# Class AB Amplifier Input Impedance Question

#### 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

#### crutschow

Joined Mar 14, 2008
31,503
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.

#### 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
31,503
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.

#### Bordodynov

Joined May 20, 2015
3,034