this circuit, the two 300 Ω resistors in parallel? or are they in series? or do i have to calculate the internal resistance for the diodes and they are a factor in the input impedance??? If not, then what is? Please help
 

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crutschow

Joined Mar 14, 2008
23,292
It's the impedance of one resistor and two diodes in series, in parallel with the other resistor and two diodes in series .
You use the incremental resistance of the diodes at their operating current.
The incremental (dynamic) impedance of one diode is roughly 26/I when I is it operating current in mA.

This, of course, assumes that the input impedance of the Darlington transistors is high enough to have a negligible effect on the amp input impedance.
 

Thread Starter

Jhon Paul Jaspe

Joined May 13, 2017
23
It's the impedance of one resistor and two diodes in series, in parallel with the other resistor and two diodes in series .
You use the incremental resistance of the diodes at their operating current.
The incremental (dynamic) impedance of one diode is roughly 26/I when I is it operating current in mA.

This, of course, assumes that the input impedance of the Darlington transistors is high enough to have a negligible effect on the amp input impedance.
the resistance for the diodes i got was below 0.2 Ω, can i neglect this? also, is it possible to convert the darlington transistors into their re models? if so, then how would i go to solving that, are these two transistors i used in Common Collector configuration?
 

crutschow

Joined Mar 14, 2008
23,292
the resistance for the diodes i got was below 0.2 Ω, can i neglect this?
Probably.
The ohmic resistance of a diode is added to the dynamic resistance as calculated by the formula I posted.
is it possible to convert the darlington transistors into their re models? if so, then how would i go to solving that, are these two transistors i used in Common Collector configuration?
You'd have to look at their data sheets.
 
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